Path: utzoo!dptcdc!jarvis.csri.toronto.edu!mailrus!purdue!decwrl!sun!pitstop!sundc!seismo!uunet!portal!cup.portal.com!bcase
From: bcase@cup.portal.com (Brian bcase Case)
Newsgroups: comp.arch
Subject: Re: One aspect of bandwidth (backplane bus)
Message-ID: <17297@cup.portal.com>
Date: 16 Apr 89 17:41:03 GMT
References: <407@bnr-fos.UUCP> <17500@obiwan.mips.COM>
Organization: The Portal System (TM)
Lines: 36

>>In a recent series of articles about address modes and other topics,
>>some posters claim that memory bandwidth is not a problem - to quote
>>Brian Case, "bandwidth can be had in abundance". I happen to think that
>>we do not enough bandwidth now. What to other people think?
>
>Seems to me that backplanes (or, in general terms, processor-to-
>main-storage data trunks) are nearly pooped out.  At least for the
>kinds of air cooled, medium-priced computers that tend to have
>microprocessors for CPUs.

Yes, I couldn't agree more.  Look at the fastest backplane buses around.
Ardent's is a "measly" 256 megabytes (now there's a nice round number!)
per second.  That's really good for a backplane, but kinda awful for a
CPU<-->Memory interconnect.  Backplane bandwidth is relatively hard to
improve.  I once worked on a CPU design whose ECL backplane would have
had 1.2 or so Gbytes/sec of bandwidth.  This would have just kept up
with cache misses on two *integer* processors (or was it one?  the
instructions were quite wide)!  And the latency was nothing to write home
about (but commensurate with the technology).

If I remember correctly, what I was referring to with the slightly-out-
of-context-quote "bandwidth can be had in abundance" was tightly-coupled,
or even on-chip, CPU<-->Memory interconnect.  Even then, the statement
sounds a little overexuberant, I guess.  Still, we have a box full of tools
for increasing bandwidth, but hardly anthing besides "shrink the process!"
for decreasing latency.  At least on-chip RAM will scale with the CPU.

>And that's just for the very next generation of air cooled, medium
>priced CPU --- 60 VUP performance is eminently forseeable in the next
>6 to 24 months (schedule varies depending upon who you talk to :-).
>Soon thereafter it will be 1994, when you can buy 125-250 VUP micros
>for cheap, and when the few remaining software issues concerning
>32-way multiprocessors will be solved :-).

This is one set of reasons why I like uniprocessors.  Nice, simple,
easy to understand uniprocessors.