You're trying to read absurd huge message sizes considering you're busy testing the memory bandwidth of your system in this manner.
As soon as the message gets larger than your CPU's caching system it has to copy the message several times via your RAM, falls outside CPU's L2 or L3 cache and bandwidth drops.
This has nothing to do with OpenMPI i'd say. On Thu, 10 Mar 2016, BRADLEY, PETER C PW wrote:
I’m curious what causes the hump in the pingpong bandwidth curve when running on shared memory. Here’s an example running on a fairly antiquated single-socket 4 core laptop with linux (2.6.32 kernel). Is this a cache effect? Something in OpenMPI itself, or a combination? Macintosh HD:Users:up:Pictures:bandwidth_onepair_onenode.png Pete
