223. Spawn Example

223.1. Manager-worker Example Using MPI_COMM_SPAWN

/* manager */ 
#include "mpi.h" 
int main(int argc, char *argv[]) 
{ 
   int world_size, universe_size, *universe_sizep, flag; 
   MPI_Comm everyone;           /* intercommunicator */ 
   char worker_program[100]; 
 
   MPI_Init(&argc, &argv); 
   MPI_Comm_size(MPI_COMM_WORLD, &world_size); 
 
   if (world_size != 1)    error("Top heavy with management"); 
 
   MPI_Comm_get_attr(MPI_COMM_WORLD, MPI_UNIVERSE_SIZE,  
                     &universe_sizep, &flag);  
   if (!flag) { 
        printf("This MPI does not support UNIVERSE_SIZE. How many\n\ 
processes total?"); 
        scanf("%d", &universe_size); 
   } else universe_size = *universe_sizep; 
   if (universe_size == 1) error("No room to start workers"); 
 
   /*  
    * Now spawn the workers. Note that there is a run-time determination 
    * of what type of worker to spawn, and presumably this calculation must 
    * be done at run time and cannot be calculated before starting 
    * the program. If everything is known when the application is  
    * first started, it is generally better to start them all at once 
    * in a single MPI_COMM_WORLD.  
    */ 
 
   choose_worker_program(worker_program); 
   MPI_Comm_spawn(worker_program, MPI_ARGV_NULL, universe_size-1,  
             MPI_INFO_NULL, 0, MPI_COMM_SELF, &everyone,  
             MPI_ERRCODES_IGNORE); 
   /* 
    * Parallel code here. The communicator "everyone" can be used 
    * to communicate with the spawned processes, which have ranks 0,.. 
    * MPI_UNIVERSE_SIZE-1 in the remote group of the intercommunicator 
    * "everyone". 
    */ 
 
   MPI_Finalize(); 
   return 0; 
} 

/* worker */ 
 
#include "mpi.h" 
int main(int argc, char *argv[]) 
{ 
   int size; 
   MPI_Comm parent; 
   MPI_Init(&argc, &argv); 
   MPI_Comm_get_parent(&parent); 
   if (parent == MPI_COMM_NULL) error("No parent!"); 
   MPI_Comm_remote_size(parent, &size); 
   if (size != 1) error("Something's wrong with the parent"); 
 
   /* 
    * Parallel code here.  
    * The manager is represented as the process with rank 0 in (the remote 
    * group of) the parent communicator.  If the workers need to communicate 
    * among themselves, they can use MPI_COMM_WORLD. 
    */ 
 
   MPI_Finalize(); 
   return 0; 
}