﻿---
title: Thread pool settings
description: A node uses several thread pools to manage memory consumption. Queues associated with many of the thread pools enable pending requests to be held instead...
url: https://www.elastic.co/elastic/docs-builder/docs/2642/reference/elasticsearch/configuration-reference/thread-pool-settings
products:
  - Elasticsearch
applies_to:
  - Self-managed Elastic deployments: Generally available
---

# Thread pool settings
A node uses several thread pools to manage memory consumption. Queues associated with many of the thread pools enable pending requests to be held instead of discarded.
There are several thread pools, but the important ones include:
<definitions>
  <definition term="generic">
    For generic operations (for example, background node discovery). Thread pool type is `scaling`.
  </definition>
</definitions>


<definitions>
  <definition term="search">
    For count/search operations at the shard level. Used also by fetch and other search related operations  Thread pool type is `fixed` with a size of `int((`[`# of allocated processors`](#node.processors)` * 3) / 2) + 1`, and queue_size of `1000`.
  </definition>
  <definition term="search_throttled">
    For count/search/suggest/get operations on `search_throttled indices`. Thread pool type is `fixed` with a size of `1`, and queue_size of `100`.
  </definition>
  <definition term="search_coordination">
    For lightweight search-related coordination operations. Thread pool type is `fixed` with a size of `(`[`# of allocated processors`](#node.processors)`) / 2`, and queue_size of `1000`.
  </definition>
  <definition term="get">
    For get operations. Thread pool type is `fixed` with a size of `int((`[`# of allocated processors`](#node.processors)` * 3) / 2) + 1`, and queue_size of `1000`.
  </definition>
  <definition term="analyze">
    For analyze requests. Thread pool type is `fixed` with a size of `1`, queue size of `16`.
  </definition>
  <definition term="write">
    For write operations and ingest processors. Thread pool type is `fixed` with a size of [`# of allocated processors`](#node.processors), queue_size of `max(10000, (`[`# of allocated processors`](#node.processors)`* 750))`. The maximum size for this pool is `1 + `[`# of allocated processors`](#node.processors).
    <note>
      In Elastic Stack 9.0 and earlier, the `write` thread pool was also used for bulk requests.
      In Elastic Stack 9.1 and earlier, the queue_size was 10000.
    </note>
  </definition>
  <definition term="write_coordination Elastic Stack: Generally available since 9.1">
    For bulk request coordination operations. Thread pool type is `fixed` with a size of [`# of allocated processors`](#node.processors), queue_size of `10000`. The maximum size for this pool is `1 + `[`# of allocated processors`](#node.processors).
  </definition>
  <definition term="snapshot">
    For snapshot/restore operations. Thread pool type is `scaling` with a keep-alive of `5m`. On nodes with at least 750MB of heap the maximum size of this pool is `10` by default. On nodes with less than 750MB of heap the maximum size of this pool is `min(5, (`[`# of allocated processors`](#node.processors)`) / 2)` by default.
  </definition>
  <definition term="snapshot_meta">
    For snapshot repository metadata read operations. Thread pool type is `scaling` with a keep-alive of `5m` and a max of `min(50, (`[`# of allocated processors`](#node.processors)`* 3))`.
  </definition>
  <definition term="warmer">
    For segment warm-up operations. Thread pool type is `scaling` with a keep-alive of `5m` and a max of `min(5, (`[`# of allocated processors`](#node.processors)`) / 2)`.
  </definition>
  <definition term="refresh">
    For refresh operations. Thread pool type is `scaling` with a keep-alive of `5m` and a max of `min(10, (`[`# of allocated processors`](#node.processors)`) / 2)`.
  </definition>
  <definition term="fetch_shard_started">
    For listing shard states. Thread pool type is `scaling` with keep-alive of `5m` and a default maximum size of `2 * `[`# of allocated processors`](#node.processors).
  </definition>
  <definition term="fetch_shard_store">
    For listing shard stores. Thread pool type is `scaling` with keep-alive of `5m` and a default maximum size of `2 * `[`# of allocated processors`](#node.processors).
  </definition>
  <definition term="flush">
    For [flush](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-indices-flush) and [translog](https://www.elastic.co/elastic/docs-builder/docs/2642/reference/elasticsearch/index-settings/translog) `fsync` operations. Thread pool type is `scaling` with a keep-alive of `5m` and a default maximum size of `min(5, (`[`# of allocated processors`](#node.processors)`) / 2)`.
  </definition>
  <definition term="merge">
    For [merge](https://www.elastic.co/guide/en/elasticsearch/reference/current/index-modules-merge.html) operations of all the shards on the node. Thread pool type is `scaling` with a keep-alive of `5m` and a default maximum size of [`# of allocated processors`](#node.processors).
  </definition>
  <definition term="force_merge">
    For waiting on blocking [force merge](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-indices-forcemerge) operations. Thread pool type is `fixed` with a size of `max(1, (`[`# of allocated processors`](#node.processors)`) / 8)` and an unbounded queue size.
  </definition>
  <definition term="management">
    For cluster management. Thread pool type is `scaling` with a keep-alive of `5m` and a default maximum size of `5`.
  </definition>
  <definition term="system_read">
    For read operations on system indices. Thread pool type is `fixed` with a default maximum size of `min(5, (`[`# of allocated processors`](#node.processors)`) / 2)`.
  </definition>
  <definition term="system_write">
    For write operations on system indices. Thread pool type is `fixed` with a default maximum size of `min(5, (`[`# of allocated processors`](#node.processors)`) / 2)`.
  </definition>
  <definition term="system_write_coordination Elastic Stack: Generally available since 9.1">
    For bulk request coordination operations on system indices. Thread pool type is `fixed` with a default maximum size of `min(5, (`[`# of allocated processors`](#node.processors)`) / 2)`.
  </definition>
  <definition term="system_critical_read">
    For critical read operations on system indices. Thread pool type is `fixed` with a default maximum size of `min(5, (`[`# of allocated processors`](#node.processors)`) / 2)`.
  </definition>
  <definition term="system_critical_write">
    For critical write operations on system indices. Thread pool type is `fixed` with a default maximum size of `min(5, (`[`# of allocated processors`](#node.processors)`) / 2)`.
  </definition>
  <definition term="watcher">
    For [watch executions](https://docs-v3-preview.elastic.dev/elastic/docs-builder/docs/2642/explore-analyze/alerts-cases/watcher). Thread pool type is `fixed` with a default maximum size of `min(5 * (`[`# of allocated processors`](#node.processors)`), 50)` and queue_size of `1000`.
  </definition>
  <definition term="esql_worker">
    Executes [ES|QL](https://www.elastic.co/elastic/docs-builder/docs/2642/reference/query-languages/esql) operations. Thread pool type is `fixed` with a size of `int((`[`# of allocated processors`](#node.processors) ` * 3) / 2) + 1`, and queue_size of `1000`.
  </definition>
</definitions>

Thread pool settings are [Static](https://docs-v3-preview.elastic.dev/elastic/docs-builder/docs/2642/deploy-manage/stack-settings#static-cluster-setting) and can be changed by editing `elasticsearch.yml`. Changing a specific thread pool can be done by setting its type-specific parameters; for example, changing the number of threads in the `write` thread pool:
```yaml
thread_pool:
    write:
        size: 30
```


## Thread pool types

The following are the types of thread pools and their respective parameters:

### `fixed`

A `fixed` thread pool holds a fixed number of threads as determined by the `size` parameter. If a task is submitted to a `fixed` thread pool and there are fewer than `size` busy threads in the pool then the task will execute immediately. If all the threads are busy when a task is submitted then it will be held in a queue for later execution. The `queue_size` parameter controls the maximum size of this queue. A `queue_size` of `-1` means that the queue is unbounded, but most `fixed` thread pools specify a bound on their queue size by default. If a bounded queue is full then it will reject further work, which typically causes the corresponding requests to fail.
```yaml
thread_pool:
    write:
        size: 30
        queue_size: 1000
```


### `scaling`

The `scaling` thread pool holds a dynamic number of threads. This number is proportional to the workload and varies between the value of the `core` and `max` parameters.
The `keep_alive` parameter determines how long a thread should be kept around in the thread pool without it doing any work.
If a task is submitted to a `scaling` thread pool when its maximum number of threads are already busy with other tasks, the new task will be held in a queue for later execution. The queue in a `scaling` thread pool is always unbounded.
```yaml
thread_pool:
    warmer:
        core: 1
        max: 8
        keep_alive: 2m
```


## Allocated processors setting

The number of processors is automatically detected, and the thread pool settings are automatically set based on it. In some cases it can be useful to override the number of detected processors. This can be done by explicitly setting the `node.processors` setting. This setting is bounded by the number of available processors and accepts floating point numbers, which can be useful in environments where the Elasticsearch nodes are configured to run with CPU limits, such as cpu shares or quota under `Cgroups`.
```yaml
node.processors: 2
```

There are a few use-cases for explicitly overriding the `node.processors` setting:
1. If you are running multiple instances of Elasticsearch on the same host but want Elasticsearch to size its thread pools as if it only has a fraction of the CPU, you should override the `node.processors` setting to the desired fraction, for example, if you’re running two instances of Elasticsearch on a 16-core machine, set `node.processors` to 8. Note that this is an expert-level use case and there’s a lot more involved than just setting the `node.processors` setting as there are other considerations like changing the number of garbage collector threads, pinning processes to cores, and so on.
2. Sometimes the number of processors is wrongly detected and in such cases explicitly setting the `node.processors` setting will workaround such issues.

In order to check the number of processors detected, use the nodes info API with the `os` flag.