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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: "'Manfred Koizar'" <mkoi-pg(at)aon(dot)at>
Cc: "'Tom Lane'" <tgl(at)sss(dot)pgh(dot)pa(dot)us>,"'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 13:49:47
Message-ID: 9EB50F1A91413F4FA63019487FCD251D113368@WEBBASEDDC.webbasedltd.local (view raw or flat)
Thread:
Lists: pgsql-hackers
> -----Original Message-----
> From: Manfred Koizar [mailto:mkoi-pg(at)aon(dot)at] 
> Sent: 25 May 2005 20:25
> To: Manfred Koizar
> Cc: Tom Lane; Greg Stark; Bruce Momjian; Mark Cave-Ayland 
> (External); pgsql-hackers(at)postgresql(dot)org
> Subject: Re: [HACKERS] Cost of XLogInsert CRC calculations

(cut)

> The most important figure is, that at MaxSpeed (/O2) 2x32 is 
> almost twice as fast as CRC64 while only being marginally 
> slower than CRC32.
> 
> Servus
>  Manfred


Hi Manfred,

Sorry about taking a while to respond on this one - the hard drive on my
laptop crashed :(. I repeated your tests on my P4 laptop with gcc 3.2.3 and
reproduced the results below:


Opt	32     32a    32b    2x32   64     64a      64b
--------------------------------------------------------
O1	4.91   4.86   5.43   6.00   11.4   11.39    11.39
O2	4.96   4.94   4.69   5.18   15.86  18.75    24.73
O3	4.82   4.83   4.64   5.18   15.14  13.77    14.73

                           ^^^^^^^^^^^^

So in summary I would say:

	- Calculating a CRC64 using 2 x 32 int can be 3 times as fast as
using 1 x 64 int on
	my 32-bit Intel laptop with gcc.
	
	- The time difference between CRC32 and CRC64 is about 0.5s in the
worse case
	shown during testing, so staying with CRC64 would not inflict too
great a penalty.


Kind regards,

Mark.

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