Allgatherv

Allgatherv, commonly written MPI_Allgatherv, is a collective communication operation in the Message Passing Interface (MPI). It gathers data from all processes in a communicator and distributes the concatenated result to all processes, allowing each process to contribute and receive a different amount of data.

The operation takes the following parameters: sendbuf, sendcount, and sendtype specify the local data to send from each process; recvbuf, recvcounts, displs, and recvtype describe how much data each process will receive and where it will be placed in the receive buffer on every process; comm is the communicator containing the participating processes. recvcounts is an array of length equal to the number of processes, where recvcounts[i] is the number of elements to receive from process i; displs is an array of the same length specifying the starting offset in recvbuf for the data from process i. The total size of the receive buffer must be the sum of all recvcounts.

Each process contributes sendcount elements of type sendtype from sendbuf (or may use MPI_IN_PLACE to indicate that the input data is already present in the receive buffer). After the call, every process owns the complete concatenation of data from all processes, with blocks placed at positions defined by displs and sized by recvcounts. The operation differs from MPI_Allgather in that Allgatherv allows varying receive counts from different processes.

Allgatherv is a collective operation; all processes in the communicator must participate. It is commonly used in parallel algorithms where process data sizes differ and a uniform Allgather would be inefficient or impossible.