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From: martelli@cadlab.sublink.ORG (Alex Martelli)
Newsgroups: comp.arch
Subject: Re: RISC vs. CISC -- SPECmarks
Message-ID: <819@cadlab.sublink.ORG>
Date: 5 May 91 09:59:05 GMT
References: <3423@charon.cwi.nl> <11602@mentor.cc.purdue.edu> <1991Apr30.163153.18568@midway.uchicago.edu> <1991May2.162909.9165@news.arc.nasa.gov>
Organization: CAD.LAB, Bologna, Italia
Lines: 22

lamaster@pioneer.arc.nasa.gov (Hugh LaMaster) writes:
	...
:I have only limited experience with the new, fast-only-in-cache, machines,
:but I have to say that the code you need to get optimum performance is
:even more non-intuitive than that for the older vector architecture machines.
:Even worse, code which was previously optimal for vector machines, and which
:was OK on a wide variety of other machines, is now pessimal for these machines.

Not really so new - I was optimizing codes for the cache in '87 for an IBM
3090 with VF... ok, there ARE problems (the curve of leading dimension of
array versus megaflops bounces up and down wildly and unpredictably for many,
many 'normal' patterns of memory access - FAR from intuitive!), but I
believe there is by now enough experience in the high-performance-Fortran
crowd to have accounted for these effects - a guy from NAG, for example, was
working on matrix-block-oriented implementation of much BLAS stuff, and
already in '87 he was getting essentially the same performance as assembler
handcoded ESSL routines... so I'm prettyy sure the optimization techniques are
by now, under numerical-programming-specialists' belts!  Whether the average
Fortran-using scientist will ever digest them is another matter, but then,
let's count this as just more fuel to the fire for the argument that anybody
who is not using specialist-coded subroutine libraries for as large a part of
his/her code as possible is probably going about it the wrong way!