Xref: utzoo comp.lang.c:34413 comp.lang.fortran:4230
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From: ds@juniper09.cray.com (David Sielaff)
Newsgroups: comp.lang.c,comp.lang.fortran
Subject: Re: Fortran vs. C for numerical work (SUMMARY)
Message-ID: <100916.14288@timbuk.cray.com>
Date: 1 Dec 90 07:57:16 GMT
References: <9458:Nov2721:51:5590@kramden.acf.nyu.edu> <2392:Nov2902:59:0590@kramden.acf.nyu.edu> <7339@lanl.gov> <6690:Nov3006:15:3890@kramden.acf.nyu.edu>
Organization: Cray Research, Inc., Eagan, MN
Lines: 32
In article <6690:Nov3006:15:3890@kramden.acf.nyu.edu> brnstnd@kramden.acf.nyu.edu (Dan Bernstein) writes:
>Several of you have been missing the crucial point.
>
>Say there's a 300 to 1 ratio of steps through a matrix to random jumps.
>On a Convex or Cray or similar vector computer, those 300 steps will run
>20 times faster. Suddenly it's just a 15-1 ratio, and a slow instruction
>outside the loop begins to compete in total runtime with a fast
>floating-point multiplication inside the loop.
>
>Anyone who doesn't think shaving a day or two off a two-week computation
>is worthwhile shouldn't be talking about efficiency.
>
>In article <7339@lanl.gov> ttw@lanl.gov (Tony Warnock) writes:
>> Model Multiplication Time Memory Latency
>> YMP 5 clock periods 18 clock periods
>> XMP 4 clock periods 14 clock periods
>> CRAY-1 6 clock periods 11 clock periods
>
>Um, I don't believe those numbers. Floating-point multiplications and
>24-bit multiplications might run that fast, but 32-bit multiplications?
>Do all your matrices really fit in 16MB?
On late-model X-MP's and all Y-MP's, those times are correct for
32 bit integer multiplications. The change (from 24 to 32 bit
multiplies) corresponds to when the address space on the
Cray 1/X-MP/Y-MP line was bumped up from 24 bits to 32 bits (it was
always 32 bits on a Cray-2).
But this certainly seems to be getting an awfully long way from C ;-)
Dave Sielaff
Cray Research, Inc.