CODEWITHSUNDEEP
mpi
inJeans
inJeans

Reputation: 199

MPI Communication Pattern

I was wondering if there was a smart way to do this. Let's say I have three nodes, 0, 1, 2. And let's say each node has an array, a0, a1, a2. If the contents of each node is something like

a0 = {0, 1, 2, 1}
a1 = {1, 2, 2, 0}
a2 = {0, 0, 1, 2}

Is there a clever communication pattern so to move each number to it's corresponding node, i.e.

a0 = {0, 0, 0, 0}
a1 = {1, 1, 1, 1}
a2 = {2, 2, 2, 2}

The approach I have in mind, would involve sorting and temporary buffers, but I was wondering if there was a smarter way?

Upvotes: 0

Views: 184

Answers (1)

Zulan
Zulan

Reputation: 22660

You can use MPI_Alltoallv for this in the following way:

  1. Sort the local_data (a) by corresponding node of each element in increasing order.
  2. Create a send_displacements array such that send_displacements[r] indicates the index of the first element in the local_data that refers to node r.
  3. Create a send_counts array such that send_counts[r] equals the number of elements in local_data that correspond to node r. This can be computed send_counts[r] = send_displacements[r+1] - send_displacements[r] except for the last rank.
  4. MPI_Alltoall(send_counts, 1, MPI_INT, recv_counts, 1, MPI_INT, comm)
  5. Compute recv_displacements such that recv_displacements[r] = sum(recv_counts[r'] for all r' < r).
  6. Prepare a recv_data with sum(recv_counts) elements.
  7. MPI_Alltoallv(local_data, send_counts, send_displacements, MPI_INT, recv_data, recv_counts, recv_displacements, MPI_INT, comm)

Upvotes: 2

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