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Re: I/O on select count(*)

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Matthew Wakeling <matthew(at)flymine(dot)org>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: I/O on select count(*)
Date: 2008-05-19 14:54:06
Message-ID: 200805191454.m4JEs6400890@momjian.us (view raw or flat)
Thread:
Lists: pgsql-performance
Matthew Wakeling wrote:
> On Fri, 16 May 2008, Tom Lane wrote:
> > One additional point: this means that one transaction in every 32K
> > writing transactions *does* have to do extra work when it assigns itself
> > an XID, namely create and zero out the next page of pg_clog.  And that
> > doesn't just slow down the transaction in question, but the next few
> > guys that would like an XID but arrive on the scene while the
> > zeroing-out is still in progress.
> >
> > This probably contributes to the behavior that Simon and Josh regularly
> > complain about, that our transaction execution time is subject to
> > unpredictable spikes.  I'm not sure how to get rid of it though.
> 
> Does it really take that long to zero out 8kB of RAM? I thought CPUs were 
> really quick at doing that!

Yea, that was my assumption too.

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +

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