[mvapich-discuss] mvapich2 1.5.1p1 doesn't honour MV2_CPU_MAPPING when giving a cpu list

[Gilles Civario]

Hi,

For using efficiently codes parallelised using mixed MPI/OpenMP model, I usually attach one single MPI process to a set of cores. The OpenMP threads will thereafter inherit this attachment, and provided I chosen wisely my cpu set, run in an effective way.
I used to use MV2_CPU_MAPPING to specify the list of core to attach to, but following the move from PLPA to HWLOC, this environment variable is no longer properly honoured starting from mvapich2 1.5.
More precisely, only the first given core per MPI process is used to perform the process to core attachment. As I don't think this is intended behaviour and as I really miss this feature, I propose here a patch to allow to specify lists of core in MV2_CPU_MAPPING.
The lists are using decimal numbering of the cores. Each process on the node will be attached sequentially following there ranks to the lists given, separated by colons ':'.
Each core are specified by separating then with comas ',' or by giving ranges which extend is given by a dash '-'.
Eg: MV2_CPU_MAPPING="0,2,5-8:1,3:4" will attach MPI process of lower rank to cores {0,2,5,6,7,8}, then MPI process of following rank to cores {1,3}, and then the last one to core 4.
Please review the patch and tell me if it could be integrated.
Cheers.

Gilles




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