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From: johnw@astroatc.UUCP (John F. Wardale)
Newsgroups: comp.arch
Subject: Re: What with these Vector's anyways?
Message-ID: <363@astroatc.UUCP>
Date: Mon, 20-Jul-87 22:10:50 EDT
Article-I.D.: astroatc.363
Posted: Mon Jul 20 22:10:50 1987
Date-Received: Wed, 22-Jul-87 06:05:56 EDT
References: <218@astra.necisa.oz> <142700010@tiger.UUCP>
Reply-To: johnw@astroatc.UUCP (John F. Wardale)
Organization: Astronautics Technology Cntr, Madison, WI
Lines: 52
Summary: clarifications and nits

In article <13401@topaz.rutgers.edu> ron@topaz.rutgers.edu (Ron Natalie) writes:
>> Could someone out there explain to me what the basic idea is behind 
>> supercomputer CPU's?  ...vectors...etc.

Thanks Ron, for a generally Ecellent responce.  Just a few additions:

>  Of course, you need to have
>problems that using an array would be a speed up.  This is the ability
>to being "vectorized" that was referred to.

The requirements are hard to explain, but just using arrays (in
fortran or whatever is NOT enuf)  The data values have to be
know in time for the operators to operate on them.
ARY (I) = some_function_of ARY (I-1)  is *HARD* or impossible to
vectorize.

> ... SIMD (Single Instruction stream, Multiple Data stream)
> ... Mulitiple Instruction and Multiple Data Streams (MIMD) 
don't forget SISD (single Instr. Single Data)  (most common form)
and MISD  (in theory...I doubt any ever built one)

>CRAY 2 - Similar to X-MP, newer and faster technology
(a nit:)  A Cray-2 is NOT like an XMP, but that's a different argumnent.

Comments on Shared memory MIMD machines:  This is probably the
easiest and a fairly flexable solution.  It works great for ~10
CPUs, but it you try it with ~100 CPUs, the memory get very
overloaded (i.e. real-slow system-responce)


The code it different programs can be 
o SCALAR          (none of the following  use SISD)
o VECTORIZABLE    (see above;  use SIMD)
o PARALLELIZABLE  (well suited for MIMD machines)
o Vec or Parr     (use either)

The problem is that most pograms contain sections that are each
of the above classes.  Each type machine will do best on different
on different sections, and its usually the SCALAR portions that
contribute to the LARGEST part of the run times.
BTW:  The Cray-1 & XMP are also very-good Scalar machines!

			John W

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