On Mon, May 08, 2006 at 06:43:31PM +0800, David Xu wrote: > On Monday 08 May 2006 14:52, Kris Kennaway wrote: > > OK, David's patch fixes the umtx thundering herd (and seems to give a > > 4-6% boost). I also fixed a thundering herd in FILEDESC_UNLOCK (which > > was also waking up 2-7 CPUs at once about 30% of the time) by doing > > s/wakeup/wakeup_one/. This did not seem to give a performance impact > > on this test though. > >.... > > filedesc contention is down by a factor of 3-4, with corresponding > > reduction in the average hold time. The process lock contention > > coming from the signal delivery wakeup has also gone way down for some > > reason. > > > > I found that mysqld frequently calls alarm() in its file thr_alarm.c and > thr_kill() to send SIGALRM to its timer thread to wake it up, the timer > thread itself is being blocked in sigwait(), normally the alarm timer will > be expired in a second, so the kernel will periodically call psignal to find > a thread which can handle the signal, it means kernel has to periodically > walk through thread list with process lock and scheduler held, this is > very expensive. > > thr_kill will in most time wake up the timer thread earlier, in thr_kill > syscall, kernel has to walk through thread list to find a thread whose > thread is matching the given id, the function thread_find() > uses a linear searching algorithm, it is slow, if there are lots of thread in > the process, the process lock will be holden too long, I think that's the > reason why you have seen so many process lock contention, if you > define USE_ALARM_THREAD in mysql header file, the contention should > be decreased ( I hope ), patch: Hmm, with this patch mysql 4.1 seems to crash at startup. I haven't yet had time to investigate. Is anyone else seeing this? Kris
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