David,
i guess you would have expected the default mapping/binding scheme is
core instead of sockets
iirc, we decided *not* to bind to cores by default because it is "safer"
if you simply
OMP_NUM_THREADS=8 mpirun -np 2 a.out
then, a default mapping/binding scheme by core means the OpenMP threads
end up doing time sharing.
this is an honest mistake (8 cores per task were not requested), so
having a default mapping/binding scheme by socket means
OpenMP threads are spread on the socket and will likely not do time sharing.
/* if you run on a single socket, or if you run 4 tasks on a dual socket
nodes, then (some) tasks do share the socket,
and depending on how the OpenMP runtime is implemented, two threads of
two distinct tasks could end up bound/running on the same core */
Cheers,
Gilles
On 9/30/2016 3:04 AM, David Shrader wrote:
Hello All,
Would anyone know why the default mapping scheme is socket for jobs
with more than 2 ranks? Would they be able to please take some time
and explain the reasoning? Please note I am not railing against the
decision, but rather trying to gather as much information about it as
I can so as to be able to better work with my users who are just now
starting to ask questions about it. The FAQ pretty much pushes folks
to the man pages, and the mpirun man page doesn't go in to the reasoning.
Thank you for your time,
David
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