Parallel MPI program delivery send message

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Preface

Achieve the goal ：
Each even numbered process sends a message to the odd numbered process on the right , And the change message is printed by the odd number process , For example, a total of 7 A process ,0 Send a message to 1 No. and by 1 Number printing ,2 Send a message to 3 No. and by 3 Number printing ,4 Send a message to 5 No. and by 5 Number printing ,6 No. will not send messages , Because there is no corresponding receiving process .
Then compile 、 misarrangement 、 perform , And summarize the learning experience

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One 、 Experimental principle and tips

MPI The program must first #include <mpi.h>.
all MPI All related calls must be located in MPI_Init() and MPI_Finalize() Two MPI Between functions .MPI_Comm_size(MPI_COMM_WORLD,&comm_sz)MPI_Comm_rank(MPI_COMM_WORLD, &my_rank) Two MPI Function calls are used to obtain the number of processes in the parallel program and the sequence number of the current process （rank）. A typical SPMD The structure is as follows ：
if(my_rank == x)
do something;
else
do other things;

Two 、 Experiment source code

The code is as follows ：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <mpi.h> 
const int MAX_STRING = 100;
int main(void) {
    
   char       greeting[MAX_STRING];      
   int        comm_sz;                  
   int        my_rank;                 
   MPI_Init(NULL, NULL);              
   MPI_Comm_size(MPI_COMM_WORLD, &comm_sz); 
   MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);   
   if (my_rank != 0) {
    
	  sprintf(greeting, "Greetings from process %d of %d!", my_rank, comm_sz);
        MPI_Send(greeting, strlen(greeting)+1, MPI_CHAR,0, 0, MPI_COMM_WORLD);
   } else {
    
       printf("Greetings from process %d of %d!\n", my_rank, comm_sz);
	   for (int q = 1; q < comm_sz; q++){
    
      MPI_Recv(greeting, MAX_STRING, MPI_CHAR, q,0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
      printf("%s\n", greeting);
      }
   }   
   MPI_Finalize();
   return 0;
}

emmm, This is still not very good. Roll it inside , Do you want to go again

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <mpi.h> 
const int MAX_STRING = 100;
int main(void) {
    
   char       greeting[MAX_STRING];      
   int        comm_sz;                  
   int        my_rank;                  
   MPI_Init(NULL, NULL);               
   MPI_Comm_size(MPI_COMM_WORLD, &comm_sz); 
   MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
   int src, dst;
   if(my_rank % 2 != 0)
     src = my_rank - 1;
   else
     dst = my_rank + 1;
   
   if (my_rank % 2 != 0) {
    
      MPI_Recv(greeting, MAX_STRING, MPI_CHAR, src, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
      printf("Message printed by process %d: %s\n", my_rank, greeting);
   } else {
    
      if(dst != comm_sz) {
    
        sprintf(greeting, "Greetings from process %d of %d!", my_rank, comm_sz);
        MPI_Send(greeting, strlen(greeting)+1, MPI_CHAR, dst, 0, MPI_COMM_WORLD);  
      }
   }
   MPI_Finalize(); 
   return 0;
} 

3、 ... and 、 Experimental results

3、 ... and 、 Problems encountered in the experiment and solutions

1. Do not understand the compiled program , Don't use your own settings correctly comm_sz、my_rank Two variables , Cause program error .
resolvent ： Read the relevant procedures , Understanding usage , At the same time, I keep trying and improving .
2. Send a message to the odd numbered process on the right of the process that implements each even numbered process , And the function of printing messages by odd numbered processes , No further thought , Just think through ifelse Statement to distinguish , Failure to consider the possible phenomenon of process idling leads to poor implementation of the effect .
resolvent ： By increasing in advance else Statement will , Singular processes are distinguished from even processes , Then enter the task of sending and receiving in the judgment statement .

summary

Parallel programs need to pay attention to many points , It is recommended that beginners run it manually first to determine where they need to pay attention , Write pseudocode , And then re programming , The lesson of blood and tears , The reason why I wrote this , I hope it can help the latecomers .