| From: | "Steinar H(dot) Gunderson" <sgunderson(at)bigfoot(dot)com> |
|---|---|
| To: | Dave Held <dave(dot)held(at)arrayservicesgrp(dot)com> |
| Cc: | Greg Stark <gsstark(at)mit(dot)edu>, John A Meinel <john(at)arbash-meinel(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Magnus Hagander <mha(at)sollentuna(dot)net>, Ken Egervari <ken(at)upfactor(dot)com>, pgsql-performance(at)postgresql(dot)org, pgsql-hackers-win32(at)postgresql(dot)org |
| Subject: | Re: [PERFORM] Help with tuning this query (with |
| Date: | 2005-03-08 01:37:50 |
| Message-ID: | 20050308013750.GA29367@uio.no |
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| Lists: | pgsql-hackers-win32 pgsql-performance |
On Mon, Mar 07, 2005 at 06:11:34PM -0600, Dave Held wrote:
>> In which case using it would be a mistake. Since rtdsc doesn't
>> work across processors.
> It doesn't always use RDTSC. I can't find anything authoritative on
> when it does. I would assume that it would use RDTSC when available
> and something else otherwise.
RDTSC is a bad source of information for this kind of thing, as the CPU
frequency might vary. Check your QueryPerformanceFrequency() -- most likely
it will not match your clock speed. I haven't tested on a lot of machines,
but I've never seen QueryPerformanceFrequency() ever match the clock speed,
which it most probably would if it was using RDTSC. (I've been told it uses
some other kind of timer available on most motherboards, but I don't know the
details.)
/* Steinar */
--
Homepage: http://www.sesse.net/
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