Path: utzoo!mnetor!uunet!husc6!bunny!vaxine!encore!collins
From: collins@encore.UUCP (Jeff Collins)
Newsgroups: comp.arch
Subject: Re: Single/Multi Tasking position summary (was Re: Single tasking)
Message-ID: <2428@encore.UUCP>
Date: 2 Jan 88 20:51:42 GMT
References: <60@amelia.nas.nasa.gov>
Reply-To: collins@encore.UUCP (Jeff Collins)
Organization: Encore Computer Corp, Marlboro, MA
Lines: 82


In article <60@amelia.nas.nasa.gov> fouts@orville.nas.nasa.gov (Marty Fouts) writes:
>He has also raised another example of parallel processing
>enthusiasm which I try to counter when I can.  He shows that an Encore
>Multimax gives a speed of of 50x over a 750 for a particular make.  Of
>course, this looks good until you realize that he's talking about two
>different generations of processors in two different price ranges.  I
>would be interested if Barry would tell us the speedup the Multimax
>gets over doing the same make on 1 of its cpus.  I would also be
>interested in comparing the Multimax performance to that of a single
>processor system done in more recent technology than an 750, for
>instance a Sun 3/260.
>
(Note: the he mentioned  is Barry Shein at BU).

	Yes, Barry is talking about two different processors but they are the
	same generation.  The original Multimax (now called the Multimax 120)
	contains NS32032 processors.  These are approximately .75Mips, as is the
	VAX 750.  As to price, yes the Multimax that was mentioned is more 
	expensive than a 750 (but they are your basic scrap metal now).  A more
	interesting comparison would be a new VAX.  As we don't have one I can't
	give the numbers on this, but I can ask the network - how long does it
	take to compile a 4.3BSD kernel on a real VAX?  I would bet it is
	longer than 4.5 minutes (the best we can do on our fastest machine).

>His ancedote is an example of a problem which we haven't discussed
>yet in this exchange over parallelism, which is price performance.  We
>have on our floor a Convex C1 and an Alliant FX8 which I believe are
>about equivalently priced models.  Although the Alliant can parallize
>some of my algorithms which the Convex can not vectorize, they all
>tend to run faster on the Convex than on the Alliant.  I do *NOT* make
>the general claim that at a given price a parallel machine has poorer
>performance than a single processor, but for a fairly wide range of
>problems over a fairly wide price range of machines, this seems to be
>true.  This exchange of ancedotal evidence leads to the hard question:
>For which problems will a parallel processor using the best available
>implementation technology give better cost/performance than the best
>single processor?

	Vector processors are a special form of parallel processors (this 
	should start lots of flames).  Perhaps the Convex is simply implemented
	better than the Alliant?  You are using one comparison to condemn
	an entire class of machines.

>
>No.  The point is not to say no one should use parallel systems under
>any circumstances.  Like Mr. Shein, I use them regularly.  The point is
>to try to help new users form realistic expectations of the class or
>problems for which parallelism is trivial, the class for which it is
>work (and the nature of the work) and the class for which it should
>not be utilized.

	I definately agree with this, it is very important to determine the
	match between an application and the type machine to use.  As an 
	attempt to further your point, let me give my perspective on the 
	various applications and machine solutions (from an admittedly biased
	perspective).

	General timesharing: A truely parallel machine like the Encore 
	    Multimax or the (I have to say this for fairness) Sequent Balance.
	    This does NOT include the Alliant or any of the current multi-CPU
	    VAXen.  The Alliant does have a parallel hardware architecture for
	    the IPs, but the OS does not currently take maximum advantage of
	    it.  The multi-CPU VAXen, do not parallelize the most important
	    aspect of a timesharing machine - I/O.  The best testimonial that
	    I have heard for this is from the University of Oklahoma.  After
	    they installed a multi for thier student time sharing, they were
	    able to eliminate the student signup procedure (you remember that
	    you sign up a week in advance for a 2 hour time slot).  The
	    machine that they installed did not cost any more than a current
	    generation VAX.  NOTE: this is entirely transparent.

	Fine-Grained parallelism: A Convex or an Alliant (unless of couse
	    you can afford a Cray).  There are some new machines that are
	    very interesting in this solution area: Multiflow, ETA.  Probably
	    the decision here should be based on compiler technology, the
	    machines are pretty evenly matched.

	Course-Grained parallelism: This is tougher to define.  Probably the
	    best course of action here is to benchmark the application on 
	    various machines.