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Re: Why isn't SO_SNDTIMEO used?



>>>>> "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?

As a separate note, I agree it would make perfect sense to use
SO_SNDTIMEO on platforms that supports it, except for one little

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?


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