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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 15:40:49 -0500
Message-ID: <878wptv3u6.fsf@oxford.xeocode.com> (raw)
In-Reply-To: <7537.1230926505@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>

Tom Lane <tgl@sss.pgh.pa.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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