In the preceeding chapters we mostly considered all MPI nodes or OpenMP thread as being in one flat pool. However, for high performance you need to worry about affinity : the question of which process or thread is placed where, and how efficiently they can interact.
Here are some situations where you affinity becomes a concern.
This asymmetry affects both MPI processes and threads on that node.
(and at TACC tacc_affinity )
can be used to pin a thread or process to a specific core.
Figure~ 44.2 depicts a Stampede compute node , which is a two-socket Intel Sandybridge design; figure~ 44.3 shows a Stampede largemem node , which is a four-socket design. Finally, figure~ 44.4 shows a Lonestar5 compute node, a~two-socket design with 12-core Intel Haswell processors with two hardware threads each.
See chapter~ OpenMP topic: Affinity for OpenMP affinity control.