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Re: LISTEN / NOTIFY performance in 8.3

From: Joel Stevenson <joelstevenson(at)mac(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: LISTEN / NOTIFY performance in 8.3
Date: 2008-02-25 19:15:44
Message-ID: p06240810c3e8c142182c@[] (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
At 1:13 PM -0500 2/25/08, Tom Lane wrote:
>Joel Stevenson <joelstevenson(at)mac(dot)com> writes:
>>>  Also, it might be worth enabling log_lock_waits to see if the slow
>>>  notifies are due to having to wait on some lock or other.
>>  Turning on log_lock_waits shows that there is a lot of waiting for
>>  locks on the pg_listener table ala:
>Interesting.  The LISTEN/NOTIFY mechanism itself takes ExclusiveLock
>on pg_listener, but never for very long at a time (assuming pg_listener
>doesn't get horribly bloated, which we know isn't happening for you).
>Another thought that comes to mind is that maybe the delays you see
>come from these lock acquisitions getting blocked behind autovacuums of
>pg_listener.  I did not see that while trying to replicate your problem,
>but maybe the issue requires more update load on pg_listener than the
>test script can create by itself, or maybe some nondefault autovacuum
>setting is needed --- what are you using?

Default autovacuum settings.

I turned on all autovacuum logging and cranked up the test script and 
have it fork 25 consumers each running 25 iterations.  At that level 
on my machine I can get the lock waiting to exceed the 1s 
deadlock_timeout right away but the autovacuum activity on 
pg_listener is entirely absent until the end when the forked 
consumers are mostly done and disconnected.

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