Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!world!bzs
From: bzs@world.std.com (Barry Shein)
Newsgroups: comp.arch
Subject: Re: Historical architectural advances??
Message-ID: <BZS.90Nov7175350@world.std.com>
Date: 7 Nov 90 22:53:50 GMT
References: <8185@scolex.sco.COM> <1868@m1.cs.man.ac.uk> <8553@scolex.sco.COM>
	<1888@m1.cs.man.ac.uk> <42895@mips.mips.COM>
Sender: bzs@world.std.com (Barry Shein)
Organization: The World
Lines: 33
In-Reply-To: mash@mips.COM's message of 7 Nov 90 18:37:50 GMT


>What is the difference between a loosely coupled multicomputer system,
>and a tightly coupled multiprocessor computer?

Once you start talking about bus-speed LANs (e.g. Gb, ~100MB/s, which
is as fast as most mini's busses can run) then the distinction becomes
less interesting.

Other than a little latency it becomes hard to distinguish such a
system from one which you just strapped an extension cabinet into.

Without that one of the biggest problems with loosely coupled
multi-computer systems is the proper emulation of shared-memory (when
the effect desired is in fact MIMD.) Reading is easy, writing (update)
is harder. Caches make this even harder.

Of course, even with a bus-speed LAN it's not enough to just hook up a
network interface with some software. For example, if you want
anything like MIMD you'll have to do something like a copy-on-write
for every modified page in any system that might be addressable by the
other system. That sort of thing generally begs a hardware assist, and
probably wants multi-ported memory.

Anyhow, yes, you can blur the distinctions, but that doesn't mean it
is easy to do so.

With very large granularity of processing then the whole thing sort of
becomes a philosopical, epistemological question.
-- 
        -Barry Shein

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