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From: guy@sun.uucp (Guy Harris)
Newsgroups: net.arch
Subject: Re: Floating point performance & Mr. Mashey's Mythical Mhz
Message-ID: <8965@sun.uucp>
Date: Wed, 5-Nov-86 17:01:12 EST
Article-I.D.: sun.8965
Posted: Wed Nov  5 17:01:12 1986
Date-Received: Thu, 6-Nov-86 20:57:10 EST
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> I really don't care how fast the internal engine has to run to produce my
> output.  If my little Alfa Romeo is tooling down the highway at 70 MPH with
> an internal engine cycle time of 3100 RPM, and I get passed by a Ferrari
> doing 110 MPH with an internal engine speed of 4900 RPM, who is going
> faster?  Certainly not me, not matter how you multiply the numbers!
> My MPH/RPM is a little higher, but I got my doors blown off nonetheless.

Yes, but what if:

	1) Horsepower, say, were linearly proportional to RPM

	2) The horsepower need by both cars to sustain a particular
	   speed were the same

	3) Your Alfa had a redline of 20,000 RPM, while the Ferrari had a
	   redline of 6000 RPM

	4) "All other things are equal"

Then just step on the gas hard enough to get near the redline, and blow the
Ferrari's doors off.

I believe Mashey's thesis is that this is more-or-less the proper analogy;
the maximum clock rate possible is mainly a function of the chip technology,
not the architecture, so an architecture that gets more work done per clock
tick can ultimately be made to run faster than ones that get less work done
per clock tick.  I shall voice no opinion on whether this is the case or not
(I don't know enough to *have* an opinion on this) , but will just let the
chip designers battle it out.

> So if I am able to build some bizarre semi-synchronous architecture with a
> 2 GHz clock rate, does it mean my machine is slower (when you divide out the
> clock in MFlops/MHz)?  I don't think so.

Since MFlops/MHz is !N*O*T! a measure of machine speed, and was never
intended as such by Mashey, the machine is neither faster nor slower "when
you divide out the clock in MFlops/MHz".  If you don't divide out the clock,
no, it doesn't mean your machine is slower.  Nobody would argue that it did.

> If we are looking for an esoteric comparison of architectural efficiency,
> *then* perhaps we have a reasonable metric here.

Well, what did you *think* MFlops/MHz was intended as?  It *was* intended
for comparing architectural efficiency!

Please, people, before you flame this measure as absurd, make sure you're
not flaming it for not being a measure of raw speed; it wasn't *intended* to
be a measure of raw speed.  *You*, the end-user, may not be interested in
architectural efficiency, but may only be interested in "how fast something
gets your job done"; the person who has to design and build that something,
however, is going to be interested in architectural efficiency.
-- 
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com (or guy@sun.arpa)