Task queue backlog

A backlogged task queue can prevent tasks from completing and put the cluster into an unhealthy state. Resource constraints, a large number of tasks being triggered at once, and long running tasks can all contribute to a backlogged task queue.

Check the thread pool status

A depleted thread pool can result in rejected requests.

Thread pool depletion might be restricted to a specific data tier. If hot spotting is occuring, one node might experience depletion faster than other nodes, leading to performance issues and a growing task backlog.

You can use the cat thread pool API to see the number of active threads in each thread pool and how many tasks are queued, how many have been rejected, and how many have completed.

resp = client.cat.thread_pool(
    v=True,
    s="t,n",
    h="type,name,node_name,active,queue,rejected,completed",
)
print(resp)

response = client.cat.thread_pool(
  v: true,
  s: 't,n',
  h: 'type,name,node_name,active,queue,rejected,completed'
)
puts response

const response = await client.cat.threadPool({
  v: "true",
  s: "t,n",
  h: "type,name,node_name,active,queue,rejected,completed",
});
console.log(response);

GET /_cat/thread_pool?v&s=t,n&h=type,name,node_name,active,queue,rejected,completed

The active and queue statistics are instantaneous while the rejected and completed statistics are cumulative from node startup.

Inspect the hot threads on each node

If a particular thread pool queue is backed up, you can periodically poll the Nodes hot threads API to determine if the thread has sufficient resources to progress and gauge how quickly it is progressing.

resp = client.nodes.hot_threads()
print(resp)

response = client.nodes.hot_threads
puts response

const response = await client.nodes.hotThreads();
console.log(response);

GET /_nodes/hot_threads

Look for long running node tasks

Long-running tasks can also cause a backlog. You can use the task management API to get information about the node tasks that are running. Check the running_time_in_nanos to identify tasks that are taking an excessive amount of time to complete.

GET /_tasks?pretty=true&human=true&detailed=true

If a particular action is suspected, you can filter the tasks further. The most common long-running tasks are bulk index- or search-related.

The API response may contain additional tasks columns, including description and header, which provides the task parameters, target, and requestor. You can use this information to perform further diagnosis.

Look for long running cluster tasks

A task backlog might also appear as a delay in synchronizing the cluster state. You can use the cluster pending tasks API to get information about the pending cluster state sync tasks that are running.

GET /_cluster/pending_tasks

Check the timeInQueue to identify tasks that are taking an excessive amount of time to complete.

Increase available resources

If tasks are progressing slowly and the queue is backing up, you might need to take steps to Reduce CPU usage.

In some cases, increasing the thread pool size might help. For example, the force_merge thread pool defaults to a single thread. Increasing the size to 2 might help reduce a backlog of force merge requests.

Cancel stuck tasks

If you find the active task’s hot thread isn’t progressing and there’s a backlog, consider canceling the task.