From: Hannu Krosing <hannu@krosing.net>
To: Greg Smith <greg@2ndquadrant.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Peter Hussey <peter@labkey.com>
Cc: pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: Questions on query planner, join types, and work_mem
Date: Wed, 04 Aug 2010 22:51:44 +0300
Message-ID: <1280951504.3735.93.camel@hvost> (raw)
In-Reply-To: <4C59BCA8.2060603@2ndquadrant.com>
References: <AANLkTikMwVZKDYLitnsK31R=VNOuTH8NtmDHDGkBR7k8@mail.gmail.com>
<18551.1280275502@sss.pgh.pa.us>
<AANLkTimXMR0jMdw+66E4Xx=SqNV37Ne9WfkFjPFZVjbV@mail.gmail.com>
<25534.1280498632@sss.pgh.pa.us>
<AANLkTik3oY1=F_uC843c84LxBLNdHxZaWdfjxrAtyBeu@mail.gmail.com>
<1280819031.14579.38.camel@hvost>
<AANLkTik6UBt2ezVFzdCM8iaUPkra_7f-Hs_AmGZgvFhh@mail.gmail.com>
<1280944268.3735.12.camel@hvost>
<22961.1280944836@sss.pgh.pa.us>
<1280947287.3735.56.camel@hvost>
<1280948634.3735.69.camel@hvost>
<4C59BCA8.2060603@2ndquadrant.com>
On Wed, 2010-08-04 at 15:16 -0400, Greg Smith wrote:
> Hannu Krosing wrote:
> > There was ample space for keeping the indexes in linux cache (it has 1GB
> > cached currently) though the system may have decided to start writing it
> > to disk, so I suspect that most of the time was spent copying random
> > index pages back and forth between shared buffers and disk cache.
> >
>
> Low shared_buffers settings will result in the same pages more often
> being written multiple times per checkpoint,
Do you mean "written to disk", or written out from shared_buffers to
disk cache ?
> particularly index pages,
> which is less efficient than keeping in the database cache and updating
> them there. This is a slightly different issue than just the overhead
> of copying them back and forth; by keeping them in cache, you actually
> reduce writes to the OS cache.
That's what I meant. Both writes to and read from the OS cache take a
significant amount of time once you are not doing real disk I/O.
> What I do to quantify that is...well,
> the attached shows it better than I can describe; only works on 9.0 or
> later as it depends on a feature I added for this purpose there. It
> measures exactly how much buffer cache churn happened during a test, in
> this case creating a pgbench database.
>
> --
> Greg Smith 2ndQuadrant US Baltimore, MD
> PostgreSQL Training, Services and Support
> greg@2ndQuadrant.com www.2ndQuadrant.us
>
>
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To: pgsql-performance@postgresql.org
Cc: hannu@krosing.net, greg@2ndquadrant.com, tgl@sss.pgh.pa.us, robertmhaas@gmail.com, peter@labkey.com
Subject: Re: Questions on query planner, join types, and work_mem
In-Reply-To: <1280951504.3735.93.camel@hvost>
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