🚀 go-pugleaf

In article <2683@sdcsvax.UCSD.EDU> ucbvax!ames-pioneer.arpa!ames!eugene@sdcsvax (Eugene Miya N.) writes:
>I am a direct measurement type for several reasons:
>1) I believe in the Missouri principle (show me)
>2) I and many other have been burned by people's claims for hardware
>3) Simulation has many draw backs: call them limitations or tradeoffs,
>you don't trade off in reality.
>4) Direct measurement can show interactions which simulation, etc.
>can't show (especially when asynchronous things take place on sequential
>simulations).....

Additional comments:
a) If I'm buying a computer, I'm a direct measurement type.
b) If I haven't built it yet, and need to know which way to do it, I sure like
simulation a lot.
c) If I've built it, and I have simulations, it's awfully nice to correlate 
them so I can trust the simulator more, although as Gene says, some situations
are very hard to simulate.

To illustrate the importance and value of really good simulations, one
can observe the unfortunate problem of announcing performance, and then
having to backtrack horribly when the product comes out.  I'm sure most people
have run into some of these.  Some are marketing ploys, but others appear to
be cases where people simply did not even simulate typical applications.

On the other hand, good simulation can be incredibly helpful, and it's hard to
believe people are serious about designing new computers without doing good
simulation.  As an example, this last summer, we started to be able to run
much larger programs than we'd been able to simulate, and also more
multi-tasking stuff, on M/500s.  Although the performance of the simulated
benchmarks was as expected, the bigger ones were not quite the 5X 11/780 
we were looking for.  There were a bunch of possible ways to fix this.
Our performance gurus simulated the alternatives, which led us to the
simplest improvement: double the I-cache from 8K to 16K [costs about $50
and was fairly straightforward]. It would have been difficult to quickly
pick the correct choice without such simulation, since a whole bunch of
other proposals were also considered. 

Coincidentally, a prospective customer was in benchmarking the week new boards
were to appear.  They ran a large benchmark on the 8K machine, and address
traces were also captured, which when run thru the simulator, predicted the
results they got.  Our gurus then fed the 16K size to the simulator,
and gave the customer the simulated numbers.  The next day, the new boards
arrived, and they re-ran their benchmarks, which matched the predictions
within about 1%!!  Needless to say, they were surprised.
Of course, we weren't surprised at all :-)  [UNIX timing granularity isn't
that good: if you can get within a few tenths of a second, you're lucky!]

Bottom line: simulation is necessary, but not sufficient.
-- 
-john mashey	DISCLAIMER: <generic disclaimer, I speak for me only, etc>
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