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Re: Problems with autovacuum

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Łukasz Jagiełło <lukasz(dot)jagiello(at)gforces(dot)pl>, Grzegorz Jaśkiewicz <gryzman(at)gmail(dot)com>, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Problems with autovacuum
Date: 2009-05-26 23:51:54
Message-ID: 20090526235154.GO32650@alvh.no-ip.org (view raw or flat)
Thread:
Lists: pgsql-performance
Tom Lane escribió:
> Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> > Tom Lane escribi:
> >> Hmm, maybe we need to improve the code too.  This example suggests that
> >> there needs to be some limit on the worker launch rate, even if there
> >> are so many databases that that means we don't meet naptime exactly.
> 
> > We already have a 100ms lower bound on the sleep time (see
> > launcher_determine_sleep()).  Maybe that needs to be increased?
> 
> Maybe.  I hesitate to suggest a GUC variable ;-)

Heh :-)

> One thought is that I don't trust the code implementing the minimum
> too much:
> 
> 	/* 100ms is the smallest time we'll allow the launcher to sleep */
> 	if (nap->tv_sec <= 0 && nap->tv_usec <= 100000)
> 	{
> 		nap->tv_sec = 0;
> 		nap->tv_usec = 100000;	/* 100 ms */
> 	}
> 
> What would happen if tv_sec is negative and tv_usec is say 500000?
> Maybe negative tv_sec is impossible here, but ...

I don't think it's possible to get negative tv_sec here currently, but
perhaps you're right that we could make this code more future-proof.

However I think there's a bigger problem here, which is that if the user
has set naptime too low, i.e. to a value lower than
number-of-databases * 100ms, we'll be running the (expensive)
rebuild_database_list function on each iteration ... maybe we oughta put
a lower bound on naptime based on the number of databases to avoid this
problem.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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