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From: reid@Glacier.ARPA (Brian Reid)
Newsgroups: net.arch
Subject: I don't believe your statements about multiprocessors
Message-ID: <7202@Glacier.ARPA>
Date: Wed, 8-May-85 12:46:32 EDT
Article-I.D.: Glacier.7202
Posted: Wed May  8 12:46:32 1985
Date-Received: Fri, 10-May-85 22:12:54 EDT
Distribution: net
Organization: Stanford University, Computer Systems Lab
Lines: 34

I've been watching people make claims about the wonderfulness of
multiprocessors for 20 years. I have also watched a much smaller number of
people actually build and measure the performance of multiprocessors. This
newsgroup has recently been alive with various informed and uninformed
metaphysical babbling about multiprocessors. I don't believe most of it.

I have never seen a single particle of evidence, not one number, that says
that a tightly-coupled (e.g. shared-memory) multiprocessor is in any way
better than a uniprocessor of the equivalent aggregate speed. If you know
how to build a 100-MIP uniprocessor CPU, or 10 10-MIP processors for the
same instruction set, or 100 1-MIP processors, then it is always much better
to have the uniprocessor. It might be cheaper to build the multiprocessor,
but the uniprocessor is a better computer.

For loosely-coupled architectures there are sometimes arguments about
reliability through redundancy, though they tend not to hold water in
practice because of peripherals. But for a shared-memory machine, the only
reason to build a multiprocessor instead of a uniprocessor is to make it
cheaper. Otherwise, the uniprocessor is easier to program, faster (no
synchronmization cost), has a higher burst speed, and can perform any
parallel computation that the multiprocessor can perform.

Of course it is not always possible to build a uniprocessor as fast as one
would like, so multiprocessors and vector machines have always been at the
leading edge of the speed wars, but this is not because they are better
computers but because people know how to build them.

I am always interested in seeing hard data about computer architecture (or
anything else, for that matter). I invite any of the proponents of radical
architecure multiprocessors to show me numbers demonstrating their
superiority over uniprocessors.
-- 
	Brian Reid	decwrl!glacier!reid
	Stanford	reid@SU-Glacier.ARPA