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Re: First set of OSDL Shared Mem scalability results, some

From: Curt Sampson <cjs(at)cynic(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Kevin Brown <kevin(at)sysexperts(dot)com>,pgsql-performance(at)postgresql(dot)org
Subject: Re: First set of OSDL Shared Mem scalability results, some
Date: 2004-10-25 00:30:56
Message-ID: Pine.NEB.4.58.0410250926300.1004@angelic-vtfw.cvpn.cynic.net (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
On Sun, 24 Oct 2004, Tom Lane wrote:

> > Well, one really can't know without testing, but memory copies are
> > extremely expensive if they go outside of the cache.
>
> Sure, but what about all the copying from write queue to page?

There's a pretty big difference between few-hundred-bytes-on-write and
eight-kilobytes-with-every-read memory copy.

As for the queue allocation, again, I have no data to back this up, but
I don't think it would be as bad as BufMgrLock. Not every page will have
a write queue, and a "hot" page is only going to get one once. (If a
page has a write queue, you might as well leave it with the page after
flushing it, and get rid of it only when the page leaves memory.)

I see the OS issues related to mapping that much memory as a much bigger
potential problem.

cjs
-- 
Curt Sampson  <cjs(at)cynic(dot)net>   +81 90 7737 2974   http://www.NetBSD.org
     Make up enjoying your city life...produced by BIC CAMERA

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