[mvapich-discuss] MPI_Bcast

[Krishna Chaitanya Kandalla]

Anthony,
        Previously, we had explored the IB multicast option to implement a few of the collectives in MVAPICH. However, we found that it was not a scalable option. In MVAPICH2, we implement all collective operations using either point-to-point or shared-memory based algorithms. 
        Getting back to your question, for the message sizes that you have mentioned (40K), we are currently using a shared-memory based algorithm to implement the MPI_Bcast operation. Which earlier version of MVAPICH/MVAPICH2 are you comparing these results with and what is the performance difference that you are observing? Also, you mentioned having to broadcast to one executable on each node, does your job involve running different executables on each node? Or do you mean you are having just one process running on each node? 

Thanks,
Krishna
        



Mayhall, Anthony J. (MSFC-ES53)[TBE-1] wrote:
> How is MPI_Bcast implemented in mvapich 1.4?  Can it use IB multicast?  If so, how do you turn that on?  It is currently taking our apps a lot longer to broadcast using MPI_Bcast to 10 nodes (175us for 40K) vs. 2 (46us for 40K) nodes.  We are only broadcasting to one executable on each node. A multicast should take the same amount of time regardless of number of nodes wouldn't it?  I used the multicast_test.c code to test IB multicast and it seems to not matter much how many nodes are being used as far as the timing goes.  Can the Bcast buffer be broken into multiple 2048 byte transfers in MPI_Bcast to use IB multicast?  I thought I saw in a white paper that these methods were being taken advantage of in mvapich.  
>
> Or are we just doing something wrong?
>
> Thanks,
>
> Anthony Mayhall
> Davidson Technologies, Inc.
> (256)544-7620
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