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From: shepherd@euroies.UUCP (Roger Shepherd)
Newsgroups: net.arch
Subject: Floating point performance
Message-ID: <340@euroies.UUCP>
Date: Wed, 8-Oct-86 03:12:27 EDT
Article-I.D.: euroies.340
Posted: Wed Oct  8 03:12:27 1986
Date-Received: Tue, 7-Oct-86 23:06:09 EDT
Organization: University College Dublin
Lines: 65

I was interested to read Ian Kaplan's (...!loral!ian) appeal
for microprocessors with fast floating point.  I am a little
concerned with the use of `peak' performance to characterise
the speed of part as I don't think that this necessarily
reflects the the USABLE performance of the part.  I think it
is instructive to look at MFLOPs compared with Whetstones. (A
good benchmark of performance on `typical' scientific
programs).
 
For example, Ian quotes some performance figures as
 
   Intel 80287       < 0.1 MFLOP  (say 0.95 MFLOP at 8Mhz?)
   National            0.1 MFLOP
   Motorola            0.3 MFLOP
 
To these I'll add the figure for the INMOS transputer (no
co-processor, floating point done in software)
 
   Inmos IMS T414-20   0.09 MFLOP (typical for * and /,
                                   + and - are slower!)
 
According to the figures I have to hand, these processors
compare somewhat differently when the Whetstone figures are
compared.  For example, I have single length Whetstone
figures as follows for these machines
 
                                 kWhets   MWhets/MFLOP
                                                    (normalised)
   Intel 80286/80287    (8 Mhz)   300         3.2      1.0
   NS 30032 & 32081    (10 Mhz)   128         1.3      0.4
   MC 68020 & 68881 (16 & 12.5)   755         2.5      0.8
 
   Inmos IMS T414B-20             663         7.4      2.3
 
The final column gives some feel for how effective these
processor/co-processor (just processor for the T414)
combinations are at turning MFLOPS into usable floating
point performance.
 
Also, I don't quite know why Ian likes the CLIPPER (three
chips on the picture of the (large) module I've seen) but
dislikes the NS 32310 (four chips); they seem to give the
same MFLOP rating. (Does anyone have Whetstone figures for
these two?)
 
Comparisons against Weiteks (or whatever) are also somewhat
suspect.  To use their peak data rate you have to use them in
pipelined mode, their scalar mode tends to be somewhat slower
and it might be possible to build a microprocessor system
that could feed them data and accept results at that rate.
However, if you're only using the chips in that mode I'm not
convinced that you really want all that silicon to be taken
up with a large pipelineable (?) multiplier; I'd rather have
a processor there!
 
On the same subject (sort of), what measure should be made
of the `goodness' of a floating point micro-processor? How
about MWhetstones per square centi-metre. (Or do all you
guys and girls still use inches? :-) ) Or, how about
MWhetstones per milliwatt?
-- 
Roger Shepherd
INMOS Limited, 1000 Aztec West, Almondsbury, Bristol, BS12 4SQ, GB
USENET: ...!euroies!shepherd
PHONE: +44 454 616616