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65 lines
No EOL
2 KiB
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[[task-queue-backlog]]
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=== Task queue backlog
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A backlogged task queue can prevent tasks from completing and
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put the cluster into an unhealthy state.
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Resource constraints, a large number of tasks being triggered at once,
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and long running tasks can all contribute to a backlogged task queue.
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[discrete]
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[[diagnose-task-queue-backlog]]
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==== Diagnose a task queue backlog
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**Check the thread pool status**
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A <<high-cpu-usage,depleted thread pool>> can result in <<rejected-requests,rejected requests>>.
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You can use the <<cat-thread-pool,cat thread pool API>> to
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see the number of active threads in each thread pool and
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how many tasks are queued, how many have been rejected, and how many have completed.
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[source,console]
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----
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GET /_cat/thread_pool?v&s=t,n&h=type,name,node_name,active,queue,rejected,completed
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----
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**Inspect the hot threads on each node**
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If a particular thread pool queue is backed up,
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you can periodically poll the <<cluster-nodes-hot-threads,Nodes hot threads>> API
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to determine if the thread has sufficient
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resources to progress and gauge how quickly it is progressing.
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[source,console]
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----
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GET /_nodes/hot_threads
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----
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**Look for long running tasks**
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Long-running tasks can also cause a backlog.
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You can use the <<tasks,task management>> API to get information about the tasks that are running.
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Check the `running_time_in_nanos` to identify tasks that are taking an excessive amount of time to complete.
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[source,console]
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----
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GET /_tasks?filter_path=nodes.*.tasks
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----
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[discrete]
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[[resolve-task-queue-backlog]]
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==== Resolve a task queue backlog
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**Increase available resources**
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If tasks are progressing slowly and the queue is backing up,
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you might need to take steps to <<reduce-cpu-usage>>.
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In some cases, increasing the thread pool size might help.
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For example, the `force_merge` thread pool defaults to a single thread.
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Increasing the size to 2 might help reduce a backlog of force merge requests.
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**Cancel stuck tasks**
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If you find the active task's hot thread isn't progressing and there's a backlog,
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consider canceling the task. |