From: Greg Smith <greg@2ndQuadrant.com>
To: tv@fuzzy.cz
Cc: pgsql-performance@postgresql.org
Subject: Re: strange pgbench results (as if blocked at the end)
Date: Sun, 14 Aug 2011 20:28:58 -0400
Message-ID: <4E48684A.2070909@2ndQuadrant.com> (raw)
In-Reply-To: <bdf03aa0550140de6491b3ce6d3eebd4@mail.gransy.com>
References: <2f9ba8ed6e4ed11d9a6d236cb4d2a2ec.squirrel@sq.gransy.com>
<4E45EAD3.9030102@2ndQuadrant.com>
<bdf03aa0550140de6491b3ce6d3eebd4@mail.gransy.com>
On 08/14/2011 08:51 AM, tv@fuzzy.cz wrote:
> I've increased the test duration to 10 minutes, decreased the
> checkpoint timeout to 4 minutes and a checkpoint is issued just before
> the pgbench. That way the starting position should be more or less the
> same for all runs.
>
That's basically what I settled on for pgbench-tools. Force a
checkpoint just before the test, so the beginning of each run is aligned
more consistently, then run for long enough that you're guaranteed at
least one checkpoint finishes[1] (and you might see more than one if you
fill checkpoint_segments fast enough). I never bothered trying to
compress that test cycle down by decreasing checkpoint_timeout. There's
already too many things you need to do in order to get this test working
well, and I didn't want to include a change I'd never recommend people
make on a production server in the mix.
[1] If your checkpoint behavior goes pathological, for example the
extended checkpoints possible when the background writer fsync queue
fills, it's not actually guaranteed that the checkpoint will finish
within 5 minutes after it starts. So a 10 minute run doesn't assure
you'll a checkpoint begin and end in all circumstances, but it is the
expected case.
--
Greg Smith 2ndQuadrant US greg@2ndQuadrant.com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
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To: pgsql-performance@postgresql.org
Cc: greg@2ndQuadrant.com, tv@fuzzy.cz
Subject: Re: strange pgbench results (as if blocked at the end)
In-Reply-To: <4E48684A.2070909@2ndQuadrant.com>
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