[mvapich-discuss] MPI_Bcast

[Mayhall, Anthony J. (MSFC-ES53)[TBE-1]]

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