OpenSearch/docs/reference/modules/threadpool.asciidoc

[[modules-threadpool]]
== Thread Pool

A node holds several thread pools in order to improve how threads are
managed and memory consumption within a node. There are several thread
pools, but the important ones include:

[horizontal]
`index`:: 
    For index/delete operations, defaults to `fixed`, 
    size `# of available processors`. 

`search`:: 
    For count/search operations, defaults to `fixed`,
    size `3x # of available processors`. 

`get`:: 
    For get operations, defaults to `fixed`
    size `# of available processors`.

`bulk`:: 
    For bulk operations, defaults to `fixed`
    size `# of available processors`.

`warmer`:: 
    For segment warm-up operations, defaults to `scaling`
    with a `5m` keep-alive. 

`refresh`:: 
    For refresh operations, defaults to `scaling`
    with a `5m` keep-alive. 

Changing a specific thread pool can be done by setting its type and
specific type parameters, for example, changing the `index` thread pool
to `blocking` type:

[source,js]
--------------------------------------------------
threadpool:
    index:
        type: blocking
        min: 1
        size: 30
        wait_time: 30s
--------------------------------------------------

NOTE: you can update threadpool settings live using
      <<cluster-update-settings>>.


[float]
=== Thread pool types

The following are the types of thread pools that can be used and their
respective parameters:

[float]
==== `cache`

The `cache` thread pool is an unbounded thread pool that will spawn a
thread if there are pending requests. Here is an example of how to set
it:

[source,js]
--------------------------------------------------
threadpool:
    index:
        type: cached
--------------------------------------------------

[float]
==== `fixed`

The `fixed` thread pool holds a fixed size of threads to handle the
requests with a queue (optionally bounded) for pending requests that
have no threads to service them.

The `size` parameter controls the number of threads, and defaults to the
number of cores times 5.

The `queue_size` allows to control the size of the queue of pending
requests that have no threads to execute them. By default, it is set to
`-1` which means its unbounded. When a request comes in and the queue is
full, the `reject_policy` parameter can control how it will behave. The
default, `abort`, will simply fail the request. Setting it to `caller`
will cause the request to execute on an IO thread allowing to throttle
the execution on the networking layer.

[source,js]
--------------------------------------------------
threadpool:
    index:
        type: fixed
        size: 30
        queue_size: 1000
        reject_policy: caller
--------------------------------------------------

[float]
==== `blocking`

The `blocking` pool allows to configure a `min` (defaults to `1`) and
`size` (defaults to the number of cores times 5) parameters for the
number of threads.

It also has a backlog queue with a default `queue_size` of `1000`. Once
the queue is full, it will wait for the provided `wait_time` (defaults
to `60s`) on the calling IO thread, and fail if it has not been
executed.

[source,js]
--------------------------------------------------
threadpool:
    index:
        type: blocking
        min: 1
        size: 30
        wait_time: 30s
--------------------------------------------------