On Mon, 2010-06-21 at 10:02 -0300, Glauber Costa wrote:
> On Fri, Jun 18, 2010 at 11:35:18AM -0700, Michael Davidson wrote:
> > Yes - on some Intel platforms the roundtrip time can be as much as 1800
> > clock
> > cycles - with the default test threshold of 500 cycles that always fails the
> > "roundtrip > 2 * threshold" test and we exit the loop without ever having
> > updated
> > the delta value.
> >
> > Note that the actual deta on those platforms is only a few tens of cycles.
> >
> > Most importantly, on looking more closely at the code that was added in
> > this change, it really doesn't do anything useful.(other than cause the test
> > to fail on platforms where the roundtrip time his high, even if the measured
> >
> > delta is low). It isn't as if doing a continue at that point will give you
> > another
> > shot at running the test - the loop executes a fixed number of times and
> > sets the result base on the "best" roundtrip time - continuing the loop
> > early
> > if the roundtrip time is large is pointless.
> >
> The first problem is that the patch was not applied in the way I sent.
>
> I sent:
>
> - for (i = 0; i < NUM_ITERS; i++) {
> + while (i < NUM_ITERS) {
>
> ...
>
> + i++;
>
> This would guarantee that if a high roundtrip was found, we would not
> count this as a valid execution. This is very different from what it is
> in the repository right now, which is clearly wrong.
Ok, reverted in r4654. I am sorry for the mess I made.
> Other than thta, if there is a platform where 2 * threshold is so common,
> maybe we can change it to something bigger (maybe 5?). My main point was to
> rule out
> scheduling issues in userspace. When they happen, they are likely to be quite
> big.
>
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