George Bosilca <bosi...@icl.utk.edu> 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.
