Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!gem.mps.ohio-state.edu!rpi!batcomputer!kahn
From: kahn@batcomputer.tn.cornell.edu (Shahin Kahn)
Newsgroups: comp.arch
Subject: Re: ATTACK OF KILLER MICROS
Message-ID: <9169@batcomputer.tn.cornell.edu>
Date: 28 Oct 89 00:02:44 GMT
References: <35825@lll-winken.LLNL.GOV> <2121@brazos.Rice.edu> <35897@lll-winken.LLNL.GOV> <9078@batcomputer.tn.cornell.edu> <36232@lll-winken.LLNL.GOV> <9119@batcomputer.tn.cornell.edu> <36593@lll-winken.LLNL.GOV>
Reply-To: kahn@tcgould.tn.cornell.edu (Shahin Kahn)
Organization: Cornell Theory Center, Cornell University, Ithaca NY
Lines: 91

In article <36593@lll-winken.LLNL.GOV> brooks@maddog.llnl.gov (Eugene Brooks) writes:
>>If you have scalar dominated code that fits in a workstation's memory
>One should not attempt to infer that a workstation's memory is small.
>An YMP 8/32 has 4 megawords (32 MB) available per processor.  If all you
>want is 32 MB per processor you can buy this with a killer micro for
>about 40K, simply throw it away in a year when its performance has been
>eclipsed by the next killer micro, and still have your computer time work
 
Point well taken.  One of the reasons I have little desire to 
defend or promote a YMP is precisely that.  The YMP has 128 MWords of
memory, by the way.  This is for 8 processors.
Cray-2 has 256 MWords.  (but a terrible latency, even in the s model).
(the Cray C90 is supposed to have 512 MW, and Cray-4 1000+ MWords, but
these are paper machines for now).
 
So, the point is that fp performance alone does not make a supercomputer
anymore (surprize surprize!).  My feeling these days is that one needs
a sophisticated VM system, with a large hierarchical memory system,
and first-rate I/O and networking, so that one could run a single
job very fast (the traditional domain of supers has been just this.
They've been "benchmark machines" in my opinion) but you could also
sit in a network and handle many users and many jobs. (many = say, 37)!
and on top of that, you need libraries, compilers, debuggers, editors,
profilers, etc.

The emergence of the powerful micro is welcome, indeed.  And when they
can be ganged-up and you know how to program them and have an application
that uses their strengths and does not excercise their weaknesses,,,
indeed, they are fast.  

BUT:  1) they dont have the software, libraries, compilers, etc.
      2) they often have a low bandwidth connection to a not-so-strong
            front-end
      3) they cant handle I/O so well yet.
      4) there are no standards for anything
      5) you need a pretty large job to get speed-up, anyway.

Remember, the way the guys at Sandia
got their great speed-ups was to make the jobs larger.  Much larger.
You need over 99.99% parallelism for a 1000 processoor machine!

So the parallel part of your program should be allowed to grow
(and fortunately, if the algorithm is parallelizable, 
the parallel parts tend to grow faster than the serial parts
in many cases, if not most.  Same thing with vectorizable parts
if the algorithm is vectorizable).

Except that it just so happens that when you have a large job,
it also runs much faster on a super!   (a modern super with
lots of memory, that is, not the OLD definition of super.)

My point is that there is no point in getting too excited about highly 
parallel machines. nor about fast microprocessors.  A micro is not called
a micro just because it has a microprocessor in it.  Not anymore.
It usually has a low bandwidth memory system, not much of a cache,
no much a VM system, not much of an I/O, etc.  That's what keeps the 
price down. (prices are going down for supers, too).
Its great to have a fast micro on your desk, but it'll have plenty to
do rendering the data that you got from the super!  and delivering
mail, etc. 
And if you want to gang them up, you'll end up paying exactly
as much as you would if you got a super, maybe more!  This is how
it will end-up being.  If you had all the software and all the I/O
and all the disk and all the networking, etc...  You'll have to
pay for those!  Hardware costs will not differ enough to burry it.
(I am comparing a contemporary super with a contemporary
high-end parallel system.  They could very well be the same thing
in the 5 years that was specified. All supers are multiprocessors
now and are increasing the number of processors.  So that's another
reasonn why you'll be paying exactly the same price if not more!)

Conclusion:  Like a teacher said a long time ago,,  there is 
    the law of conservation of difficulty!!    Highly parallel 
    systems will NOT be a revolutionary deal where you suddenly
    can do something much more cheaply.  It has been evolutionary.
    which is why I said: By the time micros become killers, 
    they wont be micros anymore.
    Highly parallel systems are good.  They have merit.  They are here
    to stay, etc.  Fast micros are also nice.  But lets not sensationalize
    the issues.

And by the way,, most of the japanese machines achieve their
speed by multiple functional units: more than one adder and one
multiplier.
 
And a final note about "pagemaker".  No insult was intended.
pagemaker is a sophisticated application that requires all
components of the machine from the cpu to the screen to the 
printer to font calculations, etc.  It would have been quite unimaginable 
to try to do something like that on a computer 30 years ago.
I think it was clear what I meant.