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From: Greg Smith <gsmith@gregsmith.com>
To: Matthew Wakeling <matthew@flymine.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: I/O on select count(*)
Date: Mon, 19 May 2008 11:47:38 -0400 (EDT)
Message-ID: <Pine.GSO.4.64.0805191137480.25115@westnet.com> (raw)
In-Reply-To: <Pine.LNX.4.64.0805191257020.16756@aragorn.flymine.org>
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	<Pine.LNX.4.64.0805191257020.16756@aragorn.flymine.org>

On Mon, 19 May 2008, Matthew Wakeling wrote:

> Does it really take that long to zero out 8kB of RAM? I thought CPUs were 
> really quick at doing that!

You don't get the whole CPU--you get time slices of one.  Some of the 
cases complaints have come in about have over a thousand connections all 
fighting for CPU time, and making every one of them block for one guy who 
needs to fiddle with memory for a while can be a problem.  If you're 
unlucky you won't even be on the same CPU you started on each time you get 
a little check of time, and you'll run at the speed of RAM rather than 
that of the CPU--again, fighting for RAM access with every other process 
on the server.

The real question in my mind is why this turns into a bottleneck before 
the similar task of cleaning the 16MB XLOG segment does.  I expected that 
one would need to be cracked before the CLOG switch time could possibly be 
an issue, but reports from the field seem to suggest otherwise.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD



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