Bug #60074 Query cache too big
Submitted: 10 Feb 2011 4:43 Modified: 5 Jul 2011 17:12
Reporter: Jeremy Bell Email Updates:
Status: Closed Impact on me:
None 
Category:MySQL Server: Documentation Severity:S5 (Performance)
Version:5.5 OS:Any
Assigned to: Paul DuBois CPU Architecture:Any
Tags: query cache, size, too big

[10 Feb 2011 4:43] Jeremy Bell 
Description:
The documentation on the variable "query_cache_size", or the query cache in general doesn't warn that a cache size too big can slow things down. With a cache approaching 1.1G of a total allowed 2G we saw serious lock contention issues with many processes in "Waiting for query cache lock" causing high CPU load on our master.

How to repeat:
N/A

Suggested fix:
Add a small warning to the "Query_cache_size" variable documentation along the lines of "Be careful not to set the size of the cache too big. Due to the need for threads to lock the cache during updates, you may see lock contention issues with a very large cache".
[10 Feb 2011 7:28] Valeriy Kravchuk 
I agree that http://dev.mysql.com/doc/refman/5.5/en/query-cache-configuration.html should have some notes about the influence of too big query cache size.
[5 Jul 2011 17:12] Paul DuBois 
Thank you for your bug report. This issue has been addressed in the documentation. The updated documentation will appear on our website shortly, and will be included in the next release of the relevant products.

Added the suggested text to the configuration section.