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From: gl8f@astsun9.astro.Virginia.EDU (Greg Lindahl)
Newsgroups: comp.os.misc
Subject: Re: Test for distributedness
Message-ID: <1990Mar25.181438.7338@murdoch.acc.Virginia.EDU>
Date: 25 Mar 90 18:14:38 GMT
References: <6067@star.cs.vu.nl> <2604@quiche.cs.mcgill.ca> <6106@star.cs.vu.nl> <6548@becker.UUCP> <6117@star.cs.vu.nl>
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Reply-To: gl8f@astsun9.astro.Virginia.EDU (Greg Lindahl)
Organization: Department of Astronomy, University of Virginia
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In article <6117@star.cs.vu.nl> ast@cs.vu.nl (Andy Tanenbaum) writes:

>The key issue in a distributed system is transparency.  Are the users
>aware of where things are? I propose a kind of Turing test for
>distributedness:
[...]
>It is legitimate for person 1 to start up a huge CPU-bound computation to
>see if person 2 notices any performance degradation.  On a timesharing
>system, person 2 would see this (Butler Lampson once called this a covert
>channel.)

Unfortunately, the terms of this test can easily be abused. If the
distributed system has more processors than processes, and the 2 users
start up "truely" CPU-bound programs (e.g. no paging, no I/O except to
print "I'm done after consuming 57 hours of cpu time, etc.), then the
distributed system should show no slowdown while the timesharing
system will see a large slowdown.

This abuse of the test depends on the observation that, under sane
operating systems (I exclude VM/CMS CPU partitions), a single
CPU-bound program running on an idle single-processor timesharing
system can consume essentially all of the CPU time. And that's all
there is, so multiple programs can't consume twice as much.

You might think that a "normal" program, which depends on several
resources, would provide a better test. In that case, you might expect
that several normal programs would be able to consume more total
resources than one normal program on both the timesharing and the
distributed system.

Of course, since this is ast's test, my CPU-bound program would solve
a problem which could be solved using integers between 0 and 255, such
as finding minimal Golomb rulers. None of this floating point stuff. 

:-)

Greg Lindahl
gl8f@virginia.edu                  I gave my lunch for space-sickness research.