Gather

• Gather; the all-to-one gather operation is performed in MPI using the MPI_Gather function.

  int MPI_Gather(void *sendbuf, int sendcount, MPI_Datatype senddatatype, void *recvbuf, int recvcount, MPI_Datatype recvdatatype, int target, MPI_Comm comm)
  

• Each process, including the target process, sends the data stored in the array sendbuf to the target process.
• As a result, the target process receives a total of p buffers (p is the number of processors in the communication comm).
• The data is stored in the array recvbuf of the target process, in a rank order.
• That is, the data from process with rank i are stored in the recvbuf starting at location i * sendcount (assuming that the array recvbuf is of the same type as recvdatatype).

• The data sent by each process must be of the same size and type.
• That is, MPI_Gather must be called with the sendcount and senddatatype arguments having the same values at each process.
• The information about the receive buffer, its length and type applies only for the target process and is ignored for all the other processes.
• The argument recvcount specifies the number of elements received by each process and not the total number of elements it receives.
• So, recvcount must be the same as sendcount and their datatypes must be matching.

Figure 4.9: Diagram for Gather.

• MPI also provides the MPI_Allgather function in which the data are gathered to all the processes and not only at the target process.

  int MPI_Allgather(void *sendbuf, int sendcount, MPI_Datatype senddatatype, void *recvbuf, int recvcount, MPI_Datatype recvdatatype, MPI_Comm comm)
  

• The meanings of the various parameters are similar to those for MPI_Gather;
• However, each process must now supply a recvbuf array that will store the gathered data.

Figure 4.10: Diagram for All_Gather.