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You also have 4096 (4K) as the minimum size result set.Unfortunately, mysql has no internal mechanism for defragmenting the query cache.From the perspective of your question, I would say that the justification of removing the query cache is not so much the overhead, but how Inno DB manages it.For more information on how Inno DB interacts with the query cache, please read pages 213-215 of the book "High Performance My SQL (Second Edition)".The only way to not acquire the query cache mutex in My SQL pre-5.5 is to compile My SQL without query cache. For My SQL 5.5 , to completely disable the query cache (thus, not acquiring the query cache mutex) is required to set query_cache_type=OFF at startup (not at runtime).
Oh, and the DB in question has a mix of INNODB and My ISAM.See my earlier posts on using My SQL 5.5 in conjunction with accessing multiple CPUs for Inno DB would be better.For clarification, when I said "no internal mechanism," I meant exactly that.Is the overhead of frequent invalidation ever worth it? The Query Cache will always butt heads with Inno DB.Edit: At the request of the user @Rolando My SQLDBA below, here's the info on My ISAM and INNODB. It would be nice if Inno DB's MVCC would let queries be served from the query cache if modifications do not affect repeatable reads for other transactions. Apparently, you have a lot of queries that get invalidated rather quickly and are probably not being reused.