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 5.3: 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 5.4: Diagram for All_Gather.

• In addition to the above versions of the gather operation, in which the sizes of the arrays sent by each process are the same, MPI also provides versions in which the size of the arrays can be different.
• MPI refers to these operations as the vector variants.

  int MPI_Gatherv(void *sendbuf, int sendcount, 
          MPI_Datatype senddatatype, void *recvbuf, 
          int *recvcounts, int *displs, 
          MPI_Datatype recvdatatype, int target, MPI_Comm comm) 
  
  int MPI_Allgatherv(void *sendbuf, int sendcount, 
          MPI_Datatype senddatatype, void *recvbuf, 
          int *recvcounts, int *displs, MPI_Datatype recvdatatype, 
          MPI_Comm comm)
  

• These functions allow a different number of data elements to be sent by each process by replacing the recvcount parameter with the array recvcounts.