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From: dennis@gpu.utcs.toronto.edu (Dennis Ferguson)
Date: Mon, 14-Mar-88 00:20:26 EST
Message-ID: <1988Mar14.002026.3977@gpu.utcs.toronto.edu>
Organization: Mechanical Engineering, University of Toronto
Newsgroups: comp.arch
Subject: Re: What's a Vax 11/780 MIP really?
References: <413@mn-at1.UUCP>
Reply-To: dennis@gpu.utcs.toronto.edu (Dennis Ferguson)

In article <413@mn-at1.UUCP> alan@mn-at1.UUCP (Alan Klietz) writes:
>A recent column in one of
>the Unix trade mags reveals that the Vax 780 = 1 MIPS rule-of-thumb
>may be grossly overstated.
[...]
>Therefore one VAX 780 "MIPS" is approximately 0.44 "honest-to-god"
>VAX MIPS?  (HTGV MIPS?)   Or a 126% overestimate?

This is well known.  I suspect you will get more than one reply (beside
this one) reiterating the story about how the DEC types benchmarked the
11/780 against a then-current 370 which IBM was calling a 1 MIPS machine,
found it to run about the same speed, and so for marketing purposes called
the 11/780 a "1 MIPS" computer.  Thus the "MIPS" referred to are supposed to
be native 370 MIPS, not native Vax MIPS.

This, unfortunately, is also not true, at least in my experience.  I have
found that you can match benchmark results on a 370 and a Vax pretty well
by multiplying the IBM-reported "MIPS" number by 1.8 or so (i.e. a 13 MIPS
3090 goes faster than one would otherwise be led to believe).  I really
think some marketeer at DEC just made up the 1 MIPS number so Vaxes would
look better against the IBM 370 competition.  The fact that trade magazines
are just getting around to realizing this shows what a good idea it was.

All of which matters not at all, since if you simply define a 780 to be
1 "MIPS" and measure everything against it, it all works out in the end
for many practical purposes anyway.

Dennis Ferguson
University of Toronto