Bug #76509 use atomics to avoid MUTEX CONTENTION ON LOCK_PLUGIN DUE TO QUERY REWRITE FRAMEW

• Submitted: 27 Mar 2015 14:45
• Modified: 30 Mar 2015 13:18
• Reporter: Tor Didriksen
• Status: Closed
• Category: MySQL Server: Parser
• Severity: S3 (Non-critical)
• Version: 5.7
• OS: Any
• CPU Architecture: Any

[27 Mar 2015 14:45] Tor Didriksen

Description:
The query rewrite framework needs to examine the list of installed plugins
three times during parsing:

- To know if a plugin needs a statement digest.
- To call pre-parse rewrite plugins.
- To call post-parse rewrite plugins.

All of these calls are needed even if there is no such plugin installed.
Performance degradation happens because each thread needs to take a global
mutex on the plugin list.

How to repeat:
Run sysbench with many threads, and --db-ps-mode=disable option

Suggested fix:
Use atomic counters when loading/unloading pre/post parse plugins.
Check counters in mysql_parse.

See also Bug#20784914

[30 Mar 2015 13:18] Paul DuBois

Noted in 5.7.7, 5.8.0 changelogs.

The query rewrite framework introduced in MySQL 5.7.6 produced
excesssive mutex acquisition that caused performance degradation under
some conditions.