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Re: Cost of XLogInsert CRC calculations

From: "Mark Cave-Ayland" <m(dot)cave-ayland(at)webbased(dot)co(dot)uk>
To: "'Tom Lane'" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "'Manfred Koizar'" <mkoi-pg(at)aon(dot)at>,"'Greg Stark'" <gsstark(at)mit(dot)edu>,"'Bruce Momjian'" <pgman(at)candle(dot)pha(dot)pa(dot)us>,<pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Cost of XLogInsert CRC calculations
Date: 2005-05-27 15:01:00
Message-ID: 9EB50F1A91413F4FA63019487FCD251D11336B@WEBBASEDDC.webbasedltd.local (view raw or flat)
Thread:
Lists: pgsql-hackers
> -----Original Message-----
> From: Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us]
> Sent: 27 May 2005 15:00
> To: Mark Cave-Ayland (External)
> Cc: 'Manfred Koizar'; 'Greg Stark'; 'Bruce Momjian';
> pgsql-hackers(at)postgresql(dot)org
> Subject: Re: [HACKERS] Cost of XLogInsert CRC calculations

(cut)

> Not sure I believe these numbers.  Shouldn't 2x32 be about twice as 
> slow as just one 32-bit CRC?

Also I've just quickly tested on the Xeon Linux FC1 box I used with my
original program using Manfred's program and the margin is even closer:

Opt	32     32a    32b    2x32   64     64a    64b
------------------------------------------------------
O1    2.75   2.81   2.71   3.16   3.53   3.64   7.25
O2    2.75   2.78   2.87   2.94   7.63   10.61  11.93
O3    2.84   2.85   3.03   2.99   7.63   7.64   7.71

I don't know whether gcc is just producing an inefficient CRC32 compared to
2x32 but the results seem very odd.... There must be something else we are
missing?


Kind regards,

Mark.

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