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From: hamilton@dg-rtp.dg.com (Eric Hamilton)
Newsgroups: comp.arch
Subject: Re: Historical architectural advances??
Message-ID: <1990Nov7.155719.24413@dg-rtp.dg.com>
Date: 7 Nov 90 15:57:19 GMT
References: <8185@scolex.sco.COM> <1868@m1.cs.man.ac.uk> <8553@scolex.sco.COM> <1888@m1.cs.man.ac.uk>
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Reply-To: hamilton@dg-rtp.dg.com (Eric Hamilton)
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In article <1888@m1.cs.man.ac.uk>, mshute@cs.man.ac.uk (Malcolm Shute) writes:
|> In article <8553@scolex.sco.COM> seanf (Sean Fagan) writes:
|> >In article <1868@m1.cs.man.ac.uk> mshute@cs.man.ac.uk (Malcolm
Shute) writes:
|> >>Does your classification scheme handle a loosley coupled system
|> >>(such as a department's worth of SUNs connected on a LAN)...
|> >>Does the overall system (network) qualify as being a
mainframe/supermini?
|> >Do you, your wife, and your two children qualify as a single person
with 8
|> >arms?  No.
|> 
|> What is the difference between a loosely coupled multicomputer
system,
|> and a tightly coupled multiprocessor computer?
|> 
|> Granularity of processes for scheduling, and geography of the
'cabinet'
|> size.  That is all (just about).
|> 
|> Our LAN fits into a cabinet called the Department of Computer
Science.
|> How much smaller would it have to be before you would consider it to
|> be a single multiprocesser computer?  Or, what other criterion would
|> have to be met before it crossed the threshold?
|> 
|> I am seriously interested in finding out how people in  comp.arch 
make
|> this distinction.  Is it possible that there is *no* firm dividing
line
|> between the two?  Does it matter to have an answer to the question?

It is obviously and trivially true that there are degrees of
loose coupling - in a practical sense the bandwidth and
latency characteristics of the interconnect control the
sorts of things that you use the interconnect for and hence
the tightness of the coupling.

I would like to see the term "tightly coupled" reserved for
systems in which memory is shared and coherent for all
processors and all the processors occupy the same physical
address space.  The distinguishing characteristic of these
machines is that the C construct *p=... can be used to
change shared data.  A bunch of microprocessors on the
same bus qualifies as tightly coupled in this sense.