[Beowulf] MPI programming question: Interleaved MPI_Gatherv?
gropp at mcs.anl.gov
Thu Mar 3 06:23:10 PST 2005
At 12:44 AM 3/1/2005, Michael Gauckler wrote:
>I would like to gather the data from several processes.
>Instead of the comonly used stride, I want to interleave
>Rank 0: AAAAA -> ABCDABCDABCDABCDABCD
>Rank 1: BBBBB ----^---^---^---^---^
>Rank 2: CCCCC -----^---^---^---^---^
>Rank 3: DDDDD ------^---^---^---^---^
>Since the stride of the receive type is indicated
>in multpiles of its mpi_type, no interleaving is
>possible (the smallest striping factor leads to
>Is there a way to achieve this behaviour in an
>elegant way, as MPI_Gather promises it? Or do
>I need to do Send/Recv with self-aligned offsets?
You should be able to do this with MPI_Gather by creating a new datatype on
the receiving process whose extent is the size of a single item; that will
get you the correct offset for the first element. In order to receive the
subsequent elements into the desired location, you need to use a vector
type containing the number of elements. And for this to be fast, you need
an MPI implementation that will handle the "resized" datatype efficiently
(use MPI_Type_vector to create the full datatype and
MPI_Type_create_resized to change its effective extent). If you are moving
large amounts of data, separate send/recvs are probably a better choice.
>Thank you for your help!
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