| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/regex-1.8.3/src/pool.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>pool.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../normalize.css"><link rel="stylesheet" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../ayu.css" disabled><link rel="stylesheet" href="../../dark.css" disabled><link rel="stylesheet" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script defer src="../../source-script.js"></script><script defer src="../../source-files.js"></script><script defer src="../../main.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../regex/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../regex/index.html"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span> |
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| </pre><pre class="rust"><code><span class="comment">// This module provides a relatively simple thread-safe pool of reusable |
| // objects. For the most part, it's implemented by a stack represented by a |
| // Mutex<Vec<T>>. It has one small trick: because unlocking a mutex is somewhat |
| // costly, in the case where a pool is accessed by the first thread that tried |
| // to get a value, we bypass the mutex. Here are some benchmarks showing the |
| // difference. |
| // |
| // 1) misc::anchored_literal_long_non_match 21 (18571 MB/s) |
| // 2) misc::anchored_literal_long_non_match 107 (3644 MB/s) |
| // 3) misc::anchored_literal_long_non_match 45 (8666 MB/s) |
| // 4) misc::anchored_literal_long_non_match 19 (20526 MB/s) |
| // |
| // (1) represents our baseline: the master branch at the time of writing when |
| // using the 'thread_local' crate to implement the pool below. |
| // |
| // (2) represents a naive pool implemented completely via Mutex<Vec<T>>. There |
| // is no special trick for bypassing the mutex. |
| // |
| // (3) is the same as (2), except it uses Mutex<Vec<Box<T>>>. It is twice as |
| // fast because a Box<T> is much smaller than the T we use with a Pool in this |
| // crate. So pushing and popping a Box<T> from a Vec is quite a bit faster |
| // than for T. |
| // |
| // (4) is the same as (3), but with the trick for bypassing the mutex in the |
| // case of the first-to-get thread. |
| // |
| // Why move off of thread_local? Even though (4) is a hair faster than (1) |
| // above, this was not the main goal. The main goal was to move off of |
| // thread_local and find a way to *simply* re-capture some of its speed for |
| // regex's specific case. So again, why move off of it? The *primary* reason is |
| // because of memory leaks. See https://github.com/rust-lang/regex/issues/362 |
| // for example. (Why do I want it to be simple? Well, I suppose what I mean is, |
| // "use as much safe code as possible to minimize risk and be as sure as I can |
| // be that it is correct.") |
| // |
| // My guess is that the thread_local design is probably not appropriate for |
| // regex since its memory usage scales to the number of active threads that |
| // have used a regex, where as the pool below scales to the number of threads |
| // that simultaneously use a regex. While neither case permits contraction, |
| // since we own the pool data structure below, we can add contraction if a |
| // clear use case pops up in the wild. More pressingly though, it seems that |
| // there are at least some use case patterns where one might have many threads |
| // sitting around that might have used a regex at one point. While thread_local |
| // does try to reuse space previously used by a thread that has since stopped, |
| // its maximal memory usage still scales with the total number of active |
| // threads. In contrast, the pool below scales with the total number of threads |
| // *simultaneously* using the pool. The hope is that this uses less memory |
| // overall. And if it doesn't, we can hopefully tune it somehow. |
| // |
| // It seems that these sort of conditions happen frequently |
| // in FFI inside of other more "managed" languages. This was |
| // mentioned in the issue linked above, and also mentioned here: |
| // https://github.com/BurntSushi/rure-go/issues/3. And in particular, users |
| // confirm that disabling the use of thread_local resolves the leak. |
| // |
| // There were other weaker reasons for moving off of thread_local as well. |
| // Namely, at the time, I was looking to reduce dependencies. And for something |
| // like regex, maintenance can be simpler when we own the full dependency tree. |
| |
| </span><span class="kw">use </span>std::panic::{RefUnwindSafe, UnwindSafe}; |
| <span class="kw">use </span>std::sync::atomic::{AtomicUsize, Ordering}; |
| <span class="kw">use </span>std::sync::Mutex; |
| |
| <span class="doccomment">/// An atomic counter used to allocate thread IDs. |
| </span><span class="kw">static </span>COUNTER: AtomicUsize = AtomicUsize::new(<span class="number">1</span>); |
| |
| <span class="macro">thread_local!</span>( |
| <span class="doccomment">/// A thread local used to assign an ID to a thread. |
| </span><span class="kw">static </span>THREAD_ID: usize = { |
| <span class="kw">let </span>next = COUNTER.fetch_add(<span class="number">1</span>, Ordering::Relaxed); |
| <span class="comment">// SAFETY: We cannot permit the reuse of thread IDs since reusing a |
| // thread ID might result in more than one thread "owning" a pool, |
| // and thus, permit accessing a mutable value from multiple threads |
| // simultaneously without synchronization. The intent of this panic is |
| // to be a sanity check. It is not expected that the thread ID space |
| // will actually be exhausted in practice. |
| // |
| // This checks that the counter never wraps around, since atomic |
| // addition wraps around on overflow. |
| </span><span class="kw">if </span>next == <span class="number">0 </span>{ |
| <span class="macro">panic!</span>(<span class="string">"regex: thread ID allocation space exhausted"</span>); |
| } |
| next |
| }; |
| ); |
| |
| <span class="doccomment">/// The type of the function used to create values in a pool when the pool is |
| /// empty and the caller requests one. |
| </span><span class="kw">type </span>CreateFn<T> = |
| Box<<span class="kw">dyn </span>Fn() -> T + Send + Sync + UnwindSafe + RefUnwindSafe + <span class="lifetime">'static</span>>; |
| |
| <span class="doccomment">/// A simple thread safe pool for reusing values. |
| /// |
| /// Getting a value out comes with a guard. When that guard is dropped, the |
| /// value is automatically put back in the pool. |
| /// |
| /// A Pool<T> impls Sync when T is Send (even if it's not Sync). This means |
| /// that T can use interior mutability. This is possible because a pool is |
| /// guaranteed to provide a value to exactly one thread at any time. |
| /// |
| /// Currently, a pool never contracts in size. Its size is proportional to the |
| /// number of simultaneous uses. |
| </span><span class="kw">pub struct </span>Pool<T> { |
| <span class="doccomment">/// A stack of T values to hand out. These are used when a Pool is |
| /// accessed by a thread that didn't create it. |
| </span>stack: Mutex<Vec<Box<T>>>, |
| <span class="doccomment">/// A function to create more T values when stack is empty and a caller |
| /// has requested a T. |
| </span>create: CreateFn<T>, |
| <span class="doccomment">/// The ID of the thread that owns this pool. The owner is the thread |
| /// that makes the first call to 'get'. When the owner calls 'get', it |
| /// gets 'owner_val' directly instead of returning a T from 'stack'. |
| /// See comments elsewhere for details, but this is intended to be an |
| /// optimization for the common case that makes getting a T faster. |
| /// |
| /// It is initialized to a value of zero (an impossible thread ID) as a |
| /// sentinel to indicate that it is unowned. |
| </span>owner: AtomicUsize, |
| <span class="doccomment">/// A value to return when the caller is in the same thread that created |
| /// the Pool. |
| </span>owner_val: T, |
| } |
| |
| <span class="comment">// SAFETY: Since we want to use a Pool from multiple threads simultaneously |
| // behind an Arc, we need for it to be Sync. In cases where T is sync, Pool<T> |
| // would be Sync. However, since we use a Pool to store mutable scratch space, |
| // we wind up using a T that has interior mutability and is thus itself not |
| // Sync. So what we *really* want is for our Pool<T> to by Sync even when T is |
| // not Sync (but is at least Send). |
| // |
| // The only non-sync aspect of a Pool is its 'owner_val' field, which is used |
| // to implement faster access to a pool value in the common case of a pool |
| // being accessed in the same thread in which it was created. The 'stack' field |
| // is also shared, but a Mutex<T> where T: Send is already Sync. So we only |
| // need to worry about 'owner_val'. |
| // |
| // The key is to guarantee that 'owner_val' can only ever be accessed from one |
| // thread. In our implementation below, we guarantee this by only returning the |
| // 'owner_val' when the ID of the current thread matches the ID of the thread |
| // that created the Pool. Since this can only ever be one thread, it follows |
| // that only one thread can access 'owner_val' at any point in time. Thus, it |
| // is safe to declare that Pool<T> is Sync when T is Send. |
| // |
| // NOTE: It would also be possible to make the owning thread be the *first* |
| // thread that tries to get a value out of a Pool. However, the current |
| // implementation is a little simpler and it's not clear if making the first |
| // thread (rather than the creating thread) is meaningfully better. |
| // |
| // If there is a way to achieve our performance goals using safe code, then |
| // I would very much welcome a patch. As it stands, the implementation below |
| // tries to balance safety with performance. The case where a Regex is used |
| // from multiple threads simultaneously will suffer a bit since getting a cache |
| // will require unlocking a mutex. |
| </span><span class="kw">unsafe impl</span><T: Send> Sync <span class="kw">for </span>Pool<T> {} |
| |
| <span class="kw">impl</span><T: ::std::fmt::Debug> ::std::fmt::Debug <span class="kw">for </span>Pool<T> { |
| <span class="kw">fn </span>fmt(<span class="kw-2">&</span><span class="self">self</span>, f: <span class="kw-2">&mut </span>::std::fmt::Formatter<<span class="lifetime">'_</span>>) -> ::std::fmt::Result { |
| f.debug_struct(<span class="string">"Pool"</span>) |
| .field(<span class="string">"stack"</span>, <span class="kw-2">&</span><span class="self">self</span>.stack) |
| .field(<span class="string">"owner"</span>, <span class="kw-2">&</span><span class="self">self</span>.owner) |
| .field(<span class="string">"owner_val"</span>, <span class="kw-2">&</span><span class="self">self</span>.owner_val) |
| .finish() |
| } |
| } |
| |
| <span class="doccomment">/// A guard that is returned when a caller requests a value from the pool. |
| /// |
| /// The purpose of the guard is to use RAII to automatically put the value back |
| /// in the pool once it's dropped. |
| </span><span class="attribute">#[derive(Debug)] |
| </span><span class="kw">pub struct </span>PoolGuard<<span class="lifetime">'a</span>, T: Send> { |
| <span class="doccomment">/// The pool that this guard is attached to. |
| </span>pool: <span class="kw-2">&</span><span class="lifetime">'a </span>Pool<T>, |
| <span class="doccomment">/// This is None when the guard represents the special "owned" value. In |
| /// which case, the value is retrieved from 'pool.owner_val'. |
| </span>value: <span class="prelude-ty">Option</span><Box<T>>, |
| } |
| |
| <span class="kw">impl</span><T: Send> Pool<T> { |
| <span class="doccomment">/// Create a new pool. The given closure is used to create values in the |
| /// pool when necessary. |
| </span><span class="kw">pub fn </span>new(create: CreateFn<T>) -> Pool<T> { |
| <span class="kw">let </span>owner = AtomicUsize::new(<span class="number">0</span>); |
| <span class="kw">let </span>owner_val = create(); |
| Pool { stack: Mutex::new(<span class="macro">vec!</span>[]), create, owner, owner_val } |
| } |
| |
| <span class="doccomment">/// Get a value from the pool. The caller is guaranteed to have exclusive |
| /// access to the given value. |
| /// |
| /// Note that there is no guarantee provided about which value in the |
| /// pool is returned. That is, calling get, dropping the guard (causing |
| /// the value to go back into the pool) and then calling get again is NOT |
| /// guaranteed to return the same value received in the first get call. |
| </span><span class="attribute">#[cfg_attr(feature = <span class="string">"perf-inline"</span>, inline(always))] |
| </span><span class="kw">pub fn </span>get(<span class="kw-2">&</span><span class="self">self</span>) -> PoolGuard<<span class="lifetime">'_</span>, T> { |
| <span class="comment">// Our fast path checks if the caller is the thread that "owns" this |
| // pool. Or stated differently, whether it is the first thread that |
| // tried to extract a value from the pool. If it is, then we can return |
| // a T to the caller without going through a mutex. |
| // |
| // SAFETY: We must guarantee that only one thread gets access to this |
| // value. Since a thread is uniquely identified by the THREAD_ID thread |
| // local, it follows that is the caller's thread ID is equal to the |
| // owner, then only one thread may receive this value. |
| </span><span class="kw">let </span>caller = THREAD_ID.with(|id| <span class="kw-2">*</span>id); |
| <span class="kw">let </span>owner = <span class="self">self</span>.owner.load(Ordering::Relaxed); |
| <span class="kw">if </span>caller == owner { |
| <span class="kw">return </span><span class="self">self</span>.guard_owned(); |
| } |
| <span class="self">self</span>.get_slow(caller, owner) |
| } |
| |
| <span class="doccomment">/// This is the "slow" version that goes through a mutex to pop an |
| /// allocated value off a stack to return to the caller. (Or, if the stack |
| /// is empty, a new value is created.) |
| /// |
| /// If the pool has no owner, then this will set the owner. |
| </span><span class="attribute">#[cold] |
| </span><span class="kw">fn </span>get_slow(<span class="kw-2">&</span><span class="self">self</span>, caller: usize, owner: usize) -> PoolGuard<<span class="lifetime">'_</span>, T> { |
| <span class="kw">use </span>std::sync::atomic::Ordering::Relaxed; |
| |
| <span class="kw">if </span>owner == <span class="number">0 </span>{ |
| <span class="comment">// The sentinel 0 value means this pool is not yet owned. We |
| // try to atomically set the owner. If we do, then this thread |
| // becomes the owner and we can return a guard that represents |
| // the special T for the owner. |
| </span><span class="kw">let </span>res = <span class="self">self</span>.owner.compare_exchange(<span class="number">0</span>, caller, Relaxed, Relaxed); |
| <span class="kw">if </span>res.is_ok() { |
| <span class="kw">return </span><span class="self">self</span>.guard_owned(); |
| } |
| } |
| <span class="kw">let </span><span class="kw-2">mut </span>stack = <span class="self">self</span>.stack.lock().unwrap(); |
| <span class="kw">let </span>value = <span class="kw">match </span>stack.pop() { |
| <span class="prelude-val">None </span>=> Box::new((<span class="self">self</span>.create)()), |
| <span class="prelude-val">Some</span>(value) => value, |
| }; |
| <span class="self">self</span>.guard_stack(value) |
| } |
| |
| <span class="doccomment">/// Puts a value back into the pool. Callers don't need to call this. Once |
| /// the guard that's returned by 'get' is dropped, it is put back into the |
| /// pool automatically. |
| </span><span class="kw">fn </span>put(<span class="kw-2">&</span><span class="self">self</span>, value: Box<T>) { |
| <span class="kw">let </span><span class="kw-2">mut </span>stack = <span class="self">self</span>.stack.lock().unwrap(); |
| stack.push(value); |
| } |
| |
| <span class="doccomment">/// Create a guard that represents the special owned T. |
| </span><span class="kw">fn </span>guard_owned(<span class="kw-2">&</span><span class="self">self</span>) -> PoolGuard<<span class="lifetime">'_</span>, T> { |
| PoolGuard { pool: <span class="self">self</span>, value: <span class="prelude-val">None </span>} |
| } |
| |
| <span class="doccomment">/// Create a guard that contains a value from the pool's stack. |
| </span><span class="kw">fn </span>guard_stack(<span class="kw-2">&</span><span class="self">self</span>, value: Box<T>) -> PoolGuard<<span class="lifetime">'_</span>, T> { |
| PoolGuard { pool: <span class="self">self</span>, value: <span class="prelude-val">Some</span>(value) } |
| } |
| } |
| |
| <span class="kw">impl</span><<span class="lifetime">'a</span>, T: Send> PoolGuard<<span class="lifetime">'a</span>, T> { |
| <span class="doccomment">/// Return the underlying value. |
| </span><span class="kw">pub fn </span>value(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="kw-2">&</span>T { |
| <span class="kw">match </span><span class="self">self</span>.value { |
| <span class="prelude-val">None </span>=> <span class="kw-2">&</span><span class="self">self</span>.pool.owner_val, |
| <span class="prelude-val">Some</span>(<span class="kw-2">ref </span>v) => <span class="kw-2">&**</span>v, |
| } |
| } |
| } |
| |
| <span class="kw">impl</span><<span class="lifetime">'a</span>, T: Send> Drop <span class="kw">for </span>PoolGuard<<span class="lifetime">'a</span>, T> { |
| <span class="attribute">#[cfg_attr(feature = <span class="string">"perf-inline"</span>, inline(always))] |
| </span><span class="kw">fn </span>drop(<span class="kw-2">&mut </span><span class="self">self</span>) { |
| <span class="kw">if let </span><span class="prelude-val">Some</span>(value) = <span class="self">self</span>.value.take() { |
| <span class="self">self</span>.pool.put(value); |
| } |
| } |
| } |
| |
| <span class="attribute">#[cfg(test)] |
| </span><span class="kw">mod </span>tests { |
| <span class="kw">use </span>std::panic::{RefUnwindSafe, UnwindSafe}; |
| |
| <span class="kw">use super</span>::<span class="kw-2">*</span>; |
| |
| <span class="attribute">#[test] |
| </span><span class="kw">fn </span>oibits() { |
| <span class="kw">use </span><span class="kw">crate</span>::exec::ProgramCache; |
| |
| <span class="kw">fn </span>has_oibits<T: Send + Sync + UnwindSafe + RefUnwindSafe>() {} |
| has_oibits::<Pool<ProgramCache>>(); |
| } |
| |
| <span class="comment">// Tests that Pool implements the "single owner" optimization. That is, the |
| // thread that first accesses the pool gets its own copy, while all other |
| // threads get distinct copies. |
| </span><span class="attribute">#[test] |
| </span><span class="kw">fn </span>thread_owner_optimization() { |
| <span class="kw">use </span>std::cell::RefCell; |
| <span class="kw">use </span>std::sync::Arc; |
| |
| <span class="kw">let </span>pool: Arc<Pool<RefCell<Vec<char>>>> = |
| Arc::new(Pool::new(Box::new(|| RefCell::new(<span class="macro">vec!</span>[<span class="string">'a'</span>])))); |
| pool.get().value().borrow_mut().push(<span class="string">'x'</span>); |
| |
| <span class="kw">let </span>pool1 = pool.clone(); |
| <span class="kw">let </span>t1 = std::thread::spawn(<span class="kw">move </span>|| { |
| <span class="kw">let </span>guard = pool1.get(); |
| <span class="kw">let </span>v = guard.value(); |
| v.borrow_mut().push(<span class="string">'y'</span>); |
| }); |
| |
| <span class="kw">let </span>pool2 = pool.clone(); |
| <span class="kw">let </span>t2 = std::thread::spawn(<span class="kw">move </span>|| { |
| <span class="kw">let </span>guard = pool2.get(); |
| <span class="kw">let </span>v = guard.value(); |
| v.borrow_mut().push(<span class="string">'z'</span>); |
| }); |
| |
| t1.join().unwrap(); |
| t2.join().unwrap(); |
| |
| <span class="comment">// If we didn't implement the single owner optimization, then one of |
| // the threads above is likely to have mutated the [a, x] vec that |
| // we stuffed in the pool before spawning the threads. But since |
| // neither thread was first to access the pool, and because of the |
| // optimization, we should be guaranteed that neither thread mutates |
| // the special owned pool value. |
| // |
| // (Technically this is an implementation detail and not a contract of |
| // Pool's API.) |
| </span><span class="macro">assert_eq!</span>(<span class="macro">vec!</span>[<span class="string">'a'</span>, <span class="string">'x'</span>], <span class="kw-2">*</span>pool.get().value().borrow()); |
| } |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="regex" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
