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Subject [GitHub] [singa-doc] chrishkchris edited a comment on issue #14: rearrange contents in dist-train.md
Date Sat, 04 Apr 2020 08:33:45 GMT
chrishkchris edited a comment on issue #14: rearrange contents in dist-train.md
URL: https://github.com/apache/singa-doc/pull/14#issuecomment-608995695
 
 
   > Is nccl_id passed to `train_mnist_cnn` for training with MPI?
   No, in the case of MPI, nccl_id is generated and immediately broadcasted by MPI from rank0
to every rank.  
   https://github.com/apache/singa/blob/master/src/io/communicator.cc#L102 
   ```
     if (MPIRankInGlobal == 0) ncclGetUniqueId(&id);
     MPICHECK(MPI_Bcast((void *)&id, sizeof(id), MPI_BYTE, 0, MPI_COMM_WORLD));
   ```
   However, for multiprocess what I can do is to generate it at the beginning and pass it
to python multiprocess function
   
   > For multiprocessing, num_gpu = gpu_per_node, hence we only need num_gpu?
   
    This is reserve for future use, because previously I planned using network socket to broadcast
ncclid to other nodes instead of MPI. 

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