mutex contention for the query cache

[MARK CALLAGHAN]

The query cache has a mutex that is locked while it is searched. This
is not a spin lock, so many threads will go to sleep when there is
contention. And it is made worse because work is done to create the
search key in Query_cache::send_result_to_client() (work == memory
allocation and other byte copying) after the mutex is locked.

Are there plans to fix this?
I don't have benchmark results (yet), but I am willing to bet that
this is a problem.

-- 
Mark Callaghan
mdcallag@stripped