Invalidating query cache mysql

The current code actually locks/unlocks the query_cache lock twice when the query_cache_size is zero! This can all be bypassed if the check for query_cache_size is done without holding the QC lock as in Query_cache::store_query The CPU time wasted is very less, however, in benchmark situations, every cpu cycle counts.How to repeat: Run a sysbench read-write benchmark with lots of threads.Therefore the data usually is not handled in one big chunk.The query cache allocates blocks for storing this data on demand, so when one block is filled, a new block is allocated.By using the command line option --query_cache_type=0, the user can disable the query cache permanently and avoid taking the query cache mutex. After review, it may be pushed to the relevant source trees for release in the next version.You can access the patch from: 2742 Kristofer Pettersson 2008-10-27 Bug#38551 query cache can still consume [very little] cpu time even when it is off.Description: Since the Query cache hurts some workloads, we turn if off by setting query_cache_size=0.However, the Query Cache still does some work before finding out that query_cache_size is 0 and returning.

Currently nobody wants: 1) a blocking mutex 2) a client barrier on a write.

Because memory allocation operation is costly (timewise), the query cache allocates blocks with a minimum size given by the system variable.

When a query is executed, the last result block is trimmed to the actual data size so that unused memory is freed.

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When using the Windows Configuration Wizard, the query cache may be enabled (that is, set to a nonzero value) due to the selected configuration.

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