implement MPI_scatterv

Parallel_Algorithm/MPI/Kmean_mpi.c

@@ -51,7 +51,7 @@ int main() {
  else{
  /*
  MPI_Scatter needs to access *X_all in all processes
- For non-root, we need to assign NULL for prevent memory error
+ For non-root, we need to assign NULL to prevent memory error
  */
  float* dummy_for_X_all=NULL;
  X_all = &dummy_for_X_all;
@@ -63,25 +63,56 @@ int main() {
  MPI_Bcast(&N_repeat,1,MPI_INT,0,MPI_COMM_WORLD);
  //printf("%d: %d,%d,%d,%d\n",rank,N_samples_all,N_features,N_clusters,N_repeat);
 
- /*
- Assume N_sample_all is divisible by size for now!
- TBD: use MPI_Scatterv to handle arbitrary size
- */
-
- N_samples = N_samples_all / size; 
- // printf("%d, Local samples: %d \n",rank,N_samples);
-
  if (rank==0){
  printf("Last element in global array: %f \n",X_all[N_samples_all-1][N_features-1]);
  }
 
+
+ // Naive Scatter: Assume N_sample_all is divisible by size 
+ /*
+ N_samples = N_samples_all / size; 
  X = Make2DFloatArray(N_samples,N_features);
  MPI_Scatter(*X_all, N_samples*N_features, MPI_FLOAT, *X,
  N_samples*N_features, MPI_FLOAT, 0, MPI_COMM_WORLD);
+ */
+
+ // Correct sactter: works for any numbers
+
+ int *sendcounts,*displs;
+ if (rank == 0){
+ int N_samples_slave = N_samples_all/size; //master node needs to know the data size for other nodes
+ N_samples = N_samples_all - N_samples_slave*(size-1);// the remaining data
+
+ sendcounts = (int *)malloc(size*sizeof(int)); // the number of elements to send to each processor
+ displs = (int *)malloc(size*sizeof(int)); //displacement relative to sendbuf for data sent to process i
+
+ sendcounts[0]=N_samples*N_features;
+ displs[0]=0;
+ for (i=1; i<size; i++) { 
+ displs[i] = (N_samples+(i-1)*N_samples_slave)*N_features; // continous data 
+ sendcounts[i] = N_samples_slave*N_features; 
+ } 
+ }
+ else{
+ //other nodes
+ N_samples = N_samples_all/size;
+
+ sendcounts = NULL; 
+ displs = NULL;
+ }
+
+ // This allocation works for all nodes
+ X = Make2DFloatArray(N_samples,N_features);
+
+ MPI_Scatterv(*X_all, sendcounts, displs,
+ MPI_FLOAT, *X, N_samples*N_features,
+ MPI_FLOAT, 0, MPI_COMM_WORLD);
+
+ //printf("%d, Local samples: %d \n",rank,N_samples);
 
  // check scattered results
  if (rank==size-1){
- printf("Last element after sacattering %d: %f \n",rank,X[N_samples-1][N_features-1]);
+ printf("Last element after scattering %d: %f \n",rank,X[N_samples-1][N_features-1]);
  }
 
  double iElaps1 = MPI_Wtime() - iStart1;