Re: Autovacuum cancellation

On Fri, 2007-10-26 at 10:32 +0100, Heikki Linnakangas wrote:
> Alvaro Herrera wrote:
> > /*
> >  * Look for a blocking autovacuum. There will only ever
> >  * be one, since the autovacuum workers are careful
> >  * not to operate concurrently on the same table.
> >  */
>
> I think that's a bit unaccurate. You could have multiple autovacuum
> workers operating on different tables participating in a deadlock. The
> reason that can't happen is that autovacuum never holds a lock while
> waiting for another.

I wrote that code comment; as you say it is true only when there are at
least 4 processes in the lock graph where 2+ normal backends are
deadlocking and there are 2+ autovacuums holding existing locks. The
comment should have said "If blocking is caused by an autovacuum process
then ... (there will)".

The blocking_autovacuum_proc doesn't react unless there are no hard
deadlocks, so the code works.

--
  Simon Riggs
  2ndQuadrant  http://www.2ndQuadrant.com