[Beowulf] MPI programming question: Interleaved MPI_Gatherv?
Robert G. Brown
rgb at phy.duke.edu
Thu Mar 3 13:40:07 EST 2005
On Thu, 3 Mar 2005, Rob Ross wrote:
OK, having re-reread everything, I conclude that you were completely
right after all. I misunderstood what his question was. I'm still not
certain that I understand, but if Bill has answered it it definitely
isn't what I though.
So double-whomp. I'll go sleep now.
> William Gropp wrote:
> > 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
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Robert G. Brown http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
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