proc4.cpp

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#include <mpi.h>
#include <vector>
using namespace std;

#define WORKTAG 1
#define DIETAG 2


/* Local functions */

const int numEntries = 10;
const int totCalls = 2 * 65536;
static vector< vector< double > > recvBuf;
static int clearPending( int numNodes, int myrank, MPI_Request *recvReq,
               double& tot );

static double* get_next_work_item( int numCalls )
{
    static vector< double > ret( numEntries );
    for ( int i = 0; i < numEntries; ++i )
        ret[i] = i + numCalls;

    if ( numCalls >= totCalls )
        return 0;
    return &ret[0];
}

static double doWork(double* work);

int main(int argc, char **argv)
{
  double tot = 0.0;
  double tc = totCalls;
  double ne = numEntries;
  double expectedTot =
        tc * ( ( ne * (ne - 1.0) )/2.0 ) +
        ne * ( tc * (tc - 1.0) )/2.0;
  int myrank;
  int numNodes;

  /* Initialize MPI */

  MPI_Init(&argc, &argv);

  /* Find out my identity in the default communicator */

  MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
  MPI_Comm_size(MPI_COMM_WORLD, &numNodes);

  double sendBuf[numNodes * numEntries];
  double recvBuf[ numEntries * numNodes ];

  int numCallsPerNode = totCalls / numNodes;

  int begin = myrank * numCallsPerNode;
  int end = begin + numCallsPerNode;

  int k = 0;
  for ( int i = 0; i < numCallsPerNode; ++i ) {
    double* work = get_next_work_item( i + begin );
    int targetNode = i % numNodes;
    for ( int j = 0; j < numEntries; ++j ) {
        sendBuf[targetNode * numEntries + j ] = work[j];
    }

    if ( targetNode == numNodes - 1 ) {
        for ( int j = 0; j < numNodes; ++j ) {
            MPI_Scatter( sendBuf, numEntries, MPI_DOUBLE,
                recvBuf, numEntries * (1 + j ), MPI_DOUBLE, j,
                MPI_COMM_WORLD
            );
            tot += doWork( recvBuf );
        }
    }
  }
  // Final pass to consolidate all the data
  double totBuf[ numNodes ];
  MPI_Allgather( &tot, 1, MPI_DOUBLE,
                  totBuf, 1, MPI_DOUBLE,
                  MPI_COMM_WORLD
               );
  double subtot = tot;
  tot = 0;
  for ( int i = 0; i < numNodes; ++i )
  {
         tot += totBuf[i];
  }

  cout << myrank << ": Tot = " << tot <<
          ", expected = " << expectedTot <<
          ", subtot = " << subtot << endl;

  /* Shut down MPI */

  MPI_Finalize();
  return 0;
}


static double
doWork(double* work)
{
    double tot = 0;
    for (int i =0; i < numEntries; ++i )
        tot += work[i];
}