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From: rogerk@mips.COM (Roger B.A. Klorese)
Newsgroups: comp.sys.dec
Subject: Re: Can a DECstation replace a 11/750 ?
Message-ID: <38515@mips.mips.COM>
Date: 4 May 90 17:56:45 GMT
References: <36119@prls.UUCP> <5070@hub.ucsb.edu> <1990May4.025422.8656@spock.UUCP>
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In article <1990May4.025422.8656@spock.UUCP> eric@spock.UUCP (Eric Volpe) writes:
>In article <5070@hub.ucsb.edu> aks@somewhere.ucsb.edu (Alan Stebbens) writes:
>>
>>The DS3100 really runs at 15 MIPS, which is *fifteen* times faster than a 
>>VAX/780.
>It is *not* *fifteen* times faster than a vax 780! the DS3100 is a RISC
>machine, meaning Reduced Instruction Set - Which means that each instruction
>on a RISC processor is very small and the equivalent of several instructions
>of a non-RISC processor. So, while it may be true that the DS3100 can
>execute fifteen times as many of its instructions in one second as a VAX780
>can of vax instructions, you're comparing apples and oranges. Each Vax
>instruction accomplishes perhaps the equivalent of eight DS3100 instructions.

If "MIPS" [as opposed to "Mips" ;-)] had a single useful meaning, we could
determine that one of you is right, and the other, wrong.

But I'd have to say that Eric is "wronger" than Alan.

First, the MIPS spoken of here is not the execution of actual, machine
level instructions.  It is the "multiple of <some benchmark> compared
to a VAX 11/780" measurement.

If the benchmark is Dhrystone, the ratio of 15:1 is approximately correct;
this is why DEC announced the machine as 14-15 "Dhrystone MIPS," or
"integer MIPS," or some such.

If the benchmark is the SPEC suite, the number is lower, but I remember it
as 10x, +/- 2x.

If the benchmark is the Digital Review benchmark suite, or the DEC
performance suite, or the Mips performance suite, the number is, if I
remember correctly, in the 12 range.  Both DR and DEC tend to call these
VUPs, or "VAX-equivalent units of processing."

Of course, the only meaningful system benchmark is your own actual 
workload.

But, on the other hand, it is a crock that there is an 8x instruction ratio
in most code.  Our static comparisons yield a typical ratio of about 1.4x;
dynamic comparisons vary with the code, of course, but it is never (well,
hardly ever) observed as anywhere near the 8x range.

Actual MIPS, that is, how many instructions get executed in a second, is
a measurement that can be viewed in the context of architectural efficiency,
or in the measurements for certain specific applications, like limited
embedded tasks.  But it is so rarely useful that most people do not use
it as the only number in their spec-sheets unless they have something to hide.

These VAX/RISC comparisons are *not* true MIPS, but rather, integer program
ratios, which are much more (if not completely) relevant.
-- 
ROGER B.A. KLORESE      MIPS Computer Systems, Inc.      phone: +1 408 720-2939
MS 4-02    950 DeGuigne Dr.   Sunnyvale, CA  94086   voicemail: +1 408 524-7421 
rogerk@mips.COM         {ames,decwrl,pyramid}!mips!rogerk         "I'm the NLA"
"Two guys, one cart, fresh pasta... *you* figure it out." -- Suzanne Sugarbaker