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Re: Impact of checkpoint_segments under continual load conditions

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Christopher Petrilli <petrilli(at)gmail(dot)com>
Cc: Vivek Khera <vivek(at)khera(dot)org>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Impact of checkpoint_segments under continual load conditions
Date: 2005-07-18 19:32:36
Message-ID: 16394.1121715156@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Christopher Petrilli <petrilli(at)gmail(dot)com> writes:
> http://blog.amber.org/diagrams/comparison_mysql_pgsql.png

> Notice the VERY steep drop off.

Hmm.  Whatever that is, it's not checkpoint's fault.  I would interpret
the regular upticks in the Postgres times (every several hundred
iterations) as being the effects of checkpoints.  You could probably
smooth out those peaks some with appropriate hacking on bgwriter
parameters, but that's not the issue at hand (is it?).

I have no idea at all what's causing the sudden falloff in performance
after about 10000 iterations.  COPY per se ought to be about a
constant-time operation, since APPEND is (or should be) constant-time.
What indexes, foreign keys, etc do you have on this table?  What else
was going on at the time?

			regards, tom lane

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Subject: Re: Impact of checkpoint_segments under continual load conditions
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