CODEWITHSUNDEEP
cmpi
dx_mrt
dx_mrt

Reputation: 717

error on mpi scatterv with long displacements

I'm using MPI_Scatterv to scatter a very large array:

MPI_Scatterv(list, counts, displacements, MPI_LONG, pSubList, counts[rank], MPI_LONG, MASTER, MPI_COMM_WORLD);

and I've declared the variables counts and displacements as long (the int type is not enough), however, the MPI_Scatterv only accepts int variables for counts and displacements.

Any idea on how can I somehow solve this?

Upvotes: 2

Views: 782

Answers (3)

Hristo Iliev
Hristo Iliev

Reputation: 74385

You can't substitute long for int, at least not in most MPI 2.2 implementations. Even if you create a user defined datatype as advised by Greg, you might (and mostly will) run into implementation limitations like using integer to represent offsets internally. The MPI 3.0 draft has specifically addressed this issue and requires that offset and count parameters be of long (64-bit) type in both C and Fortran bindings.

Until MPI 3.0 becomes standard and conformant implementations are out I would stick with some subcommunication scheme like distributing the data in several rounds with multiple scatters to subsets of ranks.

Upvotes: 2

Greg Inozemtsev
Greg Inozemtsev

Reputation: 4671

You could create a custom MPI datatype to represent a block of LONGs. Displacements and counts would then be multiples of that block size.

Upvotes: 0

Jay D
Jay D

Reputation: 3307

why not distribute your scatters as well so that int type will be enough ?

You can consider MPI processes running as the nodes of a Tree. And have the intermediate Tree Nodes have their own scatter/gathers of the corresponding sub-Trees.

Upvotes: 1

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