Re: CLOCK_MONOTONIC / CLOCK_UPTIME is not really monotonic between threads

Yeah, indeed false positive, app error. Sorry for noise and thanks for
feedback.

-Max
On Jun 3, 2016 10:18 AM, "Konstantin Belousov" <kostikbel_at_gmail.com> wrote:

> On Fri, Jun 03, 2016 at 08:04:29AM -0700, Maxim Sobolev wrote:
> > Konstantin,
> >
> > Thanks for taking your time looking into it and sorry for somewhat messy
> > problem report. I've been trying to fix that problem all day yesterday
> > thinking it's just application logic that is broken and indeed has been
> > able to find some bigger issues that were obscuring this one. But it got
> > very frustrating when the bug popped out anew at a seemingly lower level
> > now. The issue that triggered this is in some high level python code.
> Which
> > makes it quite difficult to narrow and isolate. There is still slight
> > chance that it's something about threading within the python that screws
> > this up somehow, however I don't quite see how that could lead to a
> > consistent result that is just off by few hundred microseconds and not in
> > some random garbage.
> >
> > So, I take from you message, that high level
> > clock_gettime(CLOCK_MONOTONIC*) is supposed to be monotonic with respect
> to
> > the wall time even when called in different threads? I always though it
> is,
> > but was not 100% sure about that and wanted to confirm it before I dive
> > deeper into this and spend more time writing a test case to expose this.
> Yes, CLOCK_MONOTONIC should be monotonic across all processors.
> Until the time travel is made possible, of course.
>
> > The test case you gave me is interesting, but somewhat low-level. What I
> > would do if it comes to it, is to make something that uses pthreads and
> > plain clock_gettime(2). Should not be too difficult to reproduce if it's
> > real issue.
> The test I give you verifies clock_gettime() in several threads going
> backward.
>
> >
> > P.S. I've also tried kern.timecounter.fast_gettime=0, made no difference.
> > Assuming it does not take a reboot to test it. Neither does
> > switching kern.timecounter.hardware, I've tested TSC-low(1000)
> > ACPI-fast(900) HPET(950) i8254(0), all are the same.
> I am almost sure this is app-level issue.
>
> To make me confident, run the test I provided.
>
>