[mvapich-discuss] mvapich2 slow with default mapping

[Götz Waschk]

Dear Mvapich2 experts,

I'm currently evaluating OpenHPC packages, including mvapich2 2.1.
I've tested the speed using the Intel MPI benchmarks and I have
noticed, that the first benchmark PingPong is behaving differently
when run with 16 cores vs. 2 cores, although only two cores are in use
and the remaining processes simply wait. The full results are in
OpenHPC's issue tracker on github:
https://github.com/openhpc/ohpc/issues/207#issuecomment-212319647

As you can see there, the configuration change to set these variables helped:

export MV2_SHOW_CPU_BINDING=1
export MV2_CPU_MAPPING=0:1:2:3:4:5:6:7

I still wonder why they have such an influence and the default setting
isn't sufficient here.

Regards,
Götz Waschk