Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uwm.edu!bionet!agate!stanford.edu!leland.Stanford.EDU!sgreene
From: sgreene@leland.Stanford.EDU (Spencer Greene)
Newsgroups: comp.arch
Subject: Re: More Snake bytes. (X/MHz)
Message-ID: <1991Apr5.213636.18363@leland.Stanford.EDU>
Date: 5 Apr 91 21:36:36 GMT
References: <569@diab.se> <1871@spim.mips.COM> <12914@goofy.Apple.COM>
Organization: EE Dept., Stanford University
Lines: 31

In article <12914@goofy.Apple.COM> russell@apple.com (Russell Williams) writes:
>To summarize, John Mashey has spoken of SPECint / MHz as a measure of 
>architecture independent of absolute MHz, while Mark Forsyth has indicated 
>that ability to build a higher MHz implementation given the same process 
>technology is also a relevant measure of an architecture, stating that 
>HP's process technology is typical. . .
>
>The unanswered question seems to be: can the HP architecture be designed 
>with fewer gate delays than others?

The issue, as has been pointed out, is one of headroom.  Even if you
agree that HP's cpu clock is *entirely* due to architecture, you cannot
conclude that HP has the same ability to linearly scale clock rate as
other vendors.  For example, it is more likely that MIPS can go from
33 Mhz to 66 than that HP can go from 66 to 132.  CPU issues aside,
everyone is using essentially the same SRAMs, board traces, etc., 
and Amdahl's Law says that after cranking the CPU clock rate to a
certain point these other issues will begin to dominate.  Not that they
*can't* be made to keep up in a 132-Mhz system, just that the expense is
prohibitive compared to other solutions.

Of course, this does not detract from the fact that HP has developed
today a system which tests some of the purported limits of 1-processor 
RISC, while other vendors talk of unexploited headroom.  However,
it does suggest that to evaluate potential in a product line beyond
the short term, we should look at the sophistication of the vendor's
multiprocessor hardware, and (perhaps more importantly) software.
Small wonder that SMP was all the rage at recent trade shows.

-----------
Spencer Greene                             sgreene@leland.stanford.edu