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SubjectRe: VM: 2.4.10 vs. 2.4.10-ac2 and qsort()
Date
On Thursday 04 October 2001 20:03, Alexei Podtelezhnikov wrote:
> Hi guys,
>
> I've already expressed my concern about using srand(1) in private e-mails.
> I think it's unscientific to use one particular random sequence. Since
> no one checked if that matters, I changed srand(1) to srand(time(NULL))
> and I'm posting my results. I don't do testing of Alan or Linus's kernels,
> but use recent Red Hat kernel. I think I've shown that it does matter.

One advantage of srand(1) is that it has a chance of being repeatable. You
don't WANT a truly random sequence, you want a sequence that simulates a
reproducable work load. That's why it makes sense to initialize the random
number generator with a known seed, so we can do it again after applying a
patch and see what kind of difference it made.

If you vary the initialization of the random number generator, your work load
isn't reproducible. It's the same TYPE of load, but not the same actual load
from one run to the next. If we're talking a 2% improvement expected out of
a particular tweak, you want to reduce random variation in the test case as
much as possible that might swamp your change. (We get enough variation
already from scheduling and random interrupts flushing cache state and
such...)

There's nothing wrong with using different seed values for srand, but testing
different VMs with different seed values has the possibility of being apples
to oranges.

If you want to run your test in a loop for a while to invoke statistics to
counter the randomness of your tests, you could do that too. But that's a
different kind of test. And one that's not nearly as convenient to run from
tweak to tweak...

Rob
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