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Re: Autovacuum cancellation

From: Gregory Stark <stark(at)enterprisedb(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Alvaro Herrera" <alvherre(at)commandprompt(dot)com>, <pgsql-patches(at)postgresql(dot)org>
Subject: Re: Autovacuum cancellation
Date: 2007-10-25 22:28:39
Message-ID: 87abq696co.fsf@oxford.xeocode.com (view raw or flat)
Thread:
Lists: pgsql-committerspgsql-hackerspgsql-patches
"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> There's some things still to be desired here: if an autovac process is
> involved in a hard deadlock, the patch doesn't favor zapping it over
> anybody else, nor consider cancelling the autovac as an alternative to
> rearranging queues for a soft deadlock.  But dealing with that will open
> cans of worms that I don't think we want to open for 8.3.

Can autovacuum actually get into a hard deadlock? Does it take more than one
lock that can block others at the same time?

I think there's a window where the process waiting directly on autovacuum
could have already fired its deadlock check before it was waiting directly on
autovacuum. But the only way I can see it happening is if another process is
cancelled before its deadlock check fires and the signals are processed out of
order. I'm not sure that's a case we really need to worry about.

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com

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