Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!amelia!orville.nas.nasa.gov!fouts
From: fouts@orville.nas.nasa.gov (Marty Fouts)
Newsgroups: comp.arch
Subject: Re: RISC is a nasty no-no!
Message-ID: <322@amelia.nas.nasa.gov>
Date: 3 Mar 88 17:57:52 GMT
References: <179@wsccs.UUCP: <696@nuchat.UUCP: <284@scdpyr.UUCP> <25699@linus.UUCP> <11199@duke.cs.duke.edu> <25723@linus.UUCP> <8332@eddie.MIT.EDU> <7482@apple.UUCP> <7514@boring.cwi.nl> <17415@think.UUCP>
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Reply-To: fouts@orville.nas.nasa.gov (Marty Fouts)
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In article <17415@think.UUCP> barmar@fafnir.think.com.UUCP (Barry Margolin) writes:
>>In article <7482@apple.UUCP> baum@apple.UUCP (Allen Baum) writes:
>> > (power of two array sizes are pretty common!),
>
>For scientific programming (the major application of supercomputers),
>is this really true?  What scientific applications naturally map onto
>power-of-two arrays?  I suspect that making arrays a power of two in
>size is a habit mostly of systems programmers, who know to make things
>fit neatly into memory pages in order to reduce paging.
>

Much scientific programming takes the form of solving partial
differential equations using finite difference approximations of the
initial equation.  By selecting the differencing delta, the programmer
controls the problem size, so there is really a range of sizes which
work.   The lower limit is the minimum necessary to achieve some kind
of numerical stability in the result.  The upper limit is the amount
of computer time available/desirable for solving the problem.

On many vector machines, the use of multiple memory banks to increase
apparent memory bandwidth dictates that arrays not be powers of two in
all dimensions, in order to avoid bank conflict when marching through
the array.  Some computations are done with arrays in which bounds are
arbitrarily set to relatively prime values.