https://github.com/open-mpi/ompi/pull/292
George.
On Thu, Nov 27, 2014 at 7:45 AM, Jeff Squyres (jsquyres) <jsquy...@cisco.com
> wrote:
> Gilles' concern is correct: we should never return timer values that go
> backwards.
>
> Perhaps the TSC-based WTIME should only be used in a process that is bound
> to a single core...?
>
> An MCA param can be used to force the switch between gettimeofday() and
> TSC, if someone really wants to take their chances with TSC when not bound
> to core (or bound to something wider than a core).
>
>
>
> On Nov 27, 2014, at 5:41 AM, Alex A. Granovsky <g...@classic.chem.msu.su>
> wrote:
>
> > AFAIK, Linux synchronizes all CPU timers on boot. The skew is normally
> no more than 50-100 CPU cycles.
> >
> > The reasons why you can observe larger differences are:
> >
> > 1) Main. The CPUs do not have "constant TSC" feature . Without this
> feature timer frequency changes across different power states of CPU or
> core.
> > 2) Secondary. Some motherboard can overclock CPUs depending on load
> using FSB clock generator. This results in CPU timers ticking faster or
> slower than expected, even with "constant TSC" feature (which is no longer
> constant again).
> >
> > Kind regards,
> > Alex Granovsky
> >
> >
> >
> > -----Original Message----- From: Gilles Gouaillardet
> > Sent: Thursday, November 27, 2014 1:13 PM
> > To: Open MPI Users
> > Subject: Re: [OMPI users] mpi_wtime implementation
> >
> > Folks,
> >
> > one drawback of retrieving time with rdtsc is that this value is core
> > specific :
> > if a task is not bound to a core, then the value returned by MPI_Wtime()
> > might go backward.
> >
> > if i run the following program with
> > taskset -c 1 ./time
> >
> > and then move it accross between cores
> > (taskset -cp 0 <pid> ; taskset -cp 2 <pid>; ...)
> > then the program can abort. in my environment, i can measure up to 150ms
> > difference.
> >
> > /* some mtt tests will abort if this condition is met */
> >
> >
> > i was unable to observe this behavior with gettimeofday()
> >
> > /* though it could occur when ntpd synchronizes the clock */
> >
> > is there any plan to make the timer function selectable via a mca param ?
> > or to automatically fallback to gettimeofday if a task is not bound on a
> > core ?
> >
> > Cheers,
> >
> > Gilles
> >
> > $ cat time.c
> > #include <stdio.h>
> > #include <mpi.h>
> >
> > int main (int argc, char *argv[]) {
> > int i;
> > double t = 0;
> > MPI_Init(&argc, &argv);
> > for (;;) {
> > double _t = MPI_Wtime();
> > if (_t < t) {
> > fprintf(stderr, "going back in time %lf < %lf\n", _t, t);
> > MPI_Abort(MPI_COMM_WORLD, 1);
> > }
> > t = _t;
> > }
> > MPI_Finalize();
> > return 0;
> > }
> >
> > On 2014/11/25 1:59, Dave Goodell (dgoodell) wrote:
> >> On Nov 24, 2014, at 12:06 AM, George Bosilca <bosi...@icl.utk.edu>
> wrote:
> >>
> >>> https://github.com/open-mpi/ompi/pull/285 is a potential answer. I
> would like to hear Dave Goodell comment on this before pushing it upstream.
> >>>
> >>> George.
> >> I'll take a look at it today. My notification settings were messed up
> when you originally CCed me on the PR, so I didn't see this until now.
> >>
> >> -Dave
> >>
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>
> --
> Jeff Squyres
> jsquy...@cisco.com
> For corporate legal information go to:
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