从接收方的角度来看，如何确保已收到使用 MPI_Isend 发送的所有消息？

从接收方端，我如何确保使用MPI_Isend发送的所有消息都已收到?- MPI不提供这个功能，你只能知道所有的MPI_Isend操作已经完成。

换句话说，基本上接收者不知道发送者会发送多少信息。但是发送者知道什么时候他们没有更多的消息要发送。那么，你能不能用一条消息通知接收方从秩n开始的消息将不会再收到任何消息?

你的代码正在处理另一个问题，你如何确保所有的MPI_Isend操作已经完成?

下面是基于您的示例的代码。我没有使用MPI_Iprobe，因为在MPI_Iprobe和if语句之间没有计算。相反，我使用了MPI_Probe。

下面的代码，确保所有的消息都已经发送，并且当一个进程从所有其他进程接收到stopTAG消息时，它将停止接收消息。

#include "mpi.h"
#include <stdbool.h>
#include "stdio.h"
#include "stdlib.h"
int main(int argc, char* argv[]) 
{
    MPI_Init(&argc, &argv);
    int rank, size;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    //ASSUMPTION: using 4 procs
    //Don't worry about this part.
    //just some stupid way to determine the receivers.
    // Irrelevant to the question.
    int recvs[3];   
    int i = 0, nei = 0; 
    for (; nei < 4; ++nei)
    {
        if (nei != rank)        
        {
            recvs[i] = nei;     
            ++i;        
        }
    }
    //Proc sending msgs to its neighbors.
    //In this case, it's all other procs. (but in my real app, it's almost never the case)
    int TAG = 0;
    int stopTAG = 1;
    int buff[4] = {555, 666, 777, 888};
    MPI_Request req[3*5];
    for (nei = 0; nei < 3; ++nei)
    {
        for (i = 0; i < 4; ++i)
        { 
            MPI_Isend(&buff[i], 1, MPI_INT, recvs[nei], TAG, 
                        MPI_COMM_WORLD, &req[nei * 5 + i]);
        }
    }
    for (nei = 0; nei < 3; ++nei) {
        MPI_Isend(NULL, 0, MPI_CHAR, recvs[nei], stopTAG, MPI_COMM_WORLD, 
                  &req[nei * 5 + 4]);
    }
    //Receiving
    int recvCount = 0;      
    MPI_Status status;
    int hasMsg = 0;     
    int num; 
    char stopArray[size];
    for (i = 0; i < size; i++) {
        stopArray[i] = 0;
    }
    stopArray[rank] = 1;
    char stop;
    int completedSends = 0;
    do  
    {
        MPI_Probe(MPI_ANY_SOURCE, MPI_ANY_TAG, MPI_COMM_WORLD, &status);
        if (status.MPI_TAG == TAG)
        {   
            MPI_Recv(&num, 1, MPI_INT, status.MPI_SOURCE, TAG, 
                        MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            ++recvCount;
            printf("Rank %d got %d from %dn", rank, num, 
                        status.MPI_SOURCE);
        }
        else if (status.MPI_TAG == stopTAG) {
            MPI_Recv(NULL, 0, MPI_CHAR, status.MPI_SOURCE, stopTAG, 
                        MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            stopArray[status.MPI_SOURCE] = 1;
        }
        stop = 1;
        for (i = 0; i < size; i++) {
            stop &= stopArray[i];
        }
        if (completedSends < (3*5)) {
            int indx;
            MPI_Status status;
            MPI_Waitany(3*5, req, &indx, &status);
            completedSends++;
        }
    }
    while (!stop && (completedSends <= 15));
    //Total number msgs received by all procs.
    //Now here's where I see the problem!!!
    int global_count;
    MPI_Allreduce(&recvCount, &global_count, 1, MPI_INT, MPI_SUM, 
                    MPI_COMM_WORLD); 
    printf("nRank %d receives %d msgs;nGlobal count = %dn", rank, 
                recvCount, global_count);
    MPI_Finalize();
    return 0;
}