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Change-Id: I5552a344bfb34b0c5bec8fd8d61388ec3ddcd49d Reviewed-on: http://gerrit.cloudera.org:8080/11424 Reviewed-by: Alex Rodoni <arodoni@cloudera.com> Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
96 lines
3.2 KiB
XML
96 lines
3.2 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!--
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Licensed to the Apache Software Foundation (ASF) under one
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or more contributor license agreements. See the NOTICE file
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distributed with this work for additional information
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regarding copyright ownership. The ASF licenses this file
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to you under the Apache License, Version 2.0 (the
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"License"); you may not use this file except in compliance
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with the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing,
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software distributed under the License is distributed on an
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"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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KIND, either express or implied. See the License for the
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specific language governing permissions and limitations
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under the License.
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-->
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept id="thread_reservation_limit">
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<title>THREAD_RESERVATION_LIMIT Query Option (<keyword keyref="impala31"/> or higher only)</title>
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<titlealts audience="PDF">
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<navtitle>THREAD_RESERVATION_LIMIT</navtitle>
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</titlealts>
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<prolog>
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<metadata>
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<data name="Category" value="Impala"/>
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<data name="Category" value="Impala Query Options"/>
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<data name="Category" value="Scalability"/>
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<data name="Category" value="Memory"/>
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<data name="Category" value="Troubleshooting"/>
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<data name="Category" value="Developers"/>
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<data name="Category" value="Data Analysts"/>
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</metadata>
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</prolog>
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<conbody>
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<p>
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The <codeph>THREAD_RESERVATION_LIMIT</codeph> query option limits the number of reserved
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threads for a query on each node. The option is intended to prevent execution of complex
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queries that can consume excessive CPU or operating system resources on a single node.
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Queries that have more threads per node than this threshold are rejected by Impala’s
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admission controller before they start executing. You can see the number of reserved
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threads for a query in its
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<xref
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href="impala_explain_plan.xml#explain_plan">explain plan</xref> in the
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“Per-Host Resource Reservation" line.
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</p>
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<p>
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For example, an Impala administrator could set a default value of
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<codeph>THREAD_RESERVATION_LIMIT=100</codeph> for a resource pool where they expect only
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relatively simple queries to run. This will reject queries that require more than 100
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reserved threads on a node, for example, queries with more than 100 fragments.
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</p>
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<p>
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You can override the default value per-query or per-session, in the same way as other
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query options, if you do not want the default <codeph>THREAD_RESERVATION_LIMIT</codeph>
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value to apply to a specific query or session.
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</p>
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<p>
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<note>
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The number of reserved threads on a node may be lower than the maximum value in the
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explain plan if not all fragments of that query are scheduled on every node.
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</note>
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</p>
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<p>
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<b>Syntax:</b> <codeph>SET THREAD_RESERVATION_LIMIT=number;</codeph>
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</p>
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<p>
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<b>Type:</b> numeric
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</p>
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<p rev="">
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<b>Default:</b> 3000
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</p>
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<p>
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<b>Added in:</b> <keyword keyref="impala31"/>
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</p>
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</conbody>
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</concept>
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