On Wed, Mar 10, 2010 at 12:29 PM, Michael Widenius <mo...@askmonty.org> wrote: > > Hi! > >>>>>> "Stewart" == Stewart Smith <stew...@flamingspork.com> writes: > > Stewart> On Fri, Aug 14, 2009 at 10:28:35AM -0700, MARK CALLAGHAN wrote: >>> I describe how read/write is done for sockets in mysqld. >>> http://www.facebook.com/note.php?note_id=122655300932 >>> >>> Can someone explain the history of this code? > > A comment about the article: > > The common path for process_alarm is described wrongly > > - By default on Linux, USE_ONE_SIGNAL_HAND is set, which means > that there is no pthread_sigmask() done for process_alarm() > > We also don't loop over all alarms, when we get an alarm; We only loop > over the alarms that have timed out (it's a priority queue), so we > find these very fast > > The alarm code is written with the assumption that there is very few > alarms, in which case the LOCK_alarm mutex is mostly hold a very short > time and process_alarm() is very seldom called. There should also be > very few re-scheduling of alarms. > > While looking at the code, I did however find one hot spot when there > is a lot of threads that could explain the contention for the mutex > that you are talking of: > > Removing the alarm is done with a full loop over all alarms. > > It should not be too hard to use the priority queue to quickly find the > alarm without having to go trough more than a fraction of the alarms. > By doing this, you could get a major speedup in the case of many > threads as the LOCK_alarm mutex is hold over a much shorter time. > > Mark, when you see the contention for the LOCK_alarm mutex, how many > threads have you been running?
I have to revisit this. I will reply again when I have data to share. > > As a separate note, I agree it would make perfect sense to use > SO_SNDTIMEO on platforms that supports it, except for one little > problem: > > We also use the thr_alarm() functionality when one uses 'kill > connection-id' in MySQL. I don't know of any easy way to gracefully > wake up a thread that is sleeping on SO_SNDTIMEO. Do you? > No, although I haven't looked and I don't do much network systems programming. -- Mark Callaghan mdcal...@gmail.com _______________________________________________ Mailing list: https://launchpad.net/~maria-discuss Post to : maria-discuss@lists.launchpad.net Unsubscribe : https://launchpad.net/~maria-discuss More help : https://help.launchpad.net/ListHelp
