Re: strange pgbench results (as if blocked at the end)

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