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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: 2009-01-02 20:40:49
Message-ID: 878wptv3u6.fsf@oxford.xeocode.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> In principle you should be able to adjust the constant so that vmstat
> shows about 50% CPU busy, and then enabling fadvise should improve
> matters significantly.

I think in practice individual queries don't interleave much cpu with i/o
work. A single random page fetch is 5ms which is an awful lot of cpu cycles to
be sinking somewhere. In practice I think this is going to be single-digit
percentages.

Aside from big sorts and such which tend not to be interleaved with i/o the
main time I've seen queries have a significant cpu load is when the data is
mostly in cache. In which case prefetching would be hard pressed to help at
all.

We could construct a synthetic case but the main point of this feature is to
make use of raid arrays that are currently going idle, not to pick up a few
percentage points for single spindle systems.

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's On-Demand Production Tuning

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Next:From: Bruce MomjianDate: 2009-01-02 20:43:41
Subject: Re: posix_fadvise v22
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Subject: Re: Significantly larger toast tables on 8.4?

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