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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