Re: Cost of XLogInsert CRC calculations

> -----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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