Thread Pool

A node holds several thread pools in order to improve how threads memory consumption are managed within a node. Many of these pools also have queues associated with them, which allow pending requests to be held instead of discarded.

There are several thread pools, but the important ones include:

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:

thread_pool:
    write:
        size: 30

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

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.

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, it will abort the request.

thread_pool:
    write:
        size: 30
        queue_size: 1000

The fixed_auto_queue_size thread pool holds a fixed size of threads to handle the requests with a bounded queue for pending requests that have no threads to service them. It’s similar to the fixed threadpool, however, the queue_size automatically adjusts according to calculations based on Little’s Law. These calculations will potentially adjust the queue_size up or down by 50 every time auto_queue_frame_size operations have been completed.

The size parameter controls the number of threads.

The queue_size allows to control the initial size of the queue of pending requests that have no threads to execute them.

The min_queue_size setting controls the minimum amount the queue_size can be adjusted to.

The max_queue_size setting controls the maximum amount the queue_size can be adjusted to.

The auto_queue_frame_size setting controls the number of operations during which measurement is taken before the queue is adjusted. It should be large enough that a single operation cannot unduly bias the calculation.

The target_response_time is a time value setting that indicates the targeted average response time for tasks in the thread pool queue. If tasks are routinely above this time, the thread pool queue will be adjusted down so that tasks are rejected.

thread_pool:
    search:
        size: 30
        queue_size: 500
        min_queue_size: 10
        max_queue_size: 1000
        auto_queue_frame_size: 2000
        target_response_time: 1s

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.

thread_pool:
    warmer:
        core: 1
        max: 8
        keep_alive: 2m

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 processors setting.

processors: 2

There are a few use-cases for explicitly overriding the processors setting:

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