| Bug #93281 | Odd memory load/increase when using --performance-schema-max-digest-length=large | ||
|---|---|---|---|
| Submitted: | 22 Nov 2018 4:03 | Modified: | 30 Nov 2018 14:26 |
| Reporter: | Roel Van de Paar | Email Updates: | |
| Status: | Closed | Impact on me: | |
| Category: | MySQL Server: Performance Schema | Severity: | S1 (Critical) |
| Version: | 8.0.12 | OS: | Any |
| Assigned to: | CPU Architecture: | Any | |
[22 Nov 2018 4:03]
Roel Van de Paar
[22 Nov 2018 4:03]
Roel Van de Paar
Before starting the server
Attachment: Before.png (image/png, text), 163.05 KiB.
[22 Nov 2018 4:03]
Roel Van de Paar
After the server is started / during server startup
Attachment: After.png (image/png, text), 171.69 KiB.
[22 Nov 2018 9:38]
Marc ALFF
Hi Roel, This variable is -- per query --, not global for the server. It is the amount of memory used for each query when storing a digest. This: --performance-schema-max-digest-length=1125899906842624 makes no sense, and here the value was truncated to the MAX for this variable, which is 1048576 (1 Mib). See the manual for this variable: https://dev.mysql.com/doc/refman/8.0/en/performance-schema-system-variables.html#sysvar_pe... Still with 1 Mib per query, and with a lot of queries stored in: - each thread statement history - the global statement history long - the global statement digest table this results in a huge memory consumption. Accepting the bug as a documentation bug, because the doc is unclear, misleading users to think this setting is a global amount of memory, resulting in incorrect configuration. Regards, -- Marc Alff
[22 Nov 2018 20:32]
Roel Van de Paar
Hi Marc, Did you test it? A blank server, just init-ed, with this option, will consume a HUGE amount of memory (think 20+ GB), when started.
[30 Nov 2018 14:26]
Paul DuBois
Posted by developer: The variable is per session, not global to the server. Updated https://dev.mysql.com/doc/refman/8.0/en/performance-schema-system-variables.html#sysvar_pe... to clarify: The maximum number of bytes of memory reserved per session for computation of normalized statement digest values in the Performance Schema.
[17 Dec 2018 23:09]
Paul DuBois
Posted by developer: https://dev.mysql.com/doc/refman/8.0/en/performance-schema-system-variables.html#sysvar_pe... The maximum number of bytes of memory reserved per statement for computation of normalized statement digest values in the Performance Schema. This variable is related to max_digest_length; see the description of that variable in Section 5.1.8, “Server System Variables” https://dev.mysql.com/doc/refman/8.0/en/performance-schema-statement-digests.html#statemen... The performance_schema_max_digest_length setting is not per session, it is per statement, and a session can store multiple statements in the events_statements_history table. A typical number of statements in this table is 10 per session, so each session will consume 10 times the memory indicated by the performance_schema_max_digest_length value, for this table alone. Also, there are many statements (and digests) collected globally, most notably in the events_statements_history_long table. Here, too, N statements stored will consume N times the memory indicated by the performance_schema_max_digest_length value.
