docs/topics/impala_thread_reservation_aggregate_limit.xml

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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="thread_reservation_aggregate_limit">

  <title>THREAD_RESERVATION_AGGREGATE_LIMIT Query Option (<keyword
      keyref="impala31"/> or higher only)</title>

  <titlealts audience="PDF">

    <navtitle>THREAD_RESERVATION_AGGREGATE_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 <codeph>THREAD_RESERVATION_AGGREGATE_LIMIT</codeph> query option limits the number of
      reserved threads for a query across all nodes on which it is executing. The option is
      intended to prevent execution of complex queries that can consume excessive CPU or
      operating system resources on a cluster. Queries that have more threads than this
      threshold are rejected by Impala’s admission controller before they start executing.
    </p>

    <p>
      For example, an Impala administrator could set a default value of
      <codeph>THREAD_RESERVATION_AGGREGATE_LIMIT=2000</codeph> for a resource pool on a 100 node
      where they expect only relatively simple queries with less than 20 threads per node to
      run. This will reject queries that require more than 2000 reserved threads across all
      nodes, for example a query with 21 fragments running on all 100 nodes of the cluster.
    </p>

    <p>
      You can override the default value per-query or per-session, in the same way as other
      query options, if you do not want the default
      <codeph>THREAD_RESERVATION_AGGREGATE_LIMIT</codeph> value to apply to a specific query or
      session.
    </p>

    <p>
      <b>Syntax:</b> <codeph>SET THREAD_RESERVATION_AGGREGATE_LIMIT=number;</codeph>
    </p>

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

    <p>
      <b>Default:</b> 0 (no limit)
    </p>

    <p>
      <b>Added in:</b> <keyword keyref="impala31"/>
    </p>

  </conbody>

</concept>