Date: Wed, 17 Mar 1999 08:07:07 +0100 (MET)
From: Michael Hanke <hanke AT nada DOT kth DOT se>
To: pgcc AT delorie DOT com
Subject: Re: Benchmarks for floating point operations
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On Tue, 16 Mar 1999, Marc Lehmann wrote:

> On Tue, Mar 16, 1999 at 08:33:48PM +0100, Axel Thimm wrote:
> > We are currently trying to see what we can drain maximally from PII for a
> > certain flop intensive application (QCD). Until now folks were using gcc 2.8.1
> > with -O2 -fomit-frame-pointer. I thought I might surprise them with egcs or
> > pgcc, but the perfomance dropped from 80 to 50 Mflop/s (?)
>
[snip] 
> x86 fp performance is veeery sensitive to environment issues.
> 
> > memory intensive (small ratio of computations per memory accesses) and perhaps
> > this is what makes the difference.
> 
> It might. Cahce line aliasing can make up to 200% difference in runtime.
If memory access is an issue, modern hierarchical memory
architectures have more influence on the flop rate than a compiler
can have. An impressing example is the atlas library (a gemm based
blas implementation) which pushed my machine from 7 mflops to 20
(double) and 35 (float) mflops, respectively. Other carefully
designed implementations of numerical standard algorithms lead also
to speed improvements. So my hint would be to see if you can make use
of tuned standard libraries. As far as I know, they are available for
blas, lapack, fftw. Maybe, you can change even your algorithm so that
they are able to take advantage of them.

Michael

+---------------------------------------------------------------+
|  Michael Hanke                Royal Institute of Technology   |
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