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Re: Why isn't SO_SNDTIMEO used?


On Wed, Mar 10, 2010 at 12:29 PM, Michael Widenius <monty@xxxxxxxxxxxx> wrote:
> Hi!
>>>>>> "Stewart" == Stewart Smith <stewart@xxxxxxxxxxxxxxxx> 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

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