Dave, gettimeofday() uses (seconds, microseconds) to represent the time, and hence, the resolution is hardcoded to 1 microsecond clock_gettine() uses (seconds, nanoseconds) and hence the resolution is hard coded to 1 nanosecond. this is the max resolution, and it could be lower than that depending on what gettimeofday() does under the hood. /* I remember there was an issue with the first software stack of MPSS, the Xeon phi o/s, and once in a while, the effective time resolution dropped to 100 Hz, and the MPI application has no way to be made aware of that */
bottom line, in OpenMPI, you should not expect a resolution higher than MPI_Wtick(), and yes, it might be (way) worst than that Cheers, Gilles On Friday, April 8, 2016, Dave Love <d.l...@liverpool.ac.uk> wrote: > George Bosilca <bosi...@icl.utk.edu <javascript:;>> writes: > > >> Other implementations of MPI have very accurate counters. > >> > > > > This might be a discutable topic. A quick survey of some of the MPI > > libraries available on a Linux cluster give the following precision for > > MPI_Wtime implementation : > > > > mpich-3.1.4: wtick = 1.000000e-06 > > Intel(R) MPI Library 5.1.1 for Linux*: wtick = 1.000000e-06 > > ompi-1.10.2: wtick = 1.000000e-06 > > ompi-master: wtick = 1.000000e-09 > > Can you trust wtick anyhow? OMPI 1.10 says it's always 10⁻⁶, which > isn't necessarily realistic even if it uses gettimeofday with a nominal > 1μs resolution. At least with some (older?) Linuxes, and possibly other > kernels, gettimeofday is limited to the kernel tick rate -- a few 100 Hz > if I recall correctly. I'd need convincing about 1ns generally for the > real time clock too. > > Anyhow, experimentally, RHEL6-packaged mpich 3.1's wtime calls > gettimeofday and not clock_gettime; likewise impi 4.1. > > Thanks for fixing ompi, by the way. > _______________________________________________ > users mailing list > us...@open-mpi.org <javascript:;> > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users > Link to this post: > http://www.open-mpi.org/community/lists/users/2016/04/28910.php
