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Re: posix_fadvise v22

From: Gregory Stark <stark(at)enterprisedb(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Greg Smith <gsmith(at)gregsmith(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, Postgres <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: posix_fadvise v22
Date: 2009-01-03 04:15:10
Message-ID: 87mye9t48h.fsf@oxford.xeocode.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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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