Re: [PERFORM] Help with tuning this query (with

> -----Original Message-----
> From: Greg Stark [mailto:gsstark(at)mit(dot)edu]
> Sent: Monday, March 07, 2005 5:15 PM
> To: Dave Held
> Cc: Greg Stark; John A Meinel; Tom Lane; Magnus Hagander; Ken
> Egervari;
> pgsql-performance(at)postgresql(dot)org; pgsql-hackers-win32(at)postgresql(dot)org
> Subject: Re: [pgsql-hackers-win32] [PERFORM] Help with tuning
> this query
> (with
>
> "Dave Held" <dave(dot)held(at)arrayservicesgrp(dot)com> writes:
>
> > > What would be really neato would be to use the rtdsc (sp?) or
> > > equivalent assembly instruction where available. Most
> > > processors provide such a thing and it would give much lower
> > > overhead and much more accurate answers.
> > >
> > > The main problem I see with this would be on multi-processor
> > > machines. (QueryPerformanceCounter does work properly on
> > > multi-processor machines, right?)
> >
> > I believe QueryPerformanceCounter() already does this.
> [...]
> Already does what?
>
> Use rtdsc?

Yes.

> 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.

> And using it via QueryPerformanceCounter would be a non-portable
> approach to using rtdsc. Much better to devise a portable
> approach that works on any architecture where something equivalent
> is available.

How do you know that QueryPerformanceCounter doesn't use RDTSC
where available, and something appropriate otherwise? I don't see
how any strategy that explicitly executes RDTSC can be called
"portable".

> Or already works on multi-processor machines? In which case, uh, ok.

According to MSDN it does work on MP systems, and they say that "it
doesn't matter which CPU gets called".

__
David B. Held
Software Engineer/Array Services Group
200 14th Ave. East, Sartell, MN 56377
320.534.3637 320.253.7800 800.752.8129