Re: pg_stat_activity.waiting_start

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Stephen Frost <sfrost(at)snowman(dot)net>
Cc: Joel Jacobson <joel(at)trustly(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: pg_stat_activity.waiting_start
Date: 2016-12-24 02:16:13
Message-ID: 9647.1482545773@sss.pgh.pa.us
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Stephen Frost <sfrost(at)snowman(dot)net> writes:
> * Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
>> The difficulty with that is it'd require a gettimeofday() call for
>> every wait start. Even on platforms where those are relatively cheap,
>> the overhead would be nasty --- and on some platforms, it'd be
>> astonishingly bad. We sweated quite a lot to get the overhead of
>> pg_stat_activity wait monitoring down to the point where it would be
>> tolerable for non-heavyweight locks, but I'm afraid this would push
>> it back into the not-tolerable range.

> Could we handle this like log_lock_waits..?

Well, that only applies to heavyweight locks, which do a gettimeofday
anyway in order to schedule the deadlock-check timeout. If you were
willing to populate this new column only for heavyweight locks, maybe it
could be done for minimal overhead. But that would be backsliding
quite a lot compared to what we just did to extend pg_stat_activity's
coverage of lock types.

regards, tom lane

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