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From: Gregory Stark <stark@enterprisedb.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Greg Smith <gsmith@gregsmith.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: ITAGAKI Takahiro <itagaki.takahiro@oss.ntt.co.jp>
Cc: Postgres <pgsql-hackers@postgresql.org>
Subject: Re: posix_fadvise v22
Date: Fri, 02 Jan 2009 23:15:10 -0500
Message-ID: <87mye9t48h.fsf@oxford.xeocode.com> (raw)
In-Reply-To: <14488.1230943899@sss.pgh.pa.us>
References: <FDDBA24E-FF4D-4654-BA75-692B3BA71B97@enterprisedb.com>
	<8347.1229012970@sss.pgh.pa.us>
	<4136ffa0812110911x4914a951o6ca502879e3dede7@mail.gmail.com>
	<87k5a6tmy7.fsf@oxford.xeocode.com>
	<603c8f070901011230t1afa6644racb715b975078054@mail.gmail.com>
	<10658.1230843315@sss.pgh.pa.us>
	<603c8f070901011843p121e40afu35c4a7bdc51c051c@mail.gmail.com>
	<Pine.GSO.4.64.0901020335150.16680@westnet.com>
	<24481.1230910405@sss.pgh.pa.us>
	<Pine.GSO.4.64.0901021416310.15834@westnet.com>
	<7537.1230926505@sss.pgh.pa.us>
	<878wptv3u6.fsf@oxford.xeocode.com>
	<14488.1230943899@sss.pgh.pa.us>


Tom Lane <tgl@sss.pgh.pa.us> writes:

> The point of the suggestion is to prove that the patch works as
> advertised.  How wide the sweet spot is for this test isn't nearly as
> interesting as proving that there *is* a sweet spot.  If you can't
> find one it suggests that either the patch or the local posix_fadvise
> doesn't work.

I posted tons of reproducible test cases with graphs of results for various
raid stripe widths a while back. There was a very slight benefit on a single
spindle at some prefetch depths but it wasn't very consistent and it varied
heavily depending on the prefetch depth.

I don't know what to make of this test. I don't know how to reproduce the same
data distribution, I have no idea what raid configuration it's been run on,
what version of what OS it's on, etc. It's quite possible posix_fadvise isn't
working on it, I don't know.

It's also possible the overhead of the extra buffer lookups and syscalls
outweight any benefit of overlapping i/o and cpu on a single spindle.

Trying to contrive a situation where a single spindle sees a significant
benefit is going to be very tricky. Avoiding caching effects and the
confounding effect of any overhead will make it hard to see a consistent
benefit without a raid array.

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's 24x7 Postgres support!



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