Improve thread pools #18613
Description
This issue is a meta-issue for collapsing several sub-issues for improving thread pools in core Elasticsearch:
- register all thread pool settings Register thread pool settings #18674
- provide hook for plugins to register a thread pool Register thread pool settings #18674
- thread pool settings should be node-level settings Register thread pool settings #18674
- remove unbounded queues Remove unbounded queues from thread pool executors #18491
- place a hard bound of 256 * max number of threads in pool for a bound on the queue Limit the maximum queue_size for fixed thread pools? #14448
- reduce the default maximum size of the generic thread pool
- bound the size of thread pools (e.g., as a function of the number of available processors)
The proposal here is that core Elasticsearch will no longer provide support for custom executors. Of course, a plugin can always create an executor on its own and so core Elasticsearch will provide an extension point for such plugins to provide stats back to core Elasticsearch for reading in node stats API and cat thread pools API.
Relates #17915, relates #18491, relates #14448, relates #15866