Increased query_cache_size, Queries slowed drastically on increased traffic I increased our servers query_cache_size to 2gb, hoping for performance benefits. After sometime the query performance would drastically decrease. Once I dug in deeper, I found the queries in State "Waiting for query cache lock ". Lowering the query cache to 512M brought the performance back to normal. Why would this happen, and why would lowering it increase performance ?
That happens because all requests to the QC are serialized. The larger QC is the more time it takes to find and invalidate entries.