docs/topics/impala_mem_limit.xml

<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="mem_limit">

  <title>MEM_LIMIT Query Option</title>

  <titlealts audience="PDF">

    <navtitle>MEM_LIMIT</navtitle>

  </titlealts>

  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="Impala Query Options"/>
      <data name="Category" value="Scalability"/>
      <data name="Category" value="Memory"/>
      <data name="Category" value="Troubleshooting"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      The MEM_LIMIT query option defines the maximum amount of memory a query can allocate on
      each node. The total memory that can be used by a query is the <codeph>MEM_LIMIT</codeph>
      times the number of nodes.
    </p>

    <p rev="">
      There are two levels of memory limit for Impala. The
      <codeph>&#8209;&#8209;mem_limit</codeph> startup option sets an overall limit for the
      <cmdname>impalad</cmdname> process (which handles multiple queries concurrently). That
      process memory limit can be expressed either as a percentage of RAM available to the
      process such as <codeph>&#8209;&#8209;mem_limit=70%</codeph> or as a fixed amount of
      memory, such as <codeph>100gb</codeph>. The memory available to the process is based on
      the host's physical memory and, since Impala 3.2, memory limits from Linux Control Groups.
      E.g. if an <cmdname>impalad</cmdname> process is running in a Docker container on a host
      with 100GB of memory, the memory available is 100GB or the Docker container's memory
      limit, whichever is less.
    </p>

    <p rev="">
      The <codeph>MEM_LIMIT</codeph> query option, which you set through
      <cmdname>impala-shell</cmdname> or the <codeph>SET</codeph> statement in a JDBC or ODBC
      application, applies to each individual query. The <codeph>MEM_LIMIT</codeph> query option
      is usually expressed as a fixed size such as <codeph>10gb</codeph>, and must always be
      less than the <cmdname>impalad</cmdname> memory limit.
    </p>

    <p rev="">
      If query processing exceeds the specified memory limit on any node, either the per-query
      limit or the <cmdname>impalad</cmdname> limit, Impala cancels the query automatically.
      Memory limits are checked periodically during query processing, so the actual memory in
      use might briefly exceed the limit without the query being cancelled.
    </p>

    <p>
      <b>Type:</b> numeric
    </p>

    <p rev="">
      <b>Units:</b> A numeric argument represents memory size in bytes; you can also use a
      suffix of <codeph>m</codeph> or <codeph>mb</codeph> for megabytes, or more commonly
      <codeph>g</codeph> or <codeph>gb</codeph> for gigabytes. If you specify a value with
      unrecognized formats, subsequent queries fail with an error.
    </p>

    <p rev="">
      <b>Default:</b> 0 (unlimited)
    </p>

    <p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>

    <p rev="">
      The <codeph>MEM_LIMIT</codeph> setting is primarily useful in a high-concurrency setting,
      or on a cluster with a workload shared between Impala and other data processing
      components. You can prevent any query from accidentally using much more memory than
      expected, which could negatively impact other Impala queries.
    </p>

    <p rev="">
      Use the output of the <codeph>SUMMARY</codeph> command in <cmdname>impala-shell</cmdname>
      to get a report of memory used for each phase of your most heavyweight queries on each
      node, and then set a <codeph>MEM_LIMIT</codeph> somewhat higher than that. See
      <xref href="impala_explain_plan.xml#perf_summary"/> for usage information about the
      <codeph>SUMMARY</codeph> command.
    </p>

    <p conref="../shared/impala_common.xml#common/example_blurb" rev=""/>

    <p rev="">
      The following examples show how to set the <codeph>MEM_LIMIT</codeph> query option using a
      fixed number of bytes, or suffixes representing gigabytes or megabytes.
    </p>

<codeblock rev="">
[localhost:21000] > set mem_limit=3000000000;
MEM_LIMIT set to 3000000000
[localhost:21000] > select 5;
Query: select 5
+---+
| 5 |
+---+
| 5 |
+---+

[localhost:21000] > set mem_limit=3g;
MEM_LIMIT set to 3g
[localhost:21000] > select 5;
Query: select 5
+---+
| 5 |
+---+
| 5 |
+---+

[localhost:21000] > set mem_limit=3gb;
MEM_LIMIT set to 3gb
[localhost:21000] > select 5;
+---+
| 5 |
+---+
| 5 |
+---+

[localhost:21000] > set mem_limit=3m;
MEM_LIMIT set to 3m
[localhost:21000] > select 5;
+---+
| 5 |
+---+
| 5 |
+---+
[localhost:21000] > set mem_limit=3mb;
MEM_LIMIT set to 3mb
[localhost:21000] > select 5;
+---+
| 5 |
+---+
</codeblock>

    <p rev="">
      The following examples show how unrecognized <codeph>MEM_LIMIT</codeph> values lead to
      errors for subsequent queries.
    </p>

<codeblock rev="">
[localhost:21000] > set mem_limit=3pb;
MEM_LIMIT set to 3pb
[localhost:21000] > select 5;
ERROR: Failed to parse query memory limit from '3pb'.

[localhost:21000] > set mem_limit=xyz;
MEM_LIMIT set to xyz
[localhost:21000] > select 5;
Query: select 5
ERROR: Failed to parse query memory limit from 'xyz'.
</codeblock>

    <p rev="">
      The following examples shows the automatic query cancellation when the
      <codeph>MEM_LIMIT</codeph> value is exceeded on any host involved in the Impala query.
      First it runs a successful query and checks the largest amount of memory used on any node
      for any stage of the query. Then it sets an artificially low <codeph>MEM_LIMIT</codeph>
      setting so that the same query cannot run.
    </p>

<codeblock rev="">
[localhost:21000] > select count(*) from customer;
Query: select count(*) from customer
+----------+
| count(*) |
+----------+
| 150000   |
+----------+

[localhost:21000] > select count(distinct c_name) from customer;
Query: select count(distinct c_name) from customer
+------------------------+
| count(distinct c_name) |
+------------------------+
| 150000                 |
+------------------------+

[localhost:21000] > summary;
+--------------+--------+--------+----------+----------+---------+------------+----------+---------------+---------------+
| Operator     | #Hosts | #Inst  | Avg Time | Max Time | #Rows   | Est. #Rows | Peak Mem | Est. Peak Mem | Detail        |
+--------------+--------+--------+----------+----------+---------+------------+----------+---------------+---------------+
| 06:AGGREGATE | 1      | 1      | 230.00ms | 230.00ms | 1       | 1          | 16.00 KB | -1 B          | FINALIZE      |
| 05:EXCHANGE  | 1      | 1      | 43.44us  | 43.44us  | 1       | 1          | 0 B      | -1 B          | UNPARTITIONED |
| 02:AGGREGATE | 1      | 1      | 227.14ms | 227.14ms | 1       | 1          | 12.00 KB | 10.00 MB      |               |
| 04:AGGREGATE | 1      | 1      | 126.27ms | 126.27ms | 150.00K | 150.00K    | 15.17 MB | 10.00 MB      |               |
| 03:EXCHANGE  | 1      | 1      | 44.07ms  | 44.07ms  | 150.00K | 150.00K    | 0 B      | 0 B           | HASH(c_name)  |
<b>| 01:AGGREGATE | 1      | 1      | 361.94ms | 361.94ms | 150.00K | 150.00K    | 23.04 MB | 10.00 MB      |               |</b>
| 00:SCAN HDFS | 1      | 1      | 43.64ms  | 43.64ms  | 150.00K | 150.00K    | 24.19 MB | 64.00 MB      | tpch.customer |
+--------------+--------+--------+----------+----------+---------+------------+----------+---------------+---------------+

[localhost:21000] > set mem_limit=15mb;
MEM_LIMIT set to 15mb
[localhost:21000] > select count(distinct c_name) from customer;
Query: select count(distinct c_name) from customer
ERROR:
Memory limit exceeded
Query did not have enough memory to get the minimum required buffers in the block manager.
</codeblock>

  </conbody>

</concept>