[Beowulf] MPI programming question: Interleaved MPI_Gatherv?

[William Gropp]

At 12:44 AM 3/1/2005, Michael Gauckler wrote:
>Dear List,
>
>I would like to gather the data from several processes.
>Instead of the comonly used stride, I want to interleave
>the data:
>
>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
>AAAAABBBBBBCCCCCDDDDD).
>
>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.

Bill


>Thank you for your help!
>
>  Michael
>
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William Gropp
http://www.mcs.anl.gov/~gropp