Increase force_merge threadpool size

[pxsalehi]

Changes the default size used for the force_merge threadpool to 1/8 of
the allocated processors, with a minimum value of 1.

Closes #84943

[elasticsearchmachine]

Hi @pxsalehi, I've created a changelog YAML for you.

[pxsalehi]

@elasticsearchmachine update branch

[pxsalehi]

@elasticmachine update branch

[pxsalehi]

The idea was to also include an integration(-like) test that would pass the number of allocated processors (rather than being limited to what the Runtime returns) to the ThreadPool, and verify the created force_merge threadpool size.

I spent some time on this, using a package private constructor to pass allocatedProcessors in the test. This would work for the force_merge threadpool, but it seems that allocatedProcessors is used in other places as well to validate the Settings that gets passed to ThreadPool, e.g., in the ExecutorBuilder. This would then make the test fail complaining about the value passed for the write threadpool, for example.

So it seems to me that there needs to be more changes, to make that kind of test possible. I'm not sure, if it is worth it or not. @henningandersen WDYT?

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[pxsalehi]

@elasticmachine run elasticsearch-ci/part-1

[henningandersen]

Docs preview

[henningandersen]

So it seems to me that there needs to be more changes, to make that kind of test possible. I'm not sure, if it is worth it or not. @henningandersen WDYT?

I think we should shortcut that then but at least validate that within the bounds of [1, Runtime.availableProcessors()], we do get the right thread pool size. You can pick a random probe point within the range.

[pxsalehi]

So it seems to me that there needs to be more changes, to make that kind of test possible. I'm not sure, if it is worth it or not. @henningandersen WDYT?

I think we should shortcut that then but at least validate that within the bounds of [1, Runtime.availableProcessors()], we do get the right thread pool size. You can pick a random probe point within the range.

Done. Please check 2aa5501.

[henningandersen]

LGTM with two small changes (provided CI passes). Thanks Pooya.

[pxsalehi]

Thanks, Henning!