From: walk@mrcnext.cso.uiuc.edu (Todd Walk)
Subject: Re: More Controversial Topics
Organization: University of Illinois at Urbana
Date: Wed, 16 Dec 1992 20:08:06 GMT

rsrodger@wam.umd.edu (Yamanari) writes:

>In article <Bz5yG0.Cqo@news.cso.uiuc.edu> walk@mrcnext.cso.uiuc.edu (Todd Walk) writes:
>>rsrodger@wam.umd.edu (Yamanari) writes:
>>>	It is my understanding, and this means that I'm draggin it out
>>>	of my memory, which is very poor these days, that this situation
>>>	really only occurs with some high number of processors--like
>>>	150 when you're talking about the Inmos T800 transputer.  However,
>>>	the last 20 or 30 processors add sufficiently little benefit
>>>	that it becomes almost pointless to add them.
>>
>>This will happen WAY before 150 or so processors on a SMP machine.  You've
>>got to remember that the processors share a main memory, so it's memory


>	You can buy T800 cards for the PC that have 4 processors.  Each
>	processor has 4 megs of memory *of it's own*.  In other words,
>	rethink your comment.

Yes, but can they do SMP?  SMP processors share a main memory and
have large caches so they minimize access to this memory.  Any board
put on the ISA bus MAY be able to do SMP, but only in a loosely coupled
fashion.  Multiple boards on the bus create great difficulties because
the ISA bus was made for only 1 bus master card (you can use more, but
they will probably interfer with each other).  Another point is that
unless NT is changed, it will NOT be able to do SMP from cards
being added to an ISA/EISA/MCA/VESA bus.

>>bandwidth that limits power and not the # of processors you can jam
>>onto a board.  About the most processors I've heard of on a SMP machine


>	What makes you think that all processors need go on one
>	board?  A backplane with a number of seperate CPU cards
>	would (and does) do nicely.

I didn't say that all processors had to be on one board, but it does
have MANY advantages ( and some disadvantages...).  Anytime you 
have the processors on seperate cards, the main memory connection has
been loosely coupled.  This doesn't keep it from being a SMP machine,
but it does make it harder to design (and the more processor card
slots, the harder to design).  

What I said before still stands, the limit on the speed of a SMP machine
is determined by the memory bandwidth and not the # of processors.

>>is 16.  Computers with more processors basically all use a distributed


>	I've heard of 32 for <$220,000 systems.  Then, of course,
>	there's things like a Connection Machine, the N-Cube, 
>	supercomputers and so on.

The Connection Machine and N-cube are NOT SMP machines, but have
a distributed architecture.
 
>>memory setup (each processor has it's own main memory) and communicate
>>by sending messages to each other.


>	And?  You can feel free to assume that this was precisely 
>	the machine I was describing.  The Inmos transputer has special
>	communicatiosn hardware (4 ?serial links/chip) for talking
>	one chip to another.  

Hmmm.  Do the processors on this board also share a main memory?
If they don't, it would be difficult and slow for them to do SMP.

>	Does SMP *specifically* refer to machines that are not
>	distributed memory?

No it doesn't, but it practice it might as well be.


					Todd Walk
					walk@mrcnext.cso.uiuc.edu

Newsgroups: comp.os.os2.advocacy,comp.os.ms-windows.advocacy