[MPICH] Implementation of MPI_Alltoallw

[Sudarshan Raghunathan]

Hi,

There seem to be two ways of implementing MPI_Alltoallw: (A) using
non-blocking sends and receives and then performing a synchronization
step and (B) using a N pair-wise exchanges (i.e., using
MPI_SendRecv's).  Is it always true that (A) results in better
performance than (B) or are there cases where a naive pairwise
exchange would win?

I'm a bit confused because MPICH seems to be using (A) whereas LAM uses (B).

Thank you in advance,
Sudarshan