24.8.8 Backport PR #68694 Thread pool: move thread creation out of lock

[ilejn]

Original PR:
ClickHouse#68694

Changelog category (leave one):

• Performance Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Optimized thread creation in the ThreadPool to minimize lock contention. Thread creation is now performed outside of the critical section to avoid delays in job scheduling and thread management under high load conditions. This leads to a much more responsive ClickHouse under heavy concurrent load.

Documentation entry for user-facing changes

The thread creation process within the ThreadPool has been modified to address potential delays caused by holding a lock during thread creation. Previously, threads were created within a locked section, which could lead to significant contention and delays, especially when thread creation is slow. To mitigate this, the thread creation has been moved outside of the critical section.

Same test https://gist.github.com/filimonov/7e7adde17421d4a9f83c6fea2be8f802 - before and after the change - results are in comment below.

It looks like a clear win giving about 10% better QPS, much better response times and better threadpool and CPU usage, despite that thread creation become even slower - it's because now several thread can be created simultaneously and kernel side contetion is bigger. But now it does not block the thread pool work anymore.

P.S. I have no idea how to test it in CI/CD, but there is a change that performance tests will show something.

[altinity-robot]

This is an automated comment for commit d8cceb3 with description of existing statuses. It's updated for the latest CI running

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Check name	Description	Status
Integration tests	The integration tests report. In parenthesis the package type is given, and in square brackets are the optional part/total tests	❌ failure
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Sign release	There's no description for the check yet, please add it to tests/ci/ci_config.py:CHECK_DESCRIPTIONS	❌ error
Stateless tests	Runs stateless functional tests for ClickHouse binaries built in various configurations -- release, debug, with sanitizers, etc	❌ failure

Successful checks

Check name	Description	Status
Builds	There's no description for the check yet, please add it to tests/ci/ci_config.py:CHECK_DESCRIPTIONS	✅ success
Compatibility check	Checks that clickhouse binary runs on distributions with old libc versions. If it fails, ask a maintainer for help	✅ success
Docker keeper image	The check to build and optionally push the mentioned image to docker hub	✅ success
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Install packages	Checks that the built packages are installable in a clear environment	✅ success
Ready for release	There's no description for the check yet, please add it to tests/ci/ci_config.py:CHECK_DESCRIPTIONS	✅ success
Stateful tests	Runs stateful functional tests for ClickHouse binaries built in various configurations -- release, debug, with sanitizers, etc	✅ success
Stress test	Runs stateless functional tests concurrently from several clients to detect concurrency-related errors	✅ success