chrishkchris edited a comment on issue #14: rearrange contents in dist-train.md
URL: https://github.com/apache/singa-doc/pull/14#issuecomment-609034750
 
 
   > Then would it be better to design the APIs in this way:
   > 
   > 1. For training with MPI
   > 
   > ```python
   >  # in mnist_mpi.py
   > if __name__  == '__main__':
   >   sgd = ...
   >   sgd = DistOpt(sgd)
   >   train_mnist(sgd, sparse, topK)
   > ```
   > 
   > 1. For training via multiprocessing
   > 
   > ```python
   >  # in mnist_multiprocessing.py
   > if __name__  == '__main__':
   >   nccl_id = ...
   >   sgd = ...
   >   sgd = DistOpt(sgd, num_gpu, nccl_id)
   >   train_mnist(sgd, sparse, topK)
   > ```
   > 
   > Even if you use socket, multiprocessing can only run on a single node, 
hence num_gpu = gpu_per_node.
   
   Sorry, I updated my comment:
   
   num_gpu is the local rank of a specific process
   gpu_per_node is the total number of ranks in a single node
   
   see 
https://github.com/apache/singa/blob/master/examples/autograd/mnist_multiprocess.py#L42
   ```
   for gpu_num in range(0, gpu_per_node):        
           process.append(multiprocessing.Process(target=train_mnist_cnn, 
args=(sgd, max_epoch, 
          batch_size, True, data_partition, gpu_num, gpu_per_node, nccl_id)))
   ```

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