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Re: [HACKERS] 'Waiting on lock'

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: "pgsql-patches" <pgsql-patches(at)postgresql(dot)org>, "Simon Riggs" <simon(at)2ndquadrant(dot)com>
Subject: Re: [HACKERS] 'Waiting on lock'
Date: 2007-06-20 16:36:25
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-patches
Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>> How you figure that?

> Well I'm not clear exactly what's going on with the semaphores here. If it's
> possible for to be printing the messages only as a result of another backend
> unlocking the semaphore then making the PGSemaphoreUnlock conditional on
> log_lock_waits means you can't enable log_lock_waits after startup and get
> deterministic behaviour because whether you get messages will depend on which
> other backend happens to wake you up.

I don't see how you arrive at that conclusion.  The message is printed
by the backend that is waiting for (or just obtained) a lock, dependent
on its own local setting of log_lock_waits, and not dependent on who
woke it up.

BTW, I just noticed that GUC allows deadlock_timeout to be set all the
way down to zero.  This seems bad --- surely the minimum value should at
least be positive?  As CVS HEAD stands, you're likely to get a lot of
spurious/useless log messages if you have log_lock_waits = true and
deadlock_timeout = 0.  Do we care?

			regards, tom lane

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