DetermineLocalSize.cpp

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/* \file DetermineLocalSize.cpp
 * \brief: routine to determine local size of a global petsc matrix or vector. 
 */
 
#include <stdio.h>
#include <math.h>
#include "../../../shared/shared.h"
 
int DetermineLocalSize(int global_size,ISSM_MPI_Comm comm){
 
   /*output: */
   int  local_size;
 
   /*intermediary: */
   int  i;
   int  row_rest;
   int* num_local_rows=NULL;
 
   /*from MPI: */
   int num_procs;
   int my_rank;
 
   /*recover my_rank*/
   ISSM_MPI_Comm_rank(comm,&my_rank);
   ISSM_MPI_Comm_size(comm,&num_procs);
 
   /* TODO replace the following with ->
 
   local_size=global_size/num_procs; // integer division
   if (global_size%num_procs>my_rank) local_size++; // distribute the remainder
   return local_size;
 
   <- to here  */
 
   /*We are  not bound by any library, just use what seems most logical*/
   num_local_rows=xNew<int>(num_procs);    
 
   for (i=0;i<num_procs;i++){
      /*Here, we use floor. We under distribute rows. The rows 
        left  are then redistributed, therefore resulting in a 
        more even distribution.*/
      num_local_rows[i]=(int)floor((double)global_size/(double)num_procs); 
   }
 
   /*There may be some rows left. Distribute evenly.*/ 
   row_rest=global_size - num_procs*(int)floor((double)global_size/(double)num_procs);
   for (i=0;i<row_rest;i++){
      num_local_rows[i]++;
   }
   local_size=num_local_rows[my_rank];
 
   /*free ressources: */
   xDelete<int>(num_local_rows);
 
   /*return size: */
   return local_size;
 
}