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apache/cloudberry/HEAD/./src/backend/access/transam/multixact.c
blob: 9b3e02a660962d51700b62bd235da8fcd84409b0 [file]
/*-------------------------------------------------------------------------
*
* multixact.c
* PostgreSQL multi-transaction-log manager
*
* The pg_multixact manager is a pg_xact-like manager that stores an array of
* MultiXactMember for each MultiXactId. It is a fundamental part of the
* shared-row-lock implementation. Each MultiXactMember is comprised of a
* TransactionId and a set of flag bits. The name is a bit historical:
* originally, a MultiXactId consisted of more than one TransactionId (except
* in rare corner cases), hence "multi". Nowadays, however, it's perfectly
* legitimate to have MultiXactIds that only include a single Xid.
*
* The meaning of the flag bits is opaque to this module, but they are mostly
* used in heapam.c to identify lock modes that each of the member transactions
* is holding on any given tuple. This module just contains support to store
* and retrieve the arrays.
*
* We use two SLRU areas, one for storing the offsets at which the data
* starts for each MultiXactId in the other one. This trick allows us to
* store variable length arrays of TransactionIds. (We could alternatively
* use one area containing counts and TransactionIds, with valid MultiXactId
* values pointing at slots containing counts; but that way seems less robust
* since it would get completely confused if someone inquired about a bogus
* MultiXactId that pointed to an intermediate slot containing an XID.)
*
* XLOG interactions: this module generates a record whenever a new OFFSETs or
* MEMBERs page is initialized to zeroes, as well as an
* XLOG_MULTIXACT_CREATE_ID record whenever a new MultiXactId is defined.
* This module ignores the WAL rule "write xlog before data," because it
* suffices that actions recording a MultiXactId in a heap xmax do follow that
* rule. The only way for the MXID to be referenced from any data page is for
* heap_lock_tuple() or heap_update() to have put it there, and each generates
* an XLOG record that must follow ours. The normal LSN interlock between the
* data page and that XLOG record will ensure that our XLOG record reaches
* disk first. If the SLRU members/offsets data reaches disk sooner than the
* XLOG records, we do not care; after recovery, no xmax will refer to it. On
* the flip side, to ensure that all referenced entries _do_ reach disk, this
* module's XLOG records completely rebuild the data entered since the last
* checkpoint. We flush and sync all dirty OFFSETs and MEMBERs pages to disk
* before each checkpoint is considered complete.
*
* Like clog.c, and unlike subtrans.c, we have to preserve state across
* crashes and ensure that MXID and offset numbering increases monotonically
* across a crash. We do this in the same way as it's done for transaction
* IDs: the WAL record is guaranteed to contain evidence of every MXID we
* could need to worry about, and we just make sure that at the end of
* replay, the next-MXID and next-offset counters are at least as large as
* anything we saw during replay.
*
* We are able to remove segments no longer necessary by carefully tracking
* each table's used values: during vacuum, any multixact older than a certain
* value is removed; the cutoff value is stored in pg_class. The minimum value
* across all tables in each database is stored in pg_database, and the global
* minimum across all databases is part of pg_control and is kept in shared
* memory. Whenever that minimum is advanced, the SLRUs are truncated.
*
* When new multixactid values are to be created, care is taken that the
* counter does not fall within the wraparound horizon considering the global
* minimum value.
*
* Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* src/backend/access/transam/multixact.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/multixact.h"
#include "access/slru.h"
#include "access/transam.h"
#include "access/twophase.h"
#include "access/twophase_rmgr.h"
#include "access/xact.h"
#include "access/xloginsert.h"
#include "access/xlogutils.h"
#include "catalog/pg_type.h"
#include "commands/dbcommands.h"
#include "funcapi.h"
#include "lib/ilist.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "postmaster/autovacuum.h"
#include "storage/lmgr.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "utils/builtins.h"
#include "utils/memutils.h"
#include "utils/snapmgr.h"
/*
* Defines for MultiXactOffset page sizes. A page is the same BLCKSZ as is
* used everywhere else in Postgres.
*
* Note: because MultiXactOffsets are 32 bits and wrap around at 0xFFFFFFFF,
* MultiXact page numbering also wraps around at
* 0xFFFFFFFF/MULTIXACT_OFFSETS_PER_PAGE, and segment numbering at
* 0xFFFFFFFF/MULTIXACT_OFFSETS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need
* take no explicit notice of that fact in this module, except when comparing
* segment and page numbers in TruncateMultiXact (see
* MultiXactOffsetPagePrecedes).
*/
/* We need four bytes per offset */
#define MULTIXACT_OFFSETS_PER_PAGE (BLCKSZ / sizeof(MultiXactOffset))
#define MultiXactIdToOffsetPage(xid) \
((xid) / (MultiXactOffset) MULTIXACT_OFFSETS_PER_PAGE)
#define MultiXactIdToOffsetEntry(xid) \
((xid) % (MultiXactOffset) MULTIXACT_OFFSETS_PER_PAGE)
#define MultiXactIdToOffsetSegment(xid) (MultiXactIdToOffsetPage(xid) / SLRU_PAGES_PER_SEGMENT)
/*
* The situation for members is a bit more complex: we store one byte of
* additional flag bits for each TransactionId. To do this without getting
* into alignment issues, we store four bytes of flags, and then the
* corresponding 4 Xids. Each such 5-word (20-byte) set we call a "group", and
* are stored as a whole in pages. Thus, with 8kB BLCKSZ, we keep 409 groups
* per page. This wastes 12 bytes per page, but that's OK -- simplicity (and
* performance) trumps space efficiency here.
*
* Note that the "offset" macros work with byte offset, not array indexes, so
* arithmetic must be done using "char *" pointers.
*/
/* We need eight bits per xact, so one xact fits in a byte */
#define MXACT_MEMBER_BITS_PER_XACT 8
#define MXACT_MEMBER_FLAGS_PER_BYTE 1
#define MXACT_MEMBER_XACT_BITMASK ((1 << MXACT_MEMBER_BITS_PER_XACT) - 1)
/* how many full bytes of flags are there in a group? */
#define MULTIXACT_FLAGBYTES_PER_GROUP 4
#define MULTIXACT_MEMBERS_PER_MEMBERGROUP \
(MULTIXACT_FLAGBYTES_PER_GROUP * MXACT_MEMBER_FLAGS_PER_BYTE)
/* size in bytes of a complete group */
#define MULTIXACT_MEMBERGROUP_SIZE \
(sizeof(TransactionId) * MULTIXACT_MEMBERS_PER_MEMBERGROUP + MULTIXACT_FLAGBYTES_PER_GROUP)
#define MULTIXACT_MEMBERGROUPS_PER_PAGE (BLCKSZ / MULTIXACT_MEMBERGROUP_SIZE)
#define MULTIXACT_MEMBERS_PER_PAGE \
(MULTIXACT_MEMBERGROUPS_PER_PAGE * MULTIXACT_MEMBERS_PER_MEMBERGROUP)
/*
* Because the number of items per page is not a divisor of the last item
* number (member 0xFFFFFFFF), the last segment does not use the maximum number
* of pages, and moreover the last used page therein does not use the same
* number of items as previous pages. (Another way to say it is that the
* 0xFFFFFFFF member is somewhere in the middle of the last page, so the page
* has some empty space after that item.)
*
* This constant is the number of members in the last page of the last segment.
*/
#define MAX_MEMBERS_IN_LAST_MEMBERS_PAGE \
((uint32) ((0xFFFFFFFF % MULTIXACT_MEMBERS_PER_PAGE) + 1))
/* page in which a member is to be found */
#define MXOffsetToMemberPage(xid) ((xid) / (TransactionId) MULTIXACT_MEMBERS_PER_PAGE)
#define MXOffsetToMemberSegment(xid) (MXOffsetToMemberPage(xid) / SLRU_PAGES_PER_SEGMENT)
/* Location (byte offset within page) of flag word for a given member */
#define MXOffsetToFlagsOffset(xid) \
((((xid) / (TransactionId) MULTIXACT_MEMBERS_PER_MEMBERGROUP) % \
(TransactionId) MULTIXACT_MEMBERGROUPS_PER_PAGE) * \
(TransactionId) MULTIXACT_MEMBERGROUP_SIZE)
#define MXOffsetToFlagsBitShift(xid) \
(((xid) % (TransactionId) MULTIXACT_MEMBERS_PER_MEMBERGROUP) * \
MXACT_MEMBER_BITS_PER_XACT)
/* Location (byte offset within page) of TransactionId of given member */
#define MXOffsetToMemberOffset(xid) \
(MXOffsetToFlagsOffset(xid) + MULTIXACT_FLAGBYTES_PER_GROUP + \
((xid) % MULTIXACT_MEMBERS_PER_MEMBERGROUP) * sizeof(TransactionId))
/* Multixact members wraparound thresholds. */
#define MULTIXACT_MEMBER_SAFE_THRESHOLD (MaxMultiXactOffset / 2)
#define MULTIXACT_MEMBER_DANGER_THRESHOLD \
(MaxMultiXactOffset - MaxMultiXactOffset / 4)
#define PreviousMultiXactId(xid) \
((xid) == FirstMultiXactId ? MaxMultiXactId : (xid) - 1)
/*
* Links to shared-memory data structures for MultiXact control
*/
static SlruCtlData MultiXactOffsetCtlData;
static SlruCtlData MultiXactMemberCtlData;
#define MultiXactOffsetCtl (&MultiXactOffsetCtlData)
#define MultiXactMemberCtl (&MultiXactMemberCtlData)
/*
* MultiXact state shared across all backends. All this state is protected
* by MultiXactGenLock. (We also use MultiXactOffsetSLRULock and
* MultiXactMemberSLRULock to guard accesses to the two sets of SLRU
* buffers. For concurrency's sake, we avoid holding more than one of these
* locks at a time.)
*/
typedef struct MultiXactStateData
{
/* next-to-be-assigned MultiXactId */
MultiXactId nextMXact;
/* next-to-be-assigned offset */
MultiXactOffset nextOffset;
/* Have we completed multixact startup? */
bool finishedStartup;
/*
* Oldest multixact that is still potentially referenced by a relation.
* Anything older than this should not be consulted. These values are
* updated by vacuum.
*/
MultiXactId oldestMultiXactId;
Oid oldestMultiXactDB;
/*
* Oldest multixact offset that is potentially referenced by a multixact
* referenced by a relation. We don't always know this value, so there's
* a flag here to indicate whether or not we currently do.
*/
MultiXactOffset oldestOffset;
bool oldestOffsetKnown;
/* support for anti-wraparound measures */
MultiXactId multiVacLimit;
MultiXactId multiWarnLimit;
MultiXactId multiStopLimit;
MultiXactId multiWrapLimit;
/* support for members anti-wraparound measures */
MultiXactOffset offsetStopLimit; /* known if oldestOffsetKnown */
/*
* Per-backend data starts here. We have two arrays stored in the area
* immediately following the MultiXactStateData struct. Each is indexed by
* BackendId.
*
* In both arrays, there's a slot for all normal backends (1..MaxBackends)
* followed by a slot for max_prepared_xacts prepared transactions. Valid
* BackendIds start from 1; element zero of each array is never used.
*
* OldestMemberMXactId[k] is the oldest MultiXactId each backend's current
* transaction(s) could possibly be a member of, or InvalidMultiXactId
* when the backend has no live transaction that could possibly be a
* member of a MultiXact. Each backend sets its entry to the current
* nextMXact counter just before first acquiring a shared lock in a given
* transaction, and clears it at transaction end. (This works because only
* during or after acquiring a shared lock could an XID possibly become a
* member of a MultiXact, and that MultiXact would have to be created
* during or after the lock acquisition.)
*
* OldestVisibleMXactId[k] is the oldest MultiXactId each backend's
* current transaction(s) think is potentially live, or InvalidMultiXactId
* when not in a transaction or not in a transaction that's paid any
* attention to MultiXacts yet. This is computed when first needed in a
* given transaction, and cleared at transaction end. We can compute it
* as the minimum of the valid OldestMemberMXactId[] entries at the time
* we compute it (using nextMXact if none are valid). Each backend is
* required not to attempt to access any SLRU data for MultiXactIds older
* than its own OldestVisibleMXactId[] setting; this is necessary because
* the relevant SLRU data can be concurrently truncated away.
*
* The oldest valid value among all of the OldestMemberMXactId[] and
* OldestVisibleMXactId[] entries is considered by vacuum as the earliest
* possible value still having any live member transaction -- OldestMxact.
* Any value older than that is typically removed from tuple headers, or
* "frozen" via being replaced with a new xmax. VACUUM can sometimes even
* remove an individual MultiXact xmax whose value is >= its OldestMxact
* cutoff, though typically only when no individual member XID is still
* running. See FreezeMultiXactId for full details.
*
* Whenever VACUUM advances relminmxid, then either its OldestMxact cutoff
* or the oldest extant Multi remaining in the table is used as the new
* pg_class.relminmxid value (whichever is earlier). The minimum of all
* relminmxid values in each database is stored in pg_database.datminmxid.
* In turn, the minimum of all of those values is stored in pg_control.
* This is used as the truncation point for pg_multixact when unneeded
* segments get removed by vac_truncate_clog() during vacuuming.
*/
MultiXactId perBackendXactIds[FLEXIBLE_ARRAY_MEMBER];
} MultiXactStateData;
/*
* Last element of OldestMemberMXactId and OldestVisibleMXactId arrays.
* Valid elements are (1..MaxOldestSlot); element 0 is never used.
*/
#define MaxOldestSlot (MaxBackends + max_prepared_xacts)
/* Pointers to the state data in shared memory */
static MultiXactStateData *MultiXactState;
static MultiXactId *OldestMemberMXactId;
static MultiXactId *OldestVisibleMXactId;
/*
* Definitions for the backend-local MultiXactId cache.
*
* We use this cache to store known MultiXacts, so we don't need to go to
* SLRU areas every time.
*
* The cache lasts for the duration of a single transaction, the rationale
* for this being that most entries will contain our own TransactionId and
* so they will be uninteresting by the time our next transaction starts.
* (XXX not clear that this is correct --- other members of the MultiXact
* could hang around longer than we did. However, it's not clear what a
* better policy for flushing old cache entries would be.) FIXME actually
* this is plain wrong now that multixact's may contain update Xids.
*
* We allocate the cache entries in a memory context that is deleted at
* transaction end, so we don't need to do retail freeing of entries.
*/
typedef struct mXactCacheEnt
{
MultiXactId multi;
int nmembers;
dlist_node node;
MultiXactMember members[FLEXIBLE_ARRAY_MEMBER];
} mXactCacheEnt;
#define MAX_CACHE_ENTRIES 256
static dclist_head MXactCache = DCLIST_STATIC_INIT(MXactCache);
static MemoryContext MXactContext = NULL;
#ifdef MULTIXACT_DEBUG
#define debug_elog2(a,b) elog(a,b)
#define debug_elog3(a,b,c) elog(a,b,c)
#define debug_elog4(a,b,c,d) elog(a,b,c,d)
#define debug_elog5(a,b,c,d,e) elog(a,b,c,d,e)
#define debug_elog6(a,b,c,d,e,f) elog(a,b,c,d,e,f)
#else
#define debug_elog2(a,b)
#define debug_elog3(a,b,c)
#define debug_elog4(a,b,c,d)
#define debug_elog5(a,b,c,d,e)
#define debug_elog6(a,b,c,d,e,f)
#endif
/* internal MultiXactId management */
static void MultiXactIdSetOldestVisible(void);
static void RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
int nmembers, MultiXactMember *members);
static MultiXactId GetNewMultiXactId(int nmembers, MultiXactOffset *offset);
/* MultiXact cache management */
static int mxactMemberComparator(const void *arg1, const void *arg2);
static MultiXactId mXactCacheGetBySet(int nmembers, MultiXactMember *members);
static int mXactCacheGetById(MultiXactId multi, MultiXactMember **members);
static void mXactCachePut(MultiXactId multi, int nmembers,
MultiXactMember *members);
static char *mxstatus_to_string(MultiXactStatus status);
/* management of SLRU infrastructure */
static int ZeroMultiXactOffsetPage(int pageno, bool writeXlog);
static int ZeroMultiXactMemberPage(int pageno, bool writeXlog);
static bool MultiXactOffsetPagePrecedes(int page1, int page2);
static bool MultiXactMemberPagePrecedes(int page1, int page2);
static bool MultiXactOffsetPrecedes(MultiXactOffset offset1,
MultiXactOffset offset2);
static void ExtendMultiXactOffset(MultiXactId multi);
static void ExtendMultiXactMember(MultiXactOffset offset, int nmembers);
static bool MultiXactOffsetWouldWrap(MultiXactOffset boundary,
MultiXactOffset start, uint32 distance);
static bool SetOffsetVacuumLimit(bool is_startup);
static bool find_multixact_start(MultiXactId multi, MultiXactOffset *result);
static void WriteMZeroPageXlogRec(int pageno, uint8 info);
static void WriteMTruncateXlogRec(Oid oldestMultiDB,
MultiXactId startTruncOff,
MultiXactId endTruncOff,
MultiXactOffset startTruncMemb,
MultiXactOffset endTruncMemb);
/*
* MultiXactIdCreate
* Construct a MultiXactId representing two TransactionIds.
*
* The two XIDs must be different, or be requesting different statuses.
*
* NB - we don't worry about our local MultiXactId cache here, because that
* is handled by the lower-level routines.
*/
MultiXactId
MultiXactIdCreate(TransactionId xid1, MultiXactStatus status1,
TransactionId xid2, MultiXactStatus status2)
{
MultiXactId newMulti;
MultiXactMember members[2];
Assert(TransactionIdIsValid(xid1));
Assert(TransactionIdIsValid(xid2));
Assert(!TransactionIdEquals(xid1, xid2) || (status1 != status2));
/* MultiXactIdSetOldestMember() must have been called already. */
Assert(MultiXactIdIsValid(OldestMemberMXactId[MyBackendId]));
/*
* Note: unlike MultiXactIdExpand, we don't bother to check that both XIDs
* are still running. In typical usage, xid2 will be our own XID and the
* caller just did a check on xid1, so it'd be wasted effort.
*/
members[0].xid = xid1;
members[0].status = status1;
members[1].xid = xid2;
members[1].status = status2;
newMulti = MultiXactIdCreateFromMembers(2, members);
debug_elog3(DEBUG2, "Create: %s",
mxid_to_string(newMulti, 2, members));
return newMulti;
}
/*
* MultiXactIdExpand
* Add a TransactionId to a pre-existing MultiXactId.
*
* If the TransactionId is already a member of the passed MultiXactId with the
* same status, just return it as-is.
*
* Note that we do NOT actually modify the membership of a pre-existing
* MultiXactId; instead we create a new one. This is necessary to avoid
* a race condition against code trying to wait for one MultiXactId to finish;
* see notes in heapam.c.
*
* NB - we don't worry about our local MultiXactId cache here, because that
* is handled by the lower-level routines.
*
* Note: It is critical that MultiXactIds that come from an old cluster (i.e.
* one upgraded by pg_upgrade from a cluster older than this feature) are not
* passed in.
*/
MultiXactId
MultiXactIdExpand(MultiXactId multi, TransactionId xid, MultiXactStatus status)
{
MultiXactId newMulti;
MultiXactMember *members;
MultiXactMember *newMembers;
int nmembers;
int i;
int j;
Assert(MultiXactIdIsValid(multi));
Assert(TransactionIdIsValid(xid));
/* MultiXactIdSetOldestMember() must have been called already. */
Assert(MultiXactIdIsValid(OldestMemberMXactId[MyBackendId]));
debug_elog5(DEBUG2, "Expand: received multi %u, xid %u status %s",
multi, xid, mxstatus_to_string(status));
/*
* Note: we don't allow for old multis here. The reason is that the only
* caller of this function does a check that the multixact is no longer
* running.
*/
nmembers = GetMultiXactIdMembers(multi, &members, false, false);
if (nmembers < 0)
{
MultiXactMember member;
/*
* The MultiXactId is obsolete. This can only happen if all the
* MultiXactId members stop running between the caller checking and
* passing it to us. It would be better to return that fact to the
* caller, but it would complicate the API and it's unlikely to happen
* too often, so just deal with it by creating a singleton MultiXact.
*/
member.xid = xid;
member.status = status;
newMulti = MultiXactIdCreateFromMembers(1, &member);
debug_elog4(DEBUG2, "Expand: %u has no members, create singleton %u",
multi, newMulti);
return newMulti;
}
/*
* If the TransactionId is already a member of the MultiXactId with the
* same status, just return the existing MultiXactId.
*/
for (i = 0; i < nmembers; i++)
{
if (TransactionIdEquals(members[i].xid, xid) &&
(members[i].status == status))
{
debug_elog4(DEBUG2, "Expand: %u is already a member of %u",
xid, multi);
pfree(members);
return multi;
}
}
/*
* Determine which of the members of the MultiXactId are still of
* interest. This is any running transaction, and also any transaction
* that grabbed something stronger than just a lock and was committed. (An
* update that aborted is of no interest here; and having more than one
* update Xid in a multixact would cause errors elsewhere.)
*
* Removing dead members is not just an optimization: freezing of tuples
* whose Xmax are multis depends on this behavior.
*
* Note we have the same race condition here as above: j could be 0 at the
* end of the loop.
*/
newMembers = (MultiXactMember *)
palloc(sizeof(MultiXactMember) * (nmembers + 1));
for (i = 0, j = 0; i < nmembers; i++)
{
if (TransactionIdIsInProgress(members[i].xid) ||
(ISUPDATE_from_mxstatus(members[i].status) &&
TransactionIdDidCommit(members[i].xid)))
{
newMembers[j].xid = members[i].xid;
newMembers[j++].status = members[i].status;
}
}
newMembers[j].xid = xid;
newMembers[j++].status = status;
newMulti = MultiXactIdCreateFromMembers(j, newMembers);
pfree(members);
pfree(newMembers);
debug_elog3(DEBUG2, "Expand: returning new multi %u", newMulti);
return newMulti;
}
/*
* MultiXactIdIsRunning
* Returns whether a MultiXactId is "running".
*
* We return true if at least one member of the given MultiXactId is still
* running. Note that a "false" result is certain not to change,
* because it is not legal to add members to an existing MultiXactId.
*
* Caller is expected to have verified that the multixact does not come from
* a pg_upgraded share-locked tuple.
*/
bool
MultiXactIdIsRunning(MultiXactId multi, bool isLockOnly)
{
MultiXactMember *members;
int nmembers;
int i;
debug_elog3(DEBUG2, "IsRunning %u?", multi);
/*
* "false" here means we assume our callers have checked that the given
* multi cannot possibly come from a pg_upgraded database.
*/
nmembers = GetMultiXactIdMembers(multi, &members, false, isLockOnly);
if (nmembers <= 0)
{
debug_elog2(DEBUG2, "IsRunning: no members");
return false;
}
/*
* Checking for myself is cheap compared to looking in shared memory;
* return true if any live subtransaction of the current top-level
* transaction is a member.
*
* This is not needed for correctness, it's just a fast path.
*/
for (i = 0; i < nmembers; i++)
{
if (TransactionIdIsCurrentTransactionId(members[i].xid))
{
debug_elog3(DEBUG2, "IsRunning: I (%d) am running!", i);
pfree(members);
return true;
}
}
/*
* This could be made faster by having another entry point in procarray.c,
* walking the PGPROC array only once for all the members. But in most
* cases nmembers should be small enough that it doesn't much matter.
*/
for (i = 0; i < nmembers; i++)
{
if (TransactionIdIsInProgress(members[i].xid))
{
debug_elog4(DEBUG2, "IsRunning: member %d (%u) is running",
i, members[i].xid);
pfree(members);
return true;
}
}
pfree(members);
debug_elog3(DEBUG2, "IsRunning: %u is not running", multi);
return false;
}
/*
* MultiXactIdSetOldestMember
* Save the oldest MultiXactId this transaction could be a member of.
*
* We set the OldestMemberMXactId for a given transaction the first time it's
* going to do some operation that might require a MultiXactId (tuple lock,
* update or delete). We need to do this even if we end up using a
* TransactionId instead of a MultiXactId, because there is a chance that
* another transaction would add our XID to a MultiXactId.
*
* The value to set is the next-to-be-assigned MultiXactId, so this is meant to
* be called just before doing any such possibly-MultiXactId-able operation.
*/
void
MultiXactIdSetOldestMember(void)
{
if (!MultiXactIdIsValid(OldestMemberMXactId[MyBackendId]))
{
MultiXactId nextMXact;
/*
* You might think we don't need to acquire a lock here, since
* fetching and storing of TransactionIds is probably atomic, but in
* fact we do: suppose we pick up nextMXact and then lose the CPU for
* a long time. Someone else could advance nextMXact, and then
* another someone else could compute an OldestVisibleMXactId that
* would be after the value we are going to store when we get control
* back. Which would be wrong.
*
* Note that a shared lock is sufficient, because it's enough to stop
* someone from advancing nextMXact; and nobody else could be trying
* to write to our OldestMember entry, only reading (and we assume
* storing it is atomic.)
*/
LWLockAcquire(MultiXactGenLock, LW_SHARED);
/*
* We have to beware of the possibility that nextMXact is in the
* wrapped-around state. We don't fix the counter itself here, but we
* must be sure to store a valid value in our array entry.
*/
nextMXact = MultiXactState->nextMXact;
if (nextMXact < FirstMultiXactId)
nextMXact = FirstMultiXactId;
OldestMemberMXactId[MyBackendId] = nextMXact;
LWLockRelease(MultiXactGenLock);
debug_elog4(DEBUG2, "MultiXact: setting OldestMember[%d] = %u",
MyBackendId, nextMXact);
}
}
/*
* MultiXactIdSetOldestVisible
* Save the oldest MultiXactId this transaction considers possibly live.
*
* We set the OldestVisibleMXactId for a given transaction the first time
* it's going to inspect any MultiXactId. Once we have set this, we are
* guaranteed that SLRU data for MultiXactIds >= our own OldestVisibleMXactId
* won't be truncated away.
*
* The value to set is the oldest of nextMXact and all the valid per-backend
* OldestMemberMXactId[] entries. Because of the locking we do, we can be
* certain that no subsequent call to MultiXactIdSetOldestMember can set
* an OldestMemberMXactId[] entry older than what we compute here. Therefore
* there is no live transaction, now or later, that can be a member of any
* MultiXactId older than the OldestVisibleMXactId we compute here.
*/
static void
MultiXactIdSetOldestVisible(void)
{
if (!MultiXactIdIsValid(OldestVisibleMXactId[MyBackendId]))
{
MultiXactId oldestMXact;
int i;
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
/*
* We have to beware of the possibility that nextMXact is in the
* wrapped-around state. We don't fix the counter itself here, but we
* must be sure to store a valid value in our array entry.
*/
oldestMXact = MultiXactState->nextMXact;
if (oldestMXact < FirstMultiXactId)
oldestMXact = FirstMultiXactId;
for (i = 1; i <= MaxOldestSlot; i++)
{
MultiXactId thisoldest = OldestMemberMXactId[i];
if (MultiXactIdIsValid(thisoldest) &&
MultiXactIdPrecedes(thisoldest, oldestMXact))
oldestMXact = thisoldest;
}
OldestVisibleMXactId[MyBackendId] = oldestMXact;
LWLockRelease(MultiXactGenLock);
debug_elog4(DEBUG2, "MultiXact: setting OldestVisible[%d] = %u",
MyBackendId, oldestMXact);
}
}
/*
* ReadNextMultiXactId
* Return the next MultiXactId to be assigned, but don't allocate it
*/
MultiXactId
ReadNextMultiXactId(void)
{
MultiXactId mxid;
/* XXX we could presumably do this without a lock. */
LWLockAcquire(MultiXactGenLock, LW_SHARED);
mxid = MultiXactState->nextMXact;
LWLockRelease(MultiXactGenLock);
if (mxid < FirstMultiXactId)
mxid = FirstMultiXactId;
return mxid;
}
/*
* ReadMultiXactIdRange
* Get the range of IDs that may still be referenced by a relation.
*/
void
ReadMultiXactIdRange(MultiXactId *oldest, MultiXactId *next)
{
LWLockAcquire(MultiXactGenLock, LW_SHARED);
*oldest = MultiXactState->oldestMultiXactId;
*next = MultiXactState->nextMXact;
LWLockRelease(MultiXactGenLock);
if (*oldest < FirstMultiXactId)
*oldest = FirstMultiXactId;
if (*next < FirstMultiXactId)
*next = FirstMultiXactId;
}
/*
* MultiXactIdCreateFromMembers
* Make a new MultiXactId from the specified set of members
*
* Make XLOG, SLRU and cache entries for a new MultiXactId, recording the
* given TransactionIds as members. Returns the newly created MultiXactId.
*
* NB: the passed members[] array will be sorted in-place.
*/
MultiXactId
MultiXactIdCreateFromMembers(int nmembers, MultiXactMember *members)
{
MultiXactId multi;
MultiXactOffset offset;
xl_multixact_create xlrec;
debug_elog3(DEBUG2, "Create: %s",
mxid_to_string(InvalidMultiXactId, nmembers, members));
/*
* See if the same set of members already exists in our cache; if so, just
* re-use that MultiXactId. (Note: it might seem that looking in our
* cache is insufficient, and we ought to search disk to see if a
* duplicate definition already exists. But since we only ever create
* MultiXacts containing our own XID, in most cases any such MultiXacts
* were in fact created by us, and so will be in our cache. There are
* corner cases where someone else added us to a MultiXact without our
* knowledge, but it's not worth checking for.)
*/
multi = mXactCacheGetBySet(nmembers, members);
if (MultiXactIdIsValid(multi))
{
debug_elog2(DEBUG2, "Create: in cache!");
return multi;
}
/* Verify that there is a single update Xid among the given members. */
{
int i;
bool has_update = false;
for (i = 0; i < nmembers; i++)
{
if (ISUPDATE_from_mxstatus(members[i].status))
{
if (has_update)
elog(ERROR, "new multixact has more than one updating member: %s",
mxid_to_string(InvalidMultiXactId, nmembers, members));
has_update = true;
}
}
}
/*
* Assign the MXID and offsets range to use, and make sure there is space
* in the OFFSETs and MEMBERs files. NB: this routine does
* START_CRIT_SECTION().
*
* Note: unlike MultiXactIdCreate and MultiXactIdExpand, we do not check
* that we've called MultiXactIdSetOldestMember here. This is because
* this routine is used in some places to create new MultiXactIds of which
* the current backend is not a member, notably during freezing of multis
* in vacuum. During vacuum, in particular, it would be unacceptable to
* keep OldestMulti set, in case it runs for long.
*/
multi = GetNewMultiXactId(nmembers, &offset);
/* Make an XLOG entry describing the new MXID. */
xlrec.mid = multi;
xlrec.moff = offset;
xlrec.nmembers = nmembers;
/*
* XXX Note: there's a lot of padding space in MultiXactMember. We could
* find a more compact representation of this Xlog record -- perhaps all
* the status flags in one XLogRecData, then all the xids in another one?
* Not clear that it's worth the trouble though.
*/
XLogBeginInsert();
XLogRegisterData((char *) (&xlrec), SizeOfMultiXactCreate);
XLogRegisterData((char *) members, nmembers * sizeof(MultiXactMember));
(void) XLogInsert(RM_MULTIXACT_ID, XLOG_MULTIXACT_CREATE_ID);
/* Now enter the information into the OFFSETs and MEMBERs logs */
RecordNewMultiXact(multi, offset, nmembers, members);
/* Done with critical section */
END_CRIT_SECTION();
/* Store the new MultiXactId in the local cache, too */
mXactCachePut(multi, nmembers, members);
debug_elog2(DEBUG2, "Create: all done");
return multi;
}
/*
* RecordNewMultiXact
* Write info about a new multixact into the offsets and members files
*
* This is broken out of MultiXactIdCreateFromMembers so that xlog replay can
* use it.
*/
static void
RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
int nmembers, MultiXactMember *members)
{
int pageno;
int prev_pageno;
int entryno;
int slotno;
MultiXactOffset *offptr;
int i;
LWLockAcquire(MultiXactOffsetSLRULock, LW_EXCLUSIVE);
pageno = MultiXactIdToOffsetPage(multi);
entryno = MultiXactIdToOffsetEntry(multi);
/*
* Note: we pass the MultiXactId to SimpleLruReadPage as the "transaction"
* to complain about if there's any I/O error. This is kinda bogus, but
* since the errors will always give the full pathname, it should be clear
* enough that a MultiXactId is really involved. Perhaps someday we'll
* take the trouble to generalize the slru.c error reporting code.
*/
slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, multi);
offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
offptr += entryno;
*offptr = offset;
MultiXactOffsetCtl->shared->page_dirty[slotno] = true;
/* Exchange our lock */
LWLockRelease(MultiXactOffsetSLRULock);
LWLockAcquire(MultiXactMemberSLRULock, LW_EXCLUSIVE);
prev_pageno = -1;
for (i = 0; i < nmembers; i++, offset++)
{
TransactionId *memberptr;
uint32 *flagsptr;
uint32 flagsval;
int bshift;
int flagsoff;
int memberoff;
Assert(members[i].status <= MultiXactStatusUpdate);
pageno = MXOffsetToMemberPage(offset);
memberoff = MXOffsetToMemberOffset(offset);
flagsoff = MXOffsetToFlagsOffset(offset);
bshift = MXOffsetToFlagsBitShift(offset);
if (pageno != prev_pageno)
{
slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true, multi);
prev_pageno = pageno;
}
memberptr = (TransactionId *)
(MultiXactMemberCtl->shared->page_buffer[slotno] + memberoff);
*memberptr = members[i].xid;
flagsptr = (uint32 *)
(MultiXactMemberCtl->shared->page_buffer[slotno] + flagsoff);
flagsval = *flagsptr;
flagsval &= ~(((1 << MXACT_MEMBER_BITS_PER_XACT) - 1) << bshift);
flagsval |= (members[i].status << bshift);
*flagsptr = flagsval;
MultiXactMemberCtl->shared->page_dirty[slotno] = true;
}
LWLockRelease(MultiXactMemberSLRULock);
}
/*
* GetNewMultiXactId
* Get the next MultiXactId.
*
* Also, reserve the needed amount of space in the "members" area. The
* starting offset of the reserved space is returned in *offset.
*
* This may generate XLOG records for expansion of the offsets and/or members
* files. Unfortunately, we have to do that while holding MultiXactGenLock
* to avoid race conditions --- the XLOG record for zeroing a page must appear
* before any backend can possibly try to store data in that page!
*
* We start a critical section before advancing the shared counters. The
* caller must end the critical section after writing SLRU data.
*/
static MultiXactId
GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
{
MultiXactId result;
MultiXactOffset nextOffset;
debug_elog3(DEBUG2, "GetNew: for %d xids", nmembers);
/* safety check, we should never get this far in a HS standby */
if (RecoveryInProgress())
elog(ERROR, "cannot assign MultiXactIds during recovery");
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
/* Handle wraparound of the nextMXact counter */
if (MultiXactState->nextMXact < FirstMultiXactId)
MultiXactState->nextMXact = FirstMultiXactId;
/* Assign the MXID */
result = MultiXactState->nextMXact;
/*----------
* Check to see if it's safe to assign another MultiXactId. This protects
* against catastrophic data loss due to multixact wraparound. The basic
* rules are:
*
* If we're past multiVacLimit or the safe threshold for member storage
* space, or we don't know what the safe threshold for member storage is,
* start trying to force autovacuum cycles.
* If we're past multiWarnLimit, start issuing warnings.
* If we're past multiStopLimit, refuse to create new MultiXactIds.
*
* Note these are pretty much the same protections in GetNewTransactionId.
*----------
*/
if (!MultiXactIdPrecedes(result, MultiXactState->multiVacLimit))
{
/*
* For safety's sake, we release MultiXactGenLock while sending
* signals, warnings, etc. This is not so much because we care about
* preserving concurrency in this situation, as to avoid any
* possibility of deadlock while doing get_database_name(). First,
* copy all the shared values we'll need in this path.
*/
MultiXactId multiWarnLimit = MultiXactState->multiWarnLimit;
MultiXactId multiStopLimit = MultiXactState->multiStopLimit;
MultiXactId multiWrapLimit = MultiXactState->multiWrapLimit;
Oid oldest_datoid = MultiXactState->oldestMultiXactDB;
LWLockRelease(MultiXactGenLock);
if (IsUnderPostmaster &&
!MultiXactIdPrecedes(result, multiStopLimit))
{
char *oldest_datname = get_database_name(oldest_datoid);
/*
* Immediately kick autovacuum into action as we're already in
* ERROR territory.
*/
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
/* complain even if that DB has disappeared */
if (oldest_datname)
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg("database is not accepting commands that generate new MultiXactIds to avoid wraparound data loss in database \"%s\"",
oldest_datname),
errhint("Execute a database-wide VACUUM in that database.\n"
"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
else
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg("database is not accepting commands that generate new MultiXactIds to avoid wraparound data loss in database with OID %u",
oldest_datoid),
errhint("Execute a database-wide VACUUM in that database.\n"
"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
}
/*
* To avoid swamping the postmaster with signals, we issue the autovac
* request only once per 64K multis generated. This still gives
* plenty of chances before we get into real trouble.
*/
if (IsUnderPostmaster && (result % 65536) == 0)
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
if (!MultiXactIdPrecedes(result, multiWarnLimit))
{
char *oldest_datname = get_database_name(oldest_datoid);
/* complain even if that DB has disappeared */
if (oldest_datname)
ereport(WARNING,
(errmsg_plural("database \"%s\" must be vacuumed before %u more MultiXactId is used",
"database \"%s\" must be vacuumed before %u more MultiXactIds are used",
multiWrapLimit - result,
oldest_datname,
multiWrapLimit - result),
errhint("Execute a database-wide VACUUM in that database.\n"
"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
else
ereport(WARNING,
(errmsg_plural("database with OID %u must be vacuumed before %u more MultiXactId is used",
"database with OID %u must be vacuumed before %u more MultiXactIds are used",
multiWrapLimit - result,
oldest_datoid,
multiWrapLimit - result),
errhint("Execute a database-wide VACUUM in that database.\n"
"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
}
/* Re-acquire lock and start over */
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
result = MultiXactState->nextMXact;
if (result < FirstMultiXactId)
result = FirstMultiXactId;
}
/* Make sure there is room for the MXID in the file. */
ExtendMultiXactOffset(result);
/*
* Reserve the members space, similarly to above. Also, be careful not to
* return zero as the starting offset for any multixact. See
* GetMultiXactIdMembers() for motivation.
*/
nextOffset = MultiXactState->nextOffset;
if (nextOffset == 0)
{
*offset = 1;
nmembers++; /* allocate member slot 0 too */
}
else
*offset = nextOffset;
/*----------
* Protect against overrun of the members space as well, with the
* following rules:
*
* If we're past offsetStopLimit, refuse to generate more multis.
* If we're close to offsetStopLimit, emit a warning.
*
* Arbitrarily, we start emitting warnings when we're 20 segments or less
* from offsetStopLimit.
*
* Note we haven't updated the shared state yet, so if we fail at this
* point, the multixact ID we grabbed can still be used by the next guy.
*
* Note that there is no point in forcing autovacuum runs here: the
* multixact freeze settings would have to be reduced for that to have any
* effect.
*----------
*/
#define OFFSET_WARN_SEGMENTS 20
if (MultiXactState->oldestOffsetKnown &&
MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit, nextOffset,
nmembers))
{
/* see comment in the corresponding offsets wraparound case */
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg("multixact \"members\" limit exceeded"),
errdetail_plural("This command would create a multixact with %u members, but the remaining space is only enough for %u member.",
"This command would create a multixact with %u members, but the remaining space is only enough for %u members.",
MultiXactState->offsetStopLimit - nextOffset - 1,
nmembers,
MultiXactState->offsetStopLimit - nextOffset - 1),
errhint("Execute a database-wide VACUUM in database with OID %u with reduced vacuum_multixact_freeze_min_age and vacuum_multixact_freeze_table_age settings.",
MultiXactState->oldestMultiXactDB)));
}
/*
* Check whether we should kick autovacuum into action, to prevent members
* wraparound. NB we use a much larger window to trigger autovacuum than
* just the warning limit. The warning is just a measure of last resort -
* this is in line with GetNewTransactionId's behaviour.
*/
if (!MultiXactState->oldestOffsetKnown ||
(MultiXactState->nextOffset - MultiXactState->oldestOffset
> MULTIXACT_MEMBER_SAFE_THRESHOLD))
{
/*
* To avoid swamping the postmaster with signals, we issue the autovac
* request only when crossing a segment boundary. With default
* compilation settings that's roughly after 50k members. This still
* gives plenty of chances before we get into real trouble.
*/
if ((MXOffsetToMemberPage(nextOffset) / SLRU_PAGES_PER_SEGMENT) !=
(MXOffsetToMemberPage(nextOffset + nmembers) / SLRU_PAGES_PER_SEGMENT))
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
}
if (MultiXactState->oldestOffsetKnown &&
MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,
nextOffset,
nmembers + MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT * OFFSET_WARN_SEGMENTS))
ereport(WARNING,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg_plural("database with OID %u must be vacuumed before %d more multixact member is used",
"database with OID %u must be vacuumed before %d more multixact members are used",
MultiXactState->offsetStopLimit - nextOffset + nmembers,
MultiXactState->oldestMultiXactDB,
MultiXactState->offsetStopLimit - nextOffset + nmembers),
errhint("Execute a database-wide VACUUM in that database with reduced vacuum_multixact_freeze_min_age and vacuum_multixact_freeze_table_age settings.")));
ExtendMultiXactMember(nextOffset, nmembers);
/*
* Critical section from here until caller has written the data into the
* just-reserved SLRU space; we don't want to error out with a partly
* written MultiXact structure. (In particular, failing to write our
* start offset after advancing nextMXact would effectively corrupt the
* previous MultiXact.)
*/
START_CRIT_SECTION();
/*
* Advance counters. As in GetNewTransactionId(), this must not happen
* until after file extension has succeeded!
*
* We don't care about MultiXactId wraparound here; it will be handled by
* the next iteration. But note that nextMXact may be InvalidMultiXactId
* or the first value on a segment-beginning page after this routine
* exits, so anyone else looking at the variable must be prepared to deal
* with either case. Similarly, nextOffset may be zero, but we won't use
* that as the actual start offset of the next multixact.
*/
(MultiXactState->nextMXact)++;
MultiXactState->nextOffset += nmembers;
LWLockRelease(MultiXactGenLock);
debug_elog4(DEBUG2, "GetNew: returning %u offset %u", result, *offset);
return result;
}
/*
* GetMultiXactIdMembers
* Return the set of MultiXactMembers that make up a MultiXactId
*
* Return value is the number of members found, or -1 if there are none,
* and *members is set to a newly palloc'ed array of members. It's the
* caller's responsibility to free it when done with it.
*
* from_pgupgrade must be passed as true if and only if only the multixact
* corresponds to a value from a tuple that was locked in a 9.2-or-older
* installation and later pg_upgrade'd (that is, the infomask is
* HEAP_LOCKED_UPGRADED). In this case, we know for certain that no members
* can still be running, so we return -1 just like for an empty multixact
* without any further checking. It would be wrong to try to resolve such a
* multixact: either the multixact is within the current valid multixact
* range, in which case the returned result would be bogus, or outside that
* range, in which case an error would be raised.
*
* In all other cases, the passed multixact must be within the known valid
* range, that is, greater to or equal than oldestMultiXactId, and less than
* nextMXact. Otherwise, an error is raised.
*
* isLockOnly must be set to true if caller is certain that the given multi
* is used only to lock tuples; can be false without loss of correctness,
* but passing a true means we can return quickly without checking for
* old updates.
*/
int
GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
bool from_pgupgrade, bool isLockOnly)
{
int pageno;
int prev_pageno;
int entryno;
int slotno;
MultiXactOffset *offptr;
MultiXactOffset offset;
int length;
int truelength;
int i;
MultiXactId oldestMXact;
MultiXactId nextMXact;
MultiXactId tmpMXact;
MultiXactOffset nextOffset;
MultiXactMember *ptr;
debug_elog3(DEBUG2, "GetMembers: asked for %u", multi);
if (!MultiXactIdIsValid(multi) || from_pgupgrade)
{
*members = NULL;
return -1;
}
/* See if the MultiXactId is in the local cache */
length = mXactCacheGetById(multi, members);
if (length >= 0)
{
debug_elog3(DEBUG2, "GetMembers: found %s in the cache",
mxid_to_string(multi, length, *members));
return length;
}
/* Set our OldestVisibleMXactId[] entry if we didn't already */
MultiXactIdSetOldestVisible();
/*
* If we know the multi is used only for locking and not for updates, then
* we can skip checking if the value is older than our oldest visible
* multi. It cannot possibly still be running.
*/
if (isLockOnly &&
MultiXactIdPrecedes(multi, OldestVisibleMXactId[MyBackendId]))
{
debug_elog2(DEBUG2, "GetMembers: a locker-only multi is too old");
*members = NULL;
return -1;
}
/*
* We check known limits on MultiXact before resorting to the SLRU area.
*
* An ID older than MultiXactState->oldestMultiXactId cannot possibly be
* useful; it has already been removed, or will be removed shortly, by
* truncation. If one is passed, an error is raised.
*
* Also, an ID >= nextMXact shouldn't ever be seen here; if it is seen, it
* implies undetected ID wraparound has occurred. This raises a hard
* error.
*
* Shared lock is enough here since we aren't modifying any global state.
* Acquire it just long enough to grab the current counter values. We may
* need both nextMXact and nextOffset; see below.
*/
LWLockAcquire(MultiXactGenLock, LW_SHARED);
oldestMXact = MultiXactState->oldestMultiXactId;
nextMXact = MultiXactState->nextMXact;
nextOffset = MultiXactState->nextOffset;
LWLockRelease(MultiXactGenLock);
if (MultiXactIdPrecedes(multi, oldestMXact))
ereport(ERROR,
(errcode(ERRCODE_INTERNAL_ERROR),
errmsg("MultiXactId %u does no longer exist -- apparent wraparound",
multi)));
if (!MultiXactIdPrecedes(multi, nextMXact))
ereport(ERROR,
(errcode(ERRCODE_INTERNAL_ERROR),
errmsg("MultiXactId %u has not been created yet -- apparent wraparound",
multi)));
/*
* Find out the offset at which we need to start reading MultiXactMembers
* and the number of members in the multixact. We determine the latter as
* the difference between this multixact's starting offset and the next
* one's. However, there are some corner cases to worry about:
*
* 1. This multixact may be the latest one created, in which case there is
* no next one to look at. In this case the nextOffset value we just
* saved is the correct endpoint.
*
* 2. The next multixact may still be in process of being filled in: that
* is, another process may have done GetNewMultiXactId but not yet written
* the offset entry for that ID. In that scenario, it is guaranteed that
* the offset entry for that multixact exists (because GetNewMultiXactId
* won't release MultiXactGenLock until it does) but contains zero
* (because we are careful to pre-zero offset pages). Because
* GetNewMultiXactId will never return zero as the starting offset for a
* multixact, when we read zero as the next multixact's offset, we know we
* have this case. We sleep for a bit and try again.
*
* 3. Because GetNewMultiXactId increments offset zero to offset one to
* handle case #2, there is an ambiguity near the point of offset
* wraparound. If we see next multixact's offset is one, is that our
* multixact's actual endpoint, or did it end at zero with a subsequent
* increment? We handle this using the knowledge that if the zero'th
* member slot wasn't filled, it'll contain zero, and zero isn't a valid
* transaction ID so it can't be a multixact member. Therefore, if we
* read a zero from the members array, just ignore it.
*
* This is all pretty messy, but the mess occurs only in infrequent corner
* cases, so it seems better than holding the MultiXactGenLock for a long
* time on every multixact creation.
*/
retry:
LWLockAcquire(MultiXactOffsetSLRULock, LW_EXCLUSIVE);
pageno = MultiXactIdToOffsetPage(multi);
entryno = MultiXactIdToOffsetEntry(multi);
slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, multi);
offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
offptr += entryno;
offset = *offptr;
Assert(offset != 0);
/*
* Use the same increment rule as GetNewMultiXactId(), that is, don't
* handle wraparound explicitly until needed.
*/
tmpMXact = multi + 1;
if (nextMXact == tmpMXact)
{
/* Corner case 1: there is no next multixact */
length = nextOffset - offset;
}
else
{
MultiXactOffset nextMXOffset;
/* handle wraparound if needed */
if (tmpMXact < FirstMultiXactId)
tmpMXact = FirstMultiXactId;
prev_pageno = pageno;
pageno = MultiXactIdToOffsetPage(tmpMXact);
entryno = MultiXactIdToOffsetEntry(tmpMXact);
if (pageno != prev_pageno)
slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, tmpMXact);
offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
offptr += entryno;
nextMXOffset = *offptr;
if (nextMXOffset == 0)
{
/* Corner case 2: next multixact is still being filled in */
LWLockRelease(MultiXactOffsetSLRULock);
CHECK_FOR_INTERRUPTS();
pg_usleep(1000L);
goto retry;
}
length = nextMXOffset - offset;
}
LWLockRelease(MultiXactOffsetSLRULock);
ptr = (MultiXactMember *) palloc(length * sizeof(MultiXactMember));
/* Now get the members themselves. */
LWLockAcquire(MultiXactMemberSLRULock, LW_EXCLUSIVE);
truelength = 0;
prev_pageno = -1;
for (i = 0; i < length; i++, offset++)
{
TransactionId *xactptr;
uint32 *flagsptr;
int flagsoff;
int bshift;
int memberoff;
pageno = MXOffsetToMemberPage(offset);
memberoff = MXOffsetToMemberOffset(offset);
if (pageno != prev_pageno)
{
slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true, multi);
prev_pageno = pageno;
}
xactptr = (TransactionId *)
(MultiXactMemberCtl->shared->page_buffer[slotno] + memberoff);
if (!TransactionIdIsValid(*xactptr))
{
/* Corner case 3: we must be looking at unused slot zero */
Assert(offset == 0);
continue;
}
flagsoff = MXOffsetToFlagsOffset(offset);
bshift = MXOffsetToFlagsBitShift(offset);
flagsptr = (uint32 *) (MultiXactMemberCtl->shared->page_buffer[slotno] + flagsoff);
ptr[truelength].xid = *xactptr;
ptr[truelength].status = (*flagsptr >> bshift) & MXACT_MEMBER_XACT_BITMASK;
truelength++;
}
LWLockRelease(MultiXactMemberSLRULock);
/* A multixid with zero members should not happen */
Assert(truelength > 0);
/*
* Copy the result into the local cache.
*/
mXactCachePut(multi, truelength, ptr);
debug_elog3(DEBUG2, "GetMembers: no cache for %s",
mxid_to_string(multi, truelength, ptr));
*members = ptr;
return truelength;
}
/*
* mxactMemberComparator
* qsort comparison function for MultiXactMember
*
* We can't use wraparound comparison for XIDs because that does not respect
* the triangle inequality! Any old sort order will do.
*/
static int
mxactMemberComparator(const void *arg1, const void *arg2)
{
MultiXactMember member1 = *(const MultiXactMember *) arg1;
MultiXactMember member2 = *(const MultiXactMember *) arg2;
if (member1.xid > member2.xid)
return 1;
if (member1.xid < member2.xid)
return -1;
if (member1.status > member2.status)
return 1;
if (member1.status < member2.status)
return -1;
return 0;
}
/*
* mXactCacheGetBySet
* returns a MultiXactId from the cache based on the set of
* TransactionIds that compose it, or InvalidMultiXactId if
* none matches.
*
* This is helpful, for example, if two transactions want to lock a huge
* table. By using the cache, the second will use the same MultiXactId
* for the majority of tuples, thus keeping MultiXactId usage low (saving
* both I/O and wraparound issues).
*
* NB: the passed members array will be sorted in-place.
*/
static MultiXactId
mXactCacheGetBySet(int nmembers, MultiXactMember *members)
{
dlist_iter iter;
debug_elog3(DEBUG2, "CacheGet: looking for %s",
mxid_to_string(InvalidMultiXactId, nmembers, members));
/* sort the array so comparison is easy */
qsort(members, nmembers, sizeof(MultiXactMember), mxactMemberComparator);
dclist_foreach(iter, &MXactCache)
{
mXactCacheEnt *entry = dclist_container(mXactCacheEnt, node,
iter.cur);
if (entry->nmembers != nmembers)
continue;
/*
* We assume the cache entries are sorted, and that the unused bits in
* "status" are zeroed.
*/
if (memcmp(members, entry->members, nmembers * sizeof(MultiXactMember)) == 0)
{
debug_elog3(DEBUG2, "CacheGet: found %u", entry->multi);
dclist_move_head(&MXactCache, iter.cur);
return entry->multi;
}
}
debug_elog2(DEBUG2, "CacheGet: not found :-(");
return InvalidMultiXactId;
}
/*
* mXactCacheGetById
* returns the composing MultiXactMember set from the cache for a
* given MultiXactId, if present.
*
* If successful, *xids is set to the address of a palloc'd copy of the
* MultiXactMember set. Return value is number of members, or -1 on failure.
*/
static int
mXactCacheGetById(MultiXactId multi, MultiXactMember **members)
{
dlist_iter iter;
debug_elog3(DEBUG2, "CacheGet: looking for %u", multi);
dclist_foreach(iter, &MXactCache)
{
mXactCacheEnt *entry = dclist_container(mXactCacheEnt, node,
iter.cur);
if (entry->multi == multi)
{
MultiXactMember *ptr;
Size size;
size = sizeof(MultiXactMember) * entry->nmembers;
ptr = (MultiXactMember *) palloc(size);
memcpy(ptr, entry->members, size);
debug_elog3(DEBUG2, "CacheGet: found %s",
mxid_to_string(multi,
entry->nmembers,
entry->members));
/*
* Note we modify the list while not using a modifiable iterator.
* This is acceptable only because we exit the iteration
* immediately afterwards.
*/
dclist_move_head(&MXactCache, iter.cur);
*members = ptr;
return entry->nmembers;
}
}
debug_elog2(DEBUG2, "CacheGet: not found");
return -1;
}
/*
* mXactCachePut
* Add a new MultiXactId and its composing set into the local cache.
*/
static void
mXactCachePut(MultiXactId multi, int nmembers, MultiXactMember *members)
{
mXactCacheEnt *entry;
debug_elog3(DEBUG2, "CachePut: storing %s",
mxid_to_string(multi, nmembers, members));
if (MXactContext == NULL)
{
/* The cache only lives as long as the current transaction */
debug_elog2(DEBUG2, "CachePut: initializing memory context");
MXactContext = AllocSetContextCreate(TopTransactionContext,
"MultiXact cache context",
ALLOCSET_SMALL_SIZES);
}
entry = (mXactCacheEnt *)
MemoryContextAlloc(MXactContext,
offsetof(mXactCacheEnt, members) +
nmembers * sizeof(MultiXactMember));
entry->multi = multi;
entry->nmembers = nmembers;
memcpy(entry->members, members, nmembers * sizeof(MultiXactMember));
/* mXactCacheGetBySet assumes the entries are sorted, so sort them */
qsort(entry->members, nmembers, sizeof(MultiXactMember), mxactMemberComparator);
dclist_push_head(&MXactCache, &entry->node);
if (dclist_count(&MXactCache) > MAX_CACHE_ENTRIES)
{
dlist_node *node;
node = dclist_tail_node(&MXactCache);
dclist_delete_from(&MXactCache, node);
entry = dclist_container(mXactCacheEnt, node, node);
debug_elog3(DEBUG2, "CachePut: pruning cached multi %u",
entry->multi);
pfree(entry);
}
}
static char *
mxstatus_to_string(MultiXactStatus status)
{
switch (status)
{
case MultiXactStatusForKeyShare:
return "keysh";
case MultiXactStatusForShare:
return "sh";
case MultiXactStatusForNoKeyUpdate:
return "fornokeyupd";
case MultiXactStatusForUpdate:
return "forupd";
case MultiXactStatusNoKeyUpdate:
return "nokeyupd";
case MultiXactStatusUpdate:
return "upd";
default:
elog(ERROR, "unrecognized multixact status %d", status);
return "";
}
}
char *
mxid_to_string(MultiXactId multi, int nmembers, MultiXactMember *members)
{
static char *str = NULL;
StringInfoData buf;
int i;
if (str != NULL)
pfree(str);
initStringInfo(&buf);
appendStringInfo(&buf, "%u %d[%u (%s)", multi, nmembers, members[0].xid,
mxstatus_to_string(members[0].status));
for (i = 1; i < nmembers; i++)
appendStringInfo(&buf, ", %u (%s)", members[i].xid,
mxstatus_to_string(members[i].status));
appendStringInfoChar(&buf, ']');
str = MemoryContextStrdup(TopMemoryContext, buf.data);
pfree(buf.data);
return str;
}
/*
* AtEOXact_MultiXact
* Handle transaction end for MultiXact
*
* This is called at top transaction commit or abort (we don't care which).
*/
void
AtEOXact_MultiXact(void)
{
/*
* Reset our OldestMemberMXactId and OldestVisibleMXactId values, both of
* which should only be valid while within a transaction.
*
* We assume that storing a MultiXactId is atomic and so we need not take
* MultiXactGenLock to do this.
*/
OldestMemberMXactId[MyBackendId] = InvalidMultiXactId;
OldestVisibleMXactId[MyBackendId] = InvalidMultiXactId;
/*
* Discard the local MultiXactId cache. Since MXactContext was created as
* a child of TopTransactionContext, we needn't delete it explicitly.
*/
MXactContext = NULL;
dclist_init(&MXactCache);
}
/*
* AtPrepare_MultiXact
* Save multixact state at 2PC transaction prepare
*
* In this phase, we only store our OldestMemberMXactId value in the two-phase
* state file.
*/
void
AtPrepare_MultiXact(void)
{
MultiXactId myOldestMember = OldestMemberMXactId[MyBackendId];
if (MultiXactIdIsValid(myOldestMember))
RegisterTwoPhaseRecord(TWOPHASE_RM_MULTIXACT_ID, 0,
&myOldestMember, sizeof(MultiXactId));
}
/*
* PostPrepare_MultiXact
* Clean up after successful PREPARE TRANSACTION
*/
void
PostPrepare_MultiXact(TransactionId xid)
{
MultiXactId myOldestMember;
/*
* Transfer our OldestMemberMXactId value to the slot reserved for the
* prepared transaction.
*/
myOldestMember = OldestMemberMXactId[MyBackendId];
if (MultiXactIdIsValid(myOldestMember))
{
BackendId dummyBackendId = TwoPhaseGetDummyBackendId(xid, false);
/*
* Even though storing MultiXactId is atomic, acquire lock to make
* sure others see both changes, not just the reset of the slot of the
* current backend. Using a volatile pointer might suffice, but this
* isn't a hot spot.
*/
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
OldestMemberMXactId[dummyBackendId] = myOldestMember;
OldestMemberMXactId[MyBackendId] = InvalidMultiXactId;
LWLockRelease(MultiXactGenLock);
}
/*
* We don't need to transfer OldestVisibleMXactId value, because the
* transaction is not going to be looking at any more multixacts once it's
* prepared.
*
* We assume that storing a MultiXactId is atomic and so we need not take
* MultiXactGenLock to do this.
*/
OldestVisibleMXactId[MyBackendId] = InvalidMultiXactId;
/*
* Discard the local MultiXactId cache like in AtEOXact_MultiXact.
*/
MXactContext = NULL;
dclist_init(&MXactCache);
}
/*
* multixact_twophase_recover
* Recover the state of a prepared transaction at startup
*/
void
multixact_twophase_recover(TransactionId xid, uint16 info,
void *recdata, uint32 len)
{
BackendId dummyBackendId = TwoPhaseGetDummyBackendId(xid, false);
MultiXactId oldestMember;
/*
* Get the oldest member XID from the state file record, and set it in the
* OldestMemberMXactId slot reserved for this prepared transaction.
*/
Assert(len == sizeof(MultiXactId));
oldestMember = *((MultiXactId *) recdata);
OldestMemberMXactId[dummyBackendId] = oldestMember;
}
/*
* multixact_twophase_postcommit
* Similar to AtEOXact_MultiXact but for COMMIT PREPARED
*/
void
multixact_twophase_postcommit(TransactionId xid, uint16 info,
void *recdata, uint32 len)
{
BackendId dummyBackendId = TwoPhaseGetDummyBackendId(xid, true);
Assert(len == sizeof(MultiXactId));
OldestMemberMXactId[dummyBackendId] = InvalidMultiXactId;
}
/*
* multixact_twophase_postabort
* This is actually just the same as the COMMIT case.
*/
void
multixact_twophase_postabort(TransactionId xid, uint16 info,
void *recdata, uint32 len)
{
multixact_twophase_postcommit(xid, info, recdata, len);
}
/*
* Initialization of shared memory for MultiXact. We use two SLRU areas,
* thus double memory. Also, reserve space for the shared MultiXactState
* struct and the per-backend MultiXactId arrays (two of those, too).
*/
Size
MultiXactShmemSize(void)
{
Size size;
/* We need 2*MaxOldestSlot + 1 perBackendXactIds[] entries */
#define SHARED_MULTIXACT_STATE_SIZE \
add_size(offsetof(MultiXactStateData, perBackendXactIds) + sizeof(MultiXactId), \
mul_size(sizeof(MultiXactId) * 2, MaxOldestSlot))
size = SHARED_MULTIXACT_STATE_SIZE;
size = add_size(size, SimpleLruShmemSize(NUM_MULTIXACTOFFSET_BUFFERS, 0));
size = add_size(size, SimpleLruShmemSize(NUM_MULTIXACTMEMBER_BUFFERS, 0));
return size;
}
void
MultiXactShmemInit(void)
{
bool found;
debug_elog2(DEBUG2, "Shared Memory Init for MultiXact");
MultiXactOffsetCtl->PagePrecedes = MultiXactOffsetPagePrecedes;
MultiXactMemberCtl->PagePrecedes = MultiXactMemberPagePrecedes;
SimpleLruInit(MultiXactOffsetCtl,
"MultiXactOffset", NUM_MULTIXACTOFFSET_BUFFERS, 0,
MultiXactOffsetSLRULock, "pg_multixact/offsets",
LWTRANCHE_MULTIXACTOFFSET_BUFFER,
SYNC_HANDLER_MULTIXACT_OFFSET);
SlruPagePrecedesUnitTests(MultiXactOffsetCtl, MULTIXACT_OFFSETS_PER_PAGE);
SimpleLruInit(MultiXactMemberCtl,
"MultiXactMember", NUM_MULTIXACTMEMBER_BUFFERS, 0,
MultiXactMemberSLRULock, "pg_multixact/members",
LWTRANCHE_MULTIXACTMEMBER_BUFFER,
SYNC_HANDLER_MULTIXACT_MEMBER);
/* doesn't call SimpleLruTruncate() or meet criteria for unit tests */
/* Initialize our shared state struct */
MultiXactState = ShmemInitStruct("Shared MultiXact State",
SHARED_MULTIXACT_STATE_SIZE,
&found);
if (!IsUnderPostmaster)
{
Assert(!found);
/* Make sure we zero out the per-backend state */
MemSet(MultiXactState, 0, SHARED_MULTIXACT_STATE_SIZE);
}
else
Assert(found);
/*
* Set up array pointers. Note that perBackendXactIds[0] is wasted space
* since we only use indexes 1..MaxOldestSlot in each array.
*/
OldestMemberMXactId = MultiXactState->perBackendXactIds;
OldestVisibleMXactId = OldestMemberMXactId + MaxOldestSlot;
}
/*
* This func must be called ONCE on system install. It creates the initial
* MultiXact segments. (The MultiXacts directories are assumed to have been
* created by initdb, and MultiXactShmemInit must have been called already.)
*/
void
BootStrapMultiXact(void)
{
int slotno;
LWLockAcquire(MultiXactOffsetSLRULock, LW_EXCLUSIVE);
/* Create and zero the first page of the offsets log */
slotno = ZeroMultiXactOffsetPage(0, false);
/* Make sure it's written out */
SimpleLruWritePage(MultiXactOffsetCtl, slotno);
Assert(!MultiXactOffsetCtl->shared->page_dirty[slotno]);
LWLockRelease(MultiXactOffsetSLRULock);
LWLockAcquire(MultiXactMemberSLRULock, LW_EXCLUSIVE);
/* Create and zero the first page of the members log */
slotno = ZeroMultiXactMemberPage(0, false);
/* Make sure it's written out */
SimpleLruWritePage(MultiXactMemberCtl, slotno);
Assert(!MultiXactMemberCtl->shared->page_dirty[slotno]);
LWLockRelease(MultiXactMemberSLRULock);
}
/*
* Initialize (or reinitialize) a page of MultiXactOffset to zeroes.
* If writeXlog is true, also emit an XLOG record saying we did this.
*
* The page is not actually written, just set up in shared memory.
* The slot number of the new page is returned.
*
* Control lock must be held at entry, and will be held at exit.
*/
static int
ZeroMultiXactOffsetPage(int pageno, bool writeXlog)
{
int slotno;
slotno = SimpleLruZeroPage(MultiXactOffsetCtl, pageno);
if (writeXlog)
WriteMZeroPageXlogRec(pageno, XLOG_MULTIXACT_ZERO_OFF_PAGE);
return slotno;
}
/*
* Ditto, for MultiXactMember
*/
static int
ZeroMultiXactMemberPage(int pageno, bool writeXlog)
{
int slotno;
slotno = SimpleLruZeroPage(MultiXactMemberCtl, pageno);
if (writeXlog)
WriteMZeroPageXlogRec(pageno, XLOG_MULTIXACT_ZERO_MEM_PAGE);
return slotno;
}
/*
* MaybeExtendOffsetSlru
* Extend the offsets SLRU area, if necessary
*
* After a binary upgrade from <= 9.2, the pg_multixact/offsets SLRU area might
* contain files that are shorter than necessary; this would occur if the old
* installation had used multixacts beyond the first page (files cannot be
* copied, because the on-disk representation is different). pg_upgrade would
* update pg_control to set the next offset value to be at that position, so
* that tuples marked as locked by such MultiXacts would be seen as visible
* without having to consult multixact. However, trying to create and use a
* new MultiXactId would result in an error because the page on which the new
* value would reside does not exist. This routine is in charge of creating
* such pages.
*/
static void
MaybeExtendOffsetSlru(void)
{
int pageno;
pageno = MultiXactIdToOffsetPage(MultiXactState->nextMXact);
LWLockAcquire(MultiXactOffsetSLRULock, LW_EXCLUSIVE);
if (!SimpleLruDoesPhysicalPageExist(MultiXactOffsetCtl, pageno))
{
int slotno;
/*
* Fortunately for us, SimpleLruWritePage is already prepared to deal
* with creating a new segment file even if the page we're writing is
* not the first in it, so this is enough.
*/
slotno = ZeroMultiXactOffsetPage(pageno, false);
SimpleLruWritePage(MultiXactOffsetCtl, slotno);
}
LWLockRelease(MultiXactOffsetSLRULock);
}
/*
* This must be called ONCE during postmaster or standalone-backend startup.
*
* StartupXLOG has already established nextMXact/nextOffset by calling
* MultiXactSetNextMXact and/or MultiXactAdvanceNextMXact, and the oldestMulti
* info from pg_control and/or MultiXactAdvanceOldest, but we haven't yet
* replayed WAL.
*/
void
StartupMultiXact(void)
{
MultiXactId multi = MultiXactState->nextMXact;
MultiXactOffset offset = MultiXactState->nextOffset;
int pageno;
/*
* Initialize offset's idea of the latest page number.
*/
pageno = MultiXactIdToOffsetPage(multi);
MultiXactOffsetCtl->shared->latest_page_number = pageno;
/*
* Initialize member's idea of the latest page number.
*/
pageno = MXOffsetToMemberPage(offset);
MultiXactMemberCtl->shared->latest_page_number = pageno;
}
/*
* This must be called ONCE at the end of startup/recovery.
*/
void
TrimMultiXact(void)
{
MultiXactId nextMXact;
MultiXactOffset offset;
MultiXactId oldestMXact;
Oid oldestMXactDB;
int pageno;
int entryno;
int flagsoff;
LWLockAcquire(MultiXactGenLock, LW_SHARED);
nextMXact = MultiXactState->nextMXact;
offset = MultiXactState->nextOffset;
oldestMXact = MultiXactState->oldestMultiXactId;
oldestMXactDB = MultiXactState->oldestMultiXactDB;
LWLockRelease(MultiXactGenLock);
/* Clean up offsets state */
LWLockAcquire(MultiXactOffsetSLRULock, LW_EXCLUSIVE);
/*
* (Re-)Initialize our idea of the latest page number for offsets.
*/
pageno = MultiXactIdToOffsetPage(nextMXact);
MultiXactOffsetCtl->shared->latest_page_number = pageno;
/*
* Zero out the remainder of the current offsets page. See notes in
* TrimCLOG() for background. Unlike CLOG, some WAL record covers every
* pg_multixact SLRU mutation. Since, also unlike CLOG, we ignore the WAL
* rule "write xlog before data," nextMXact successors may carry obsolete,
* nonzero offset values. Zero those so case 2 of GetMultiXactIdMembers()
* operates normally.
*/
entryno = MultiXactIdToOffsetEntry(nextMXact);
if (entryno != 0)
{
int slotno;
MultiXactOffset *offptr;
slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, nextMXact);
offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
offptr += entryno;
MemSet(offptr, 0, BLCKSZ - (entryno * sizeof(MultiXactOffset)));
MultiXactOffsetCtl->shared->page_dirty[slotno] = true;
}
LWLockRelease(MultiXactOffsetSLRULock);
/* And the same for members */
LWLockAcquire(MultiXactMemberSLRULock, LW_EXCLUSIVE);
/*
* (Re-)Initialize our idea of the latest page number for members.
*/
pageno = MXOffsetToMemberPage(offset);
MultiXactMemberCtl->shared->latest_page_number = pageno;
/*
* Zero out the remainder of the current members page. See notes in
* TrimCLOG() for motivation.
*/
flagsoff = MXOffsetToFlagsOffset(offset);
if (flagsoff != 0)
{
int slotno;
TransactionId *xidptr;
int memberoff;
memberoff = MXOffsetToMemberOffset(offset);
slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true, offset);
xidptr = (TransactionId *)
(MultiXactMemberCtl->shared->page_buffer[slotno] + memberoff);
MemSet(xidptr, 0, BLCKSZ - memberoff);
/*
* Note: we don't need to zero out the flag bits in the remaining
* members of the current group, because they are always reset before
* writing.
*/
MultiXactMemberCtl->shared->page_dirty[slotno] = true;
}
LWLockRelease(MultiXactMemberSLRULock);
/* signal that we're officially up */
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
MultiXactState->finishedStartup = true;
LWLockRelease(MultiXactGenLock);
/* Now compute how far away the next members wraparound is. */
SetMultiXactIdLimit(oldestMXact, oldestMXactDB, true);
}
/*
* Get the MultiXact data to save in a checkpoint record
*/
void
MultiXactGetCheckptMulti(bool is_shutdown pg_attribute_unused() ,
MultiXactId *nextMulti,
MultiXactOffset *nextMultiOffset,
MultiXactId *oldestMulti,
Oid *oldestMultiDB)
{
LWLockAcquire(MultiXactGenLock, LW_SHARED);
*nextMulti = MultiXactState->nextMXact;
*nextMultiOffset = MultiXactState->nextOffset;
*oldestMulti = MultiXactState->oldestMultiXactId;
*oldestMultiDB = MultiXactState->oldestMultiXactDB;
LWLockRelease(MultiXactGenLock);
debug_elog6(DEBUG2,
"MultiXact: checkpoint is nextMulti %u, nextOffset %u, oldestMulti %u in DB %u",
*nextMulti, *nextMultiOffset, *oldestMulti, *oldestMultiDB);
}
/*
* Perform a checkpoint --- either during shutdown, or on-the-fly
*/
void
CheckPointMultiXact(void)
{
TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_START(true);
/*
* Write dirty MultiXact pages to disk. This may result in sync requests
* queued for later handling by ProcessSyncRequests(), as part of the
* checkpoint.
*/
SimpleLruWriteAll(MultiXactOffsetCtl, true);
SimpleLruWriteAll(MultiXactMemberCtl, true);
TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_DONE(true);
}
/*
* Set the next-to-be-assigned MultiXactId and offset
*
* This is used when we can determine the correct next ID/offset exactly
* from a checkpoint record. Although this is only called during bootstrap
* and XLog replay, we take the lock in case any hot-standby backends are
* examining the values.
*/
void
MultiXactSetNextMXact(MultiXactId nextMulti,
MultiXactOffset nextMultiOffset)
{
debug_elog4(DEBUG2, "MultiXact: setting next multi to %u offset %u",
nextMulti, nextMultiOffset);
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
MultiXactState->nextMXact = nextMulti;
MultiXactState->nextOffset = nextMultiOffset;
LWLockRelease(MultiXactGenLock);
/*
* During a binary upgrade, make sure that the offsets SLRU is large
* enough to contain the next value that would be created.
*
* We need to do this pretty early during the first startup in binary
* upgrade mode: before StartupMultiXact() in fact, because this routine
* is called even before that by StartupXLOG(). And we can't do it
* earlier than at this point, because during that first call of this
* routine we determine the MultiXactState->nextMXact value that
* MaybeExtendOffsetSlru needs.
*/
if (IsBinaryUpgrade)
MaybeExtendOffsetSlru();
}
/*
* Determine the last safe MultiXactId to allocate given the currently oldest
* datminmxid (ie, the oldest MultiXactId that might exist in any database
* of our cluster), and the OID of the (or a) database with that value.
*
* is_startup is true when we are just starting the cluster, false when we
* are updating state in a running cluster. This only affects log messages.
*/
void
SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid,
bool is_startup)
{
MultiXactId multiVacLimit;
MultiXactId multiWarnLimit;
MultiXactId multiStopLimit;
MultiXactId multiWrapLimit;
MultiXactId curMulti;
bool needs_offset_vacuum;
Assert(MultiXactIdIsValid(oldest_datminmxid));
/*
* We pretend that a wrap will happen halfway through the multixact ID
* space, but that's not really true, because multixacts wrap differently
* from transaction IDs. Note that, separately from any concern about
* multixact IDs wrapping, we must ensure that multixact members do not
* wrap. Limits for that are set in SetOffsetVacuumLimit, not here.
*/
multiWrapLimit = oldest_datminmxid + (MaxMultiXactId >> 1);
if (multiWrapLimit < FirstMultiXactId)
multiWrapLimit += FirstMultiXactId;
/*
* We'll refuse to continue assigning MultiXactIds once we get within 3M
* multi of data loss. See SetTransactionIdLimit.
*/
multiStopLimit = multiWrapLimit - 3000000;
if (multiStopLimit < FirstMultiXactId)
multiStopLimit -= FirstMultiXactId;
/*
* We'll start complaining loudly when we get within 40M multis of data
* loss. This is kind of arbitrary, but if you let your gas gauge get
* down to 2% of full, would you be looking for the next gas station? We
* need to be fairly liberal about this number because there are lots of
* scenarios where most transactions are done by automatic clients that
* won't pay attention to warnings. (No, we're not gonna make this
* configurable. If you know enough to configure it, you know enough to
* not get in this kind of trouble in the first place.)
*/
multiWarnLimit = multiWrapLimit - 40000000;
if (multiWarnLimit < FirstMultiXactId)
multiWarnLimit -= FirstMultiXactId;
/*
* We'll start trying to force autovacuums when oldest_datminmxid gets to
* be more than autovacuum_multixact_freeze_max_age mxids old.
*
* Note: autovacuum_multixact_freeze_max_age is a PGC_POSTMASTER parameter
* so that we don't have to worry about dealing with on-the-fly changes in
* its value. See SetTransactionIdLimit.
*/
multiVacLimit = oldest_datminmxid + autovacuum_multixact_freeze_max_age;
if (multiVacLimit < FirstMultiXactId)
multiVacLimit += FirstMultiXactId;
/* Grab lock for just long enough to set the new limit values */
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
MultiXactState->oldestMultiXactId = oldest_datminmxid;
MultiXactState->oldestMultiXactDB = oldest_datoid;
MultiXactState->multiVacLimit = multiVacLimit;
MultiXactState->multiWarnLimit = multiWarnLimit;
MultiXactState->multiStopLimit = multiStopLimit;
MultiXactState->multiWrapLimit = multiWrapLimit;
curMulti = MultiXactState->nextMXact;
LWLockRelease(MultiXactGenLock);
/* Log the info */
ereport(DEBUG1,
(errmsg_internal("MultiXactId wrap limit is %u, limited by database with OID %u",
multiWrapLimit, oldest_datoid)));
/*
* Computing the actual limits is only possible once the data directory is
* in a consistent state. There's no need to compute the limits while
* still replaying WAL - no decisions about new multis are made even
* though multixact creations might be replayed. So we'll only do further
* checks after TrimMultiXact() has been called.
*/
if (!MultiXactState->finishedStartup)
return;
Assert(!InRecovery);
/* Set limits for offset vacuum. */
needs_offset_vacuum = SetOffsetVacuumLimit(is_startup);
/*
* If past the autovacuum force point, immediately signal an autovac
* request. The reason for this is that autovac only processes one
* database per invocation. Once it's finished cleaning up the oldest
* database, it'll call here, and we'll signal the postmaster to start
* another iteration immediately if there are still any old databases.
*/
if ((MultiXactIdPrecedes(multiVacLimit, curMulti) ||
needs_offset_vacuum) && IsUnderPostmaster)
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
/* Give an immediate warning if past the wrap warn point */
if (MultiXactIdPrecedes(multiWarnLimit, curMulti))
{
char *oldest_datname;
/*
* We can be called when not inside a transaction, for example during
* StartupXLOG(). In such a case we cannot do database access, so we
* must just report the oldest DB's OID.
*
* Note: it's also possible that get_database_name fails and returns
* NULL, for example because the database just got dropped. We'll
* still warn, even though the warning might now be unnecessary.
*/
if (IsTransactionState())
oldest_datname = get_database_name(oldest_datoid);
else
oldest_datname = NULL;
if (oldest_datname)
ereport(WARNING,
(errmsg_plural("database \"%s\" must be vacuumed before %u more MultiXactId is used",
"database \"%s\" must be vacuumed before %u more MultiXactIds are used",
multiWrapLimit - curMulti,
oldest_datname,
multiWrapLimit - curMulti),
errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
else
ereport(WARNING,
(errmsg_plural("database with OID %u must be vacuumed before %u more MultiXactId is used",
"database with OID %u must be vacuumed before %u more MultiXactIds are used",
multiWrapLimit - curMulti,
oldest_datoid,
multiWrapLimit - curMulti),
errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
}
}
/*
* Ensure the next-to-be-assigned MultiXactId is at least minMulti,
* and similarly nextOffset is at least minMultiOffset.
*
* This is used when we can determine minimum safe values from an XLog
* record (either an on-line checkpoint or an mxact creation log entry).
* Although this is only called during XLog replay, we take the lock in case
* any hot-standby backends are examining the values.
*/
void
MultiXactAdvanceNextMXact(MultiXactId minMulti,
MultiXactOffset minMultiOffset)
{
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
if (MultiXactIdPrecedes(MultiXactState->nextMXact, minMulti))
{
debug_elog3(DEBUG2, "MultiXact: setting next multi to %u", minMulti);
MultiXactState->nextMXact = minMulti;
}
if (MultiXactOffsetPrecedes(MultiXactState->nextOffset, minMultiOffset))
{
debug_elog3(DEBUG2, "MultiXact: setting next offset to %u",
minMultiOffset);
MultiXactState->nextOffset = minMultiOffset;
}
LWLockRelease(MultiXactGenLock);
}
/*
* Update our oldestMultiXactId value, but only if it's more recent than what
* we had.
*
* This may only be called during WAL replay.
*/
void
MultiXactAdvanceOldest(MultiXactId oldestMulti, Oid oldestMultiDB)
{
Assert(InRecovery);
if (MultiXactIdPrecedes(MultiXactState->oldestMultiXactId, oldestMulti))
SetMultiXactIdLimit(oldestMulti, oldestMultiDB, false);
}
/*
* Make sure that MultiXactOffset has room for a newly-allocated MultiXactId.
*
* NB: this is called while holding MultiXactGenLock. We want it to be very
* fast most of the time; even when it's not so fast, no actual I/O need
* happen unless we're forced to write out a dirty log or xlog page to make
* room in shared memory.
*/
static void
ExtendMultiXactOffset(MultiXactId multi)
{
int pageno;
/*
* No work except at first MultiXactId of a page. But beware: just after
* wraparound, the first MultiXactId of page zero is FirstMultiXactId.
*/
if (MultiXactIdToOffsetEntry(multi) != 0 &&
multi != FirstMultiXactId)
return;
pageno = MultiXactIdToOffsetPage(multi);
LWLockAcquire(MultiXactOffsetSLRULock, LW_EXCLUSIVE);
/* Zero the page and make an XLOG entry about it */
ZeroMultiXactOffsetPage(pageno, true);
LWLockRelease(MultiXactOffsetSLRULock);
}
/*
* Make sure that MultiXactMember has room for the members of a newly-
* allocated MultiXactId.
*
* Like the above routine, this is called while holding MultiXactGenLock;
* same comments apply.
*/
static void
ExtendMultiXactMember(MultiXactOffset offset, int nmembers)
{
/*
* It's possible that the members span more than one page of the members
* file, so we loop to ensure we consider each page. The coding is not
* optimal if the members span several pages, but that seems unusual
* enough to not worry much about.
*/
while (nmembers > 0)
{
int flagsoff;
int flagsbit;
uint32 difference;
/*
* Only zero when at first entry of a page.
*/
flagsoff = MXOffsetToFlagsOffset(offset);
flagsbit = MXOffsetToFlagsBitShift(offset);
if (flagsoff == 0 && flagsbit == 0)
{
int pageno;
pageno = MXOffsetToMemberPage(offset);
LWLockAcquire(MultiXactMemberSLRULock, LW_EXCLUSIVE);
/* Zero the page and make an XLOG entry about it */
ZeroMultiXactMemberPage(pageno, true);
LWLockRelease(MultiXactMemberSLRULock);
}
/*
* Compute the number of items till end of current page. Careful: if
* addition of unsigned ints wraps around, we're at the last page of
* the last segment; since that page holds a different number of items
* than other pages, we need to do it differently.
*/
if (offset + MAX_MEMBERS_IN_LAST_MEMBERS_PAGE < offset)
{
/*
* This is the last page of the last segment; we can compute the
* number of items left to allocate in it without modulo
* arithmetic.
*/
difference = MaxMultiXactOffset - offset + 1;
}
else
difference = MULTIXACT_MEMBERS_PER_PAGE - offset % MULTIXACT_MEMBERS_PER_PAGE;
/*
* Advance to next page, taking care to properly handle the wraparound
* case. OK if nmembers goes negative.
*/
nmembers -= difference;
offset += difference;
}
}
/*
* GetOldestMultiXactId
*
* Return the oldest MultiXactId that's still possibly still seen as live by
* any running transaction. Older ones might still exist on disk, but they no
* longer have any running member transaction.
*
* It's not safe to truncate MultiXact SLRU segments on the value returned by
* this function; however, it can be set as the new relminmxid for any table
* that VACUUM knows has no remaining MXIDs < the same value. It is only safe
* to truncate SLRUs when no table can possibly still have a referencing MXID.
*/
MultiXactId
GetOldestMultiXactId(void)
{
MultiXactId oldestMXact;
MultiXactId nextMXact;
int i;
/*
* This is the oldest valid value among all the OldestMemberMXactId[] and
* OldestVisibleMXactId[] entries, or nextMXact if none are valid.
*/
LWLockAcquire(MultiXactGenLock, LW_SHARED);
/*
* We have to beware of the possibility that nextMXact is in the
* wrapped-around state. We don't fix the counter itself here, but we
* must be sure to use a valid value in our calculation.
*/
nextMXact = MultiXactState->nextMXact;
if (nextMXact < FirstMultiXactId)
nextMXact = FirstMultiXactId;
oldestMXact = nextMXact;
for (i = 1; i <= MaxOldestSlot; i++)
{
MultiXactId thisoldest;
thisoldest = OldestMemberMXactId[i];
if (MultiXactIdIsValid(thisoldest) &&
MultiXactIdPrecedes(thisoldest, oldestMXact))
oldestMXact = thisoldest;
thisoldest = OldestVisibleMXactId[i];
if (MultiXactIdIsValid(thisoldest) &&
MultiXactIdPrecedes(thisoldest, oldestMXact))
oldestMXact = thisoldest;
}
LWLockRelease(MultiXactGenLock);
return oldestMXact;
}
/*
* Determine how aggressively we need to vacuum in order to prevent member
* wraparound.
*
* To do so determine what's the oldest member offset and install the limit
* info in MultiXactState, where it can be used to prevent overrun of old data
* in the members SLRU area.
*
* The return value is true if emergency autovacuum is required and false
* otherwise.
*/
static bool
SetOffsetVacuumLimit(bool is_startup)
{
MultiXactId oldestMultiXactId;
MultiXactId nextMXact;
MultiXactOffset oldestOffset = 0; /* placate compiler */
MultiXactOffset prevOldestOffset;
MultiXactOffset nextOffset;
bool oldestOffsetKnown = false;
bool prevOldestOffsetKnown;
MultiXactOffset offsetStopLimit = 0;
MultiXactOffset prevOffsetStopLimit;
/*
* NB: Have to prevent concurrent truncation, we might otherwise try to
* lookup an oldestMulti that's concurrently getting truncated away.
*/
LWLockAcquire(MultiXactTruncationLock, LW_SHARED);
/* Read relevant fields from shared memory. */
LWLockAcquire(MultiXactGenLock, LW_SHARED);
oldestMultiXactId = MultiXactState->oldestMultiXactId;
nextMXact = MultiXactState->nextMXact;
nextOffset = MultiXactState->nextOffset;
prevOldestOffsetKnown = MultiXactState->oldestOffsetKnown;
prevOldestOffset = MultiXactState->oldestOffset;
prevOffsetStopLimit = MultiXactState->offsetStopLimit;
Assert(MultiXactState->finishedStartup);
LWLockRelease(MultiXactGenLock);
/*
* Determine the offset of the oldest multixact. Normally, we can read
* the offset from the multixact itself, but there's an important special
* case: if there are no multixacts in existence at all, oldestMXact
* obviously can't point to one. It will instead point to the multixact
* ID that will be assigned the next time one is needed.
*/
if (oldestMultiXactId == nextMXact)
{
/*
* When the next multixact gets created, it will be stored at the next
* offset.
*/
oldestOffset = nextOffset;
oldestOffsetKnown = true;
}
else
{
/*
* Figure out where the oldest existing multixact's offsets are
* stored. Due to bugs in early release of PostgreSQL 9.3.X and 9.4.X,
* the supposedly-earliest multixact might not really exist. We are
* careful not to fail in that case.
*/
oldestOffsetKnown =
find_multixact_start(oldestMultiXactId, &oldestOffset);
if (oldestOffsetKnown)
ereport(DEBUG1,
(errmsg_internal("oldest MultiXactId member is at offset %u",
oldestOffset)));
else
ereport(LOG,
(errmsg("MultiXact member wraparound protections are disabled because oldest checkpointed MultiXact %u does not exist on disk",
oldestMultiXactId)));
}
LWLockRelease(MultiXactTruncationLock);
/*
* If we can, compute limits (and install them MultiXactState) to prevent
* overrun of old data in the members SLRU area. We can only do so if the
* oldest offset is known though.
*/
if (oldestOffsetKnown)
{
/* move back to start of the corresponding segment */
offsetStopLimit = oldestOffset - (oldestOffset %
(MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT));
/* always leave one segment before the wraparound point */
offsetStopLimit -= (MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);
if (!prevOldestOffsetKnown && !is_startup)
ereport(LOG,
(errmsg("MultiXact member wraparound protections are now enabled")));
ereport(DEBUG1,
(errmsg_internal("MultiXact member stop limit is now %u based on MultiXact %u",
offsetStopLimit, oldestMultiXactId)));
}
else if (prevOldestOffsetKnown)
{
/*
* If we failed to get the oldest offset this time, but we have a
* value from a previous pass through this function, use the old
* values rather than automatically forcing an emergency autovacuum
* cycle again.
*/
oldestOffset = prevOldestOffset;
oldestOffsetKnown = true;
offsetStopLimit = prevOffsetStopLimit;
}
/* Install the computed values */
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
MultiXactState->oldestOffset = oldestOffset;
MultiXactState->oldestOffsetKnown = oldestOffsetKnown;
MultiXactState->offsetStopLimit = offsetStopLimit;
LWLockRelease(MultiXactGenLock);
/*
* Do we need an emergency autovacuum? If we're not sure, assume yes.
*/
return !oldestOffsetKnown ||
(nextOffset - oldestOffset > MULTIXACT_MEMBER_SAFE_THRESHOLD);
}
/*
* Return whether adding "distance" to "start" would move past "boundary".
*
* We use this to determine whether the addition is "wrapping around" the
* boundary point, hence the name. The reason we don't want to use the regular
* 2^31-modulo arithmetic here is that we want to be able to use the whole of
* the 2^32-1 space here, allowing for more multixacts than would fit
* otherwise.
*/
static bool
MultiXactOffsetWouldWrap(MultiXactOffset boundary, MultiXactOffset start,
uint32 distance)
{
MultiXactOffset finish;
/*
* Note that offset number 0 is not used (see GetMultiXactIdMembers), so
* if the addition wraps around the UINT_MAX boundary, skip that value.
*/
finish = start + distance;
if (finish < start)
finish++;
/*-----------------------------------------------------------------------
* When the boundary is numerically greater than the starting point, any
* value numerically between the two is not wrapped:
*
* <----S----B---->
* [---) = F wrapped past B (and UINT_MAX)
* [---) = F not wrapped
* [----] = F wrapped past B
*
* When the boundary is numerically less than the starting point (i.e. the
* UINT_MAX wraparound occurs somewhere in between) then all values in
* between are wrapped:
*
* <----B----S---->
* [---) = F not wrapped past B (but wrapped past UINT_MAX)
* [---) = F wrapped past B (and UINT_MAX)
* [----] = F not wrapped
*-----------------------------------------------------------------------
*/
if (start < boundary)
return finish >= boundary || finish < start;
else
return finish >= boundary && finish < start;
}
/*
* Find the starting offset of the given MultiXactId.
*
* Returns false if the file containing the multi does not exist on disk.
* Otherwise, returns true and sets *result to the starting member offset.
*
* This function does not prevent concurrent truncation, so if that's
* required, the caller has to protect against that.
*/
static bool
find_multixact_start(MultiXactId multi, MultiXactOffset *result)
{
MultiXactOffset offset;
int pageno;
int entryno;
int slotno;
MultiXactOffset *offptr;
Assert(MultiXactState->finishedStartup);
pageno = MultiXactIdToOffsetPage(multi);
entryno = MultiXactIdToOffsetEntry(multi);
/*
* Write out dirty data, so PhysicalPageExists can work correctly.
*/
SimpleLruWriteAll(MultiXactOffsetCtl, true);
SimpleLruWriteAll(MultiXactMemberCtl, true);
if (!SimpleLruDoesPhysicalPageExist(MultiXactOffsetCtl, pageno))
return false;
/* lock is acquired by SimpleLruReadPage_ReadOnly */
slotno = SimpleLruReadPage_ReadOnly(MultiXactOffsetCtl, pageno, multi);
offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
offptr += entryno;
offset = *offptr;
LWLockRelease(MultiXactOffsetSLRULock);
*result = offset;
return true;
}
/*
* Determine how many multixacts, and how many multixact members, currently
* exist. Return false if unable to determine.
*/
static bool
ReadMultiXactCounts(uint32 *multixacts, MultiXactOffset *members)
{
MultiXactOffset nextOffset;
MultiXactOffset oldestOffset;
MultiXactId oldestMultiXactId;
MultiXactId nextMultiXactId;
bool oldestOffsetKnown;
LWLockAcquire(MultiXactGenLock, LW_SHARED);
nextOffset = MultiXactState->nextOffset;
oldestMultiXactId = MultiXactState->oldestMultiXactId;
nextMultiXactId = MultiXactState->nextMXact;
oldestOffset = MultiXactState->oldestOffset;
oldestOffsetKnown = MultiXactState->oldestOffsetKnown;
LWLockRelease(MultiXactGenLock);
if (!oldestOffsetKnown)
return false;
*members = nextOffset - oldestOffset;
*multixacts = nextMultiXactId - oldestMultiXactId;
return true;
}
/*
* Multixact members can be removed once the multixacts that refer to them
* are older than every datminmxid. autovacuum_multixact_freeze_max_age and
* vacuum_multixact_freeze_table_age work together to make sure we never have
* too many multixacts; we hope that, at least under normal circumstances,
* this will also be sufficient to keep us from using too many offsets.
* However, if the average multixact has many members, we might exhaust the
* members space while still using few enough members that these limits fail
* to trigger relminmxid advancement by VACUUM. At that point, we'd have no
* choice but to start failing multixact-creating operations with an error.
*
* To prevent that, if more than a threshold portion of the members space is
* used, we effectively reduce autovacuum_multixact_freeze_max_age and
* to a value just less than the number of multixacts in use. We hope that
* this will quickly trigger autovacuuming on the table or tables with the
* oldest relminmxid, thus allowing datminmxid values to advance and removing
* some members.
*
* As the fraction of the member space currently in use grows, we become
* more aggressive in clamping this value. That not only causes autovacuum
* to ramp up, but also makes any manual vacuums the user issues more
* aggressive. This happens because vacuum_get_cutoffs() will clamp the
* freeze table and the minimum freeze age cutoffs based on the effective
* autovacuum_multixact_freeze_max_age this function returns. In the worst
* case, we'll claim the freeze_max_age to zero, and every vacuum of any
* table will freeze every multixact.
*/
int
MultiXactMemberFreezeThreshold(void)
{
MultiXactOffset members;
uint32 multixacts;
uint32 victim_multixacts;
double fraction;
int result;
/* If we can't determine member space utilization, assume the worst. */
if (!ReadMultiXactCounts(&multixacts, &members))
return 0;
/* If member space utilization is low, no special action is required. */
if (members <= MULTIXACT_MEMBER_SAFE_THRESHOLD)
return autovacuum_multixact_freeze_max_age;
/*
* Compute a target for relminmxid advancement. The number of multixacts
* we try to eliminate from the system is based on how far we are past
* MULTIXACT_MEMBER_SAFE_THRESHOLD.
*/
fraction = (double) (members - MULTIXACT_MEMBER_SAFE_THRESHOLD) /
(MULTIXACT_MEMBER_DANGER_THRESHOLD - MULTIXACT_MEMBER_SAFE_THRESHOLD);
victim_multixacts = multixacts * fraction;
/* fraction could be > 1.0, but lowest possible freeze age is zero */
if (victim_multixacts > multixacts)
return 0;
result = multixacts - victim_multixacts;
/*
* Clamp to autovacuum_multixact_freeze_max_age, so that we never make
* autovacuum less aggressive than it would otherwise be.
*/
return Min(result, autovacuum_multixact_freeze_max_age);
}
typedef struct mxtruncinfo
{
int earliestExistingPage;
} mxtruncinfo;
/*
* SlruScanDirectory callback
* This callback determines the earliest existing page number.
*/
static bool
SlruScanDirCbFindEarliest(SlruCtl ctl, char *filename, int segpage, void *data)
{
mxtruncinfo *trunc = (mxtruncinfo *) data;
if (trunc->earliestExistingPage == -1 ||
ctl->PagePrecedes(segpage, trunc->earliestExistingPage))
{
trunc->earliestExistingPage = segpage;
}
return false; /* keep going */
}
/*
* Delete members segments [oldest, newOldest)
*
* The members SLRU can, in contrast to the offsets one, be filled to almost
* the full range at once. This means SimpleLruTruncate() can't trivially be
* used - instead the to-be-deleted range is computed using the offsets
* SLRU. C.f. TruncateMultiXact().
*/
static void
PerformMembersTruncation(MultiXactOffset oldestOffset, MultiXactOffset newOldestOffset)
{
const int maxsegment = MXOffsetToMemberSegment(MaxMultiXactOffset);
int startsegment = MXOffsetToMemberSegment(oldestOffset);
int endsegment = MXOffsetToMemberSegment(newOldestOffset);
int segment = startsegment;
/*
* Delete all the segments but the last one. The last segment can still
* contain, possibly partially, valid data.
*/
while (segment != endsegment)
{
elog(DEBUG2, "truncating multixact members segment %x", segment);
SlruDeleteSegment(MultiXactMemberCtl, segment);
/* move to next segment, handling wraparound correctly */
if (segment == maxsegment)
segment = 0;
else
segment += 1;
}
}
/*
* Delete offsets segments [oldest, newOldest)
*/
static void
PerformOffsetsTruncation(MultiXactId oldestMulti, MultiXactId newOldestMulti)
{
/*
* We step back one multixact to avoid passing a cutoff page that hasn't
* been created yet in the rare case that oldestMulti would be the first
* item on a page and oldestMulti == nextMulti. In that case, if we
* didn't subtract one, we'd trigger SimpleLruTruncate's wraparound
* detection.
*/
SimpleLruTruncate(MultiXactOffsetCtl,
MultiXactIdToOffsetPage(PreviousMultiXactId(newOldestMulti)));
}
/*
* Remove all MultiXactOffset and MultiXactMember segments before the oldest
* ones still of interest.
*
* This is only called on a primary as part of vacuum (via
* vac_truncate_clog()). During recovery truncation is done by replaying
* truncation WAL records logged here.
*
* newOldestMulti is the oldest currently required multixact, newOldestMultiDB
* is one of the databases preventing newOldestMulti from increasing.
*/
void
TruncateMultiXact(MultiXactId newOldestMulti, Oid newOldestMultiDB)
{
MultiXactId oldestMulti;
MultiXactId nextMulti;
MultiXactOffset newOldestOffset;
MultiXactOffset oldestOffset;
MultiXactOffset nextOffset;
mxtruncinfo trunc;
MultiXactId earliest;
Assert(!RecoveryInProgress());
Assert(MultiXactState->finishedStartup);
/*
* We can only allow one truncation to happen at once. Otherwise parts of
* members might vanish while we're doing lookups or similar. There's no
* need to have an interlock with creating new multis or such, since those
* are constrained by the limits (which only grow, never shrink).
*/
LWLockAcquire(MultiXactTruncationLock, LW_EXCLUSIVE);
LWLockAcquire(MultiXactGenLock, LW_SHARED);
nextMulti = MultiXactState->nextMXact;
nextOffset = MultiXactState->nextOffset;
oldestMulti = MultiXactState->oldestMultiXactId;
LWLockRelease(MultiXactGenLock);
Assert(MultiXactIdIsValid(oldestMulti));
/*
* Make sure to only attempt truncation if there's values to truncate
* away. In normal processing values shouldn't go backwards, but there's
* some corner cases (due to bugs) where that's possible.
*/
if (MultiXactIdPrecedesOrEquals(newOldestMulti, oldestMulti))
{
LWLockRelease(MultiXactTruncationLock);
return;
}
/*
* Note we can't just plow ahead with the truncation; it's possible that
* there are no segments to truncate, which is a problem because we are
* going to attempt to read the offsets page to determine where to
* truncate the members SLRU. So we first scan the directory to determine
* the earliest offsets page number that we can read without error.
*
* When nextMXact is less than one segment away from multiWrapLimit,
* SlruScanDirCbFindEarliest can find some early segment other than the
* actual earliest. (MultiXactOffsetPagePrecedes(EARLIEST, LATEST)
* returns false, because not all pairs of entries have the same answer.)
* That can also arise when an earlier truncation attempt failed unlink()
* or returned early from this function. The only consequence is
* returning early, which wastes space that we could have liberated.
*
* NB: It's also possible that the page that oldestMulti is on has already
* been truncated away, and we crashed before updating oldestMulti.
*/
trunc.earliestExistingPage = -1;
SlruScanDirectory(MultiXactOffsetCtl, SlruScanDirCbFindEarliest, &trunc);
earliest = trunc.earliestExistingPage * MULTIXACT_OFFSETS_PER_PAGE;
if (earliest < FirstMultiXactId)
earliest = FirstMultiXactId;
/* If there's nothing to remove, we can bail out early. */
if (MultiXactIdPrecedes(oldestMulti, earliest))
{
LWLockRelease(MultiXactTruncationLock);
return;
}
/*
* First, compute the safe truncation point for MultiXactMember. This is
* the starting offset of the oldest multixact.
*
* Hopefully, find_multixact_start will always work here, because we've
* already checked that it doesn't precede the earliest MultiXact on disk.
* But if it fails, don't truncate anything, and log a message.
*/
if (oldestMulti == nextMulti)
{
/* there are NO MultiXacts */
oldestOffset = nextOffset;
}
else if (!find_multixact_start(oldestMulti, &oldestOffset))
{
ereport(LOG,
(errmsg("oldest MultiXact %u not found, earliest MultiXact %u, skipping truncation",
oldestMulti, earliest)));
LWLockRelease(MultiXactTruncationLock);
return;
}
/*
* Secondly compute up to where to truncate. Lookup the corresponding
* member offset for newOldestMulti for that.
*/
if (newOldestMulti == nextMulti)
{
/* there are NO MultiXacts */
newOldestOffset = nextOffset;
}
else if (!find_multixact_start(newOldestMulti, &newOldestOffset))
{
ereport(LOG,
(errmsg("cannot truncate up to MultiXact %u because it does not exist on disk, skipping truncation",
newOldestMulti)));
LWLockRelease(MultiXactTruncationLock);
return;
}
elog(DEBUG1, "performing multixact truncation: "
"offsets [%u, %u), offsets segments [%x, %x), "
"members [%u, %u), members segments [%x, %x)",
oldestMulti, newOldestMulti,
MultiXactIdToOffsetSegment(oldestMulti),
MultiXactIdToOffsetSegment(newOldestMulti),
oldestOffset, newOldestOffset,
MXOffsetToMemberSegment(oldestOffset),
MXOffsetToMemberSegment(newOldestOffset));
/*
* Do truncation, and the WAL logging of the truncation, in a critical
* section. That way offsets/members cannot get out of sync anymore, i.e.
* once consistent the newOldestMulti will always exist in members, even
* if we crashed in the wrong moment.
*/
START_CRIT_SECTION();
/*
* Prevent checkpoints from being scheduled concurrently. This is critical
* because otherwise a truncation record might not be replayed after a
* crash/basebackup, even though the state of the data directory would
* require it.
*/
Assert((MyProc->delayChkptFlags & DELAY_CHKPT_START) == 0);
MyProc->delayChkptFlags |= DELAY_CHKPT_START;
/* WAL log truncation */
WriteMTruncateXlogRec(newOldestMultiDB,
oldestMulti, newOldestMulti,
oldestOffset, newOldestOffset);
/*
* Update in-memory limits before performing the truncation, while inside
* the critical section: Have to do it before truncation, to prevent
* concurrent lookups of those values. Has to be inside the critical
* section as otherwise a future call to this function would error out,
* while looking up the oldest member in offsets, if our caller crashes
* before updating the limits.
*/
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
MultiXactState->oldestMultiXactId = newOldestMulti;
MultiXactState->oldestMultiXactDB = newOldestMultiDB;
LWLockRelease(MultiXactGenLock);
/* First truncate members */
PerformMembersTruncation(oldestOffset, newOldestOffset);
/* Then offsets */
PerformOffsetsTruncation(oldestMulti, newOldestMulti);
MyProc->delayChkptFlags &= ~DELAY_CHKPT_START;
END_CRIT_SECTION();
LWLockRelease(MultiXactTruncationLock);
}
/*
* Decide whether a MultiXactOffset page number is "older" for truncation
* purposes. Analogous to CLOGPagePrecedes().
*
* Offsetting the values is optional, because MultiXactIdPrecedes() has
* translational symmetry.
*/
static bool
MultiXactOffsetPagePrecedes(int page1, int page2)
{
MultiXactId multi1;
MultiXactId multi2;
multi1 = ((MultiXactId) page1) * MULTIXACT_OFFSETS_PER_PAGE;
multi1 += FirstMultiXactId + 1;
multi2 = ((MultiXactId) page2) * MULTIXACT_OFFSETS_PER_PAGE;
multi2 += FirstMultiXactId + 1;
return (MultiXactIdPrecedes(multi1, multi2) &&
MultiXactIdPrecedes(multi1,
multi2 + MULTIXACT_OFFSETS_PER_PAGE - 1));
}
/*
* Decide whether a MultiXactMember page number is "older" for truncation
* purposes. There is no "invalid offset number" so use the numbers verbatim.
*/
static bool
MultiXactMemberPagePrecedes(int page1, int page2)
{
MultiXactOffset offset1;
MultiXactOffset offset2;
offset1 = ((MultiXactOffset) page1) * MULTIXACT_MEMBERS_PER_PAGE;
offset2 = ((MultiXactOffset) page2) * MULTIXACT_MEMBERS_PER_PAGE;
return (MultiXactOffsetPrecedes(offset1, offset2) &&
MultiXactOffsetPrecedes(offset1,
offset2 + MULTIXACT_MEMBERS_PER_PAGE - 1));
}
/*
* Decide which of two MultiXactIds is earlier.
*
* XXX do we need to do something special for InvalidMultiXactId?
* (Doesn't look like it.)
*/
bool
MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2)
{
int32 diff = (int32) (multi1 - multi2);
return (diff < 0);
}
/*
* MultiXactIdPrecedesOrEquals -- is multi1 logically <= multi2?
*
* XXX do we need to do something special for InvalidMultiXactId?
* (Doesn't look like it.)
*/
bool
MultiXactIdPrecedesOrEquals(MultiXactId multi1, MultiXactId multi2)
{
int32 diff = (int32) (multi1 - multi2);
return (diff <= 0);
}
/*
* Decide which of two offsets is earlier.
*/
static bool
MultiXactOffsetPrecedes(MultiXactOffset offset1, MultiXactOffset offset2)
{
int32 diff = (int32) (offset1 - offset2);
return (diff < 0);
}
/*
* Write an xlog record reflecting the zeroing of either a MEMBERs or
* OFFSETs page (info shows which)
*/
static void
WriteMZeroPageXlogRec(int pageno, uint8 info)
{
XLogBeginInsert();
XLogRegisterData((char *) (&pageno), sizeof(int));
(void) XLogInsert(RM_MULTIXACT_ID, info);
}
/*
* Write a TRUNCATE xlog record
*
* We must flush the xlog record to disk before returning --- see notes in
* TruncateCLOG().
*/
static void
WriteMTruncateXlogRec(Oid oldestMultiDB,
MultiXactId startTruncOff, MultiXactId endTruncOff,
MultiXactOffset startTruncMemb, MultiXactOffset endTruncMemb)
{
XLogRecPtr recptr;
xl_multixact_truncate xlrec;
xlrec.oldestMultiDB = oldestMultiDB;
xlrec.startTruncOff = startTruncOff;
xlrec.endTruncOff = endTruncOff;
xlrec.startTruncMemb = startTruncMemb;
xlrec.endTruncMemb = endTruncMemb;
XLogBeginInsert();
XLogRegisterData((char *) (&xlrec), SizeOfMultiXactTruncate);
recptr = XLogInsert(RM_MULTIXACT_ID, XLOG_MULTIXACT_TRUNCATE_ID);
XLogFlush(recptr);
}
/*
* MULTIXACT resource manager's routines
*/
void
multixact_redo(XLogReaderState *record)
{
uint8 info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
/* Backup blocks are not used in multixact records */
Assert(!XLogRecHasAnyBlockRefs(record));
if (info == XLOG_MULTIXACT_ZERO_OFF_PAGE)
{
int pageno;
int slotno;
memcpy(&pageno, XLogRecGetData(record), sizeof(int));
LWLockAcquire(MultiXactOffsetSLRULock, LW_EXCLUSIVE);
slotno = ZeroMultiXactOffsetPage(pageno, false);
SimpleLruWritePage(MultiXactOffsetCtl, slotno);
Assert(!MultiXactOffsetCtl->shared->page_dirty[slotno]);
LWLockRelease(MultiXactOffsetSLRULock);
}
else if (info == XLOG_MULTIXACT_ZERO_MEM_PAGE)
{
int pageno;
int slotno;
memcpy(&pageno, XLogRecGetData(record), sizeof(int));
LWLockAcquire(MultiXactMemberSLRULock, LW_EXCLUSIVE);
slotno = ZeroMultiXactMemberPage(pageno, false);
SimpleLruWritePage(MultiXactMemberCtl, slotno);
Assert(!MultiXactMemberCtl->shared->page_dirty[slotno]);
LWLockRelease(MultiXactMemberSLRULock);
}
else if (info == XLOG_MULTIXACT_CREATE_ID)
{
xl_multixact_create *xlrec =
(xl_multixact_create *) XLogRecGetData(record);
TransactionId max_xid;
int i;
/* Store the data back into the SLRU files */
RecordNewMultiXact(xlrec->mid, xlrec->moff, xlrec->nmembers,
xlrec->members);
/* Make sure nextMXact/nextOffset are beyond what this record has */
MultiXactAdvanceNextMXact(xlrec->mid + 1,
xlrec->moff + xlrec->nmembers);
/*
* Make sure nextXid is beyond any XID mentioned in the record. This
* should be unnecessary, since any XID found here ought to have other
* evidence in the XLOG, but let's be safe.
*/
max_xid = XLogRecGetXid(record);
for (i = 0; i < xlrec->nmembers; i++)
{
if (TransactionIdPrecedes(max_xid, xlrec->members[i].xid))
max_xid = xlrec->members[i].xid;
}
AdvanceNextFullTransactionIdPastXid(max_xid);
}
else if (info == XLOG_MULTIXACT_TRUNCATE_ID)
{
xl_multixact_truncate xlrec;
int pageno;
memcpy(&xlrec, XLogRecGetData(record),
SizeOfMultiXactTruncate);
elog(DEBUG1, "replaying multixact truncation: "
"offsets [%u, %u), offsets segments [%x, %x), "
"members [%u, %u), members segments [%x, %x)",
xlrec.startTruncOff, xlrec.endTruncOff,
MultiXactIdToOffsetSegment(xlrec.startTruncOff),
MultiXactIdToOffsetSegment(xlrec.endTruncOff),
xlrec.startTruncMemb, xlrec.endTruncMemb,
MXOffsetToMemberSegment(xlrec.startTruncMemb),
MXOffsetToMemberSegment(xlrec.endTruncMemb));
/* should not be required, but more than cheap enough */
LWLockAcquire(MultiXactTruncationLock, LW_EXCLUSIVE);
/*
* Advance the horizon values, so they're current at the end of
* recovery.
*/
SetMultiXactIdLimit(xlrec.endTruncOff, xlrec.oldestMultiDB, false);
PerformMembersTruncation(xlrec.startTruncMemb, xlrec.endTruncMemb);
/*
* During XLOG replay, latest_page_number isn't necessarily set up
* yet; insert a suitable value to bypass the sanity test in
* SimpleLruTruncate.
*/
pageno = MultiXactIdToOffsetPage(xlrec.endTruncOff);
MultiXactOffsetCtl->shared->latest_page_number = pageno;
PerformOffsetsTruncation(xlrec.startTruncOff, xlrec.endTruncOff);
LWLockRelease(MultiXactTruncationLock);
}
else
elog(PANIC, "multixact_redo: unknown op code %u", info);
}
Datum
pg_get_multixact_members(PG_FUNCTION_ARGS)
{
typedef struct
{
MultiXactMember *members;
int nmembers;
int iter;
} mxact;
MultiXactId mxid = PG_GETARG_TRANSACTIONID(0);
mxact *multi;
FuncCallContext *funccxt;
if (mxid < FirstMultiXactId)
ereport(ERROR,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("invalid MultiXactId: %u", mxid)));
if (SRF_IS_FIRSTCALL())
{
MemoryContext oldcxt;
TupleDesc tupdesc;
funccxt = SRF_FIRSTCALL_INIT();
oldcxt = MemoryContextSwitchTo(funccxt->multi_call_memory_ctx);
multi = palloc(sizeof(mxact));
/* no need to allow for old values here */
multi->nmembers = GetMultiXactIdMembers(mxid, &multi->members, false,
false);
multi->iter = 0;
if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
elog(ERROR, "return type must be a row type");
funccxt->tuple_desc = tupdesc;
funccxt->attinmeta = TupleDescGetAttInMetadata(tupdesc);
funccxt->user_fctx = multi;
MemoryContextSwitchTo(oldcxt);
}
funccxt = SRF_PERCALL_SETUP();
multi = (mxact *) funccxt->user_fctx;
while (multi->iter < multi->nmembers)
{
HeapTuple tuple;
char *values[2];
values[0] = psprintf("%u", multi->members[multi->iter].xid);
values[1] = mxstatus_to_string(multi->members[multi->iter].status);
tuple = BuildTupleFromCStrings(funccxt->attinmeta, values);
multi->iter++;
pfree(values[0]);
SRF_RETURN_NEXT(funccxt, HeapTupleGetDatum(tuple));
}
SRF_RETURN_DONE(funccxt);
}
/*
* Entrypoint for sync.c to sync offsets files.
*/
int
multixactoffsetssyncfiletag(const FileTag *ftag, char *path)
{
return SlruSyncFileTag(MultiXactOffsetCtl, ftag, path);
}
/*
* Entrypoint for sync.c to sync members files.
*/
int
multixactmemberssyncfiletag(const FileTag *ftag, char *path)
{
return SlruSyncFileTag(MultiXactMemberCtl, ftag, path);
}