Subject: Re: dbt-2 tuning results with postgresql-8.3.5
From: Simon Riggs <simon@2ndQuadrant.com>
To: Mark Wong <markwkm@gmail.com>
Cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>,
	Selena Deckelmann <selenamarie@gmail.com>,
	Gabrielle Roth <gorthx@gmail.com>
In-Reply-To: <70c01d1d0812201654i5f55150ep619a6c7ff094470@mail.gmail.com>
References: <70c01d1d0812201654i5f55150ep619a6c7ff094470@mail.gmail.com>
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Date: Wed, 24 Dec 2008 14:14:06 +0000
Message-Id: <1230128046.4793.1145.camel@ebony.2ndQuadrant>
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Hi Mark,

Good to see you producing results again.


On Sat, 2008-12-20 at 16:54 -0800, Mark Wong wrote:
> Here are links to how the throughput changes when increasing shared_buffers:
> 
> http://pugs.postgresql.org/node/505

Only starnge thing here is the result at 22528MB. It's the only normal
one there. Seems to be a freeze occurring on most tests around the 30
minute mark, which delays many backends and reduces writes. 

Reduction in performance as shared_buffers increases looks normal.

Increase wal_buffers, but look for something else as well. Try to get a
backtrace from when the lock up happens. It may not be Postgres?

> And another series of tests to show how throughput changes when
> checkpoint_segments are increased:
> 
> http://pugs.postgresql.org/node/503
> 
> The links go to a graphical summary and raw data.  Note that the
> maximum theoretical throughput at this scale factor is approximately
> 12000 notpm.
> 
> My first glance takes tells me that the system performance is quite
> erratic when increasing the shared_buffers.  I'm also not what to
> gather from increasing the checkpoint_segments.  Is it simply that the
> more checkpoint segments you have, the more time the database spends
> fsyncing when at a checkpoint?

I would ignore the checkpoint_segment tests because you aren't using a
realistic value of shared_buffers. I doubt any such effect is noticeable
when you use a realistic value determined from set of tests 505.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support