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| <!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
| <concept id="thread_reservation_limit"> |
| |
| <title>THREAD_RESERVATION_LIMIT Query Option (<keyword keyref="impala31"/> or higher only)</title> |
| |
| <titlealts audience="PDF"> |
| |
| <navtitle>THREAD RESERVATION 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_LIMIT</codeph> query option limits the number of reserved |
| threads for a query on each node. The option is intended to prevent execution of complex |
| queries that can consume excessive CPU or operating system resources on a single node. |
| Queries that have more threads per node than this threshold are rejected by Impala’s |
| admission controller before they start executing. You can see the number of reserved |
| threads for a query in its |
| <xref |
| href="impala_explain_plan.xml#explain_plan">explain plan</xref> in the |
| “Per-Host Resource Reservation" line. |
| </p> |
| |
| <p> |
| For example, an Impala administrator could set a default value of |
| <codeph>THREAD_RESERVATION_LIMIT=100</codeph> for a resource pool where they expect only |
| relatively simple queries to run. This will reject queries that require more than 100 |
| reserved threads on a node, for example, queries with more than 100 fragments. |
| </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_LIMIT</codeph> |
| value to apply to a specific query or session. |
| </p> |
| |
| <p> |
| <note> |
| The number of reserved threads on a node may be lower than the maximum value in the |
| explain plan if not all fragments of that query are scheduled on every node. |
| </note> |
| </p> |
| |
| <p> |
| <b>Syntax:</b> <codeph>SET THREAD_RESERVATION_LIMIT=number;</codeph> |
| </p> |
| |
| <p> |
| <b>Type:</b> numeric |
| </p> |
| |
| <p rev=""> |
| <b>Default:</b> 3000 |
| </p> |
| |
| <p> |
| <b>Added in:</b> <keyword keyref="impala31"/> |
| </p> |
| |
| </conbody> |
| |
| </concept> |