For those Who know MPI

**Chris_1980** · 11-08-2010

In the code below. I splitt the comm WORLD_COMM in 6 subgroups and Allreduce the rank before splitting.

Code:

  #include "mpi.h" 
#include <stdio.h>
int main(argc,argv) 
int argc; 
char *argv[]; { 
  int rank,rank2,group_size,group_size2,comm_size,comm_size2,new_rank, sendbuf, recvbuf, server=0;
 int ranks[1];
 int array2, array1;
 int color=5;
 MPI_Group workers_group, world_group; 
 MPI_Comm world,workers_comm; 
 MPI_Init(&argc,&argv); 
 MPI_Comm_rank(MPI_COMM_WORLD, &rank);
 MPI_Comm_size(MPI_COMM_WORLD, &comm_size);
 array1=rank;
 printf("rank= %d comm_size=%d \n",rank, comm_size);

 world=MPI_COMM_WORLD;

 MPI_Barrier(world);

 if (rank == 0){color=0;}
 if ((rank>0) && (rank<4)){color=1;}
 if ((rank>3)&& (rank<7)){color=2;}
 if ((rank>6)&& (rank<10)){color=3;}
 if ((rank>9)&& (rank<21)){color=4;}


 MPI_Comm_split(world,color,rank,&workers_comm);

 MPI_Comm_rank(workers_comm,&rank2);
 MPI_Comm_size(workers_comm,&comm_size2);


 printf("size=%d  size2= %d comm_size2=%d new rank= %d\n",group_size,group_size2, comm_size2,rank2);
 
 
 MPI_Allreduce(&array1,&array2,comm_size2,MPI_INT,MPI_SUM,workers_comm);
 

 printf("array1=%d  array2= %d comm_size2=%d new rank= %d\n",array1,array2, comm_size2,rank2);
 
 MPI_Barrier(workers_comm);
 MPI_Comm_free(&workers_comm);
 MPI_Finalize();
}

when I am using 10 processes the outcome seems ok:

Code:

rank= 1 comm_size=10 
rank= 2 comm_size=10 
rank= 3 comm_size=10 
rank= 5 comm_size=10 
rank= 4 comm_size=10 
rank= 0 comm_size=10 
rank= 6 comm_size=10 
rank= 7 comm_size=10 
rank= 9 comm_size=10 
rank= 8 comm_size=10 
size=4414736  size2= 0 comm_size2=3 new rank= 2
size=4414736  size2= 0 comm_size2=3 new rank= 0
size=4414736  size2= 0 comm_size2=3 new rank= 1
size=4414736  size2= 0 comm_size2=3 new rank= 2
size=4414736  size2= 0 comm_size2=3 new rank= 2
size=4414736  size2= 0 comm_size2=3 new rank= 1
size=4414736  size2= 0 comm_size2=3 new rank= 0
array1=2  array2= 6 comm_size2=3 new rank= 1
array1=3  array2= 6 comm_size2=3 new rank= 2
size=4414736  size2= 0 comm_size2=3 new rank= 0
array1=5  array2= 15 comm_size2=3 new rank= 1
array1=4  array2= 15 comm_size2=3 new rank= 0
size=4414736  size2= 0 comm_size2=1 new rank= 0
array1=1  array2= 6 comm_size2=3 new rank= 0
size=4414736  size2= 0 comm_size2=3 new rank= 1
array1=6  array2= 15 comm_size2=3 new rank= 2
array1=0  array2= 0 comm_size2=1 new rank= 0
array1=8  array2= 24 comm_size2=3 new rank= 1
array1=7  array2= 24 comm_size2=3 new rank= 0
array1=9  array2= 24 comm_size2=3 new rank= 2

But when i am using 14 or above I am getting strange numbers for the size and rank of the new comm (comm_size2)

Code:

rank= 1 comm_size=20 
rank= 2 comm_size=20 
rank= 3 comm_size=20 
rank= 4 comm_size=20 
rank= 5 comm_size=20 
rank= 6 comm_size=20 
rank= 8 comm_size=20 
rank= 9 comm_size=20 
rank= 11 comm_size=20 
rank= 7 comm_size=20 
rank= 10 comm_size=20 
rank= 14 comm_size=20 
rank= 15 comm_size=20 
rank= 0 comm_size=20 
rank= 16 comm_size=20 
rank= 12 comm_size=20 
rank= 17 comm_size=20 
rank= 18 comm_size=20 
rank= 19 comm_size=20 
rank= 13 comm_size=20 
size=4414736  size2= 0 comm_size2=3 new rank= 0
size=4414736  size2= 0 comm_size2=3 new rank= 2
size=4414736  size2= 0 comm_size2=10 new rank= 5
size=4414736  size2= 0 comm_size2=3 new rank= 2
size=4414736  size2= 0 comm_size2=3 new rank= 0
size=4414736  size2= 0 comm_size2=10 new rank= 2
size=4414736  size2= 0 comm_size2=10 new rank= 7
size=4414736  size2= 0 comm_size2=3 new rank= 1
array1=8  array2= 24 comm_size2=3 new rank= 1
array1=9  array2= 24 comm_size2=3 new rank= 2
size=4414736  size2= 0 comm_size2=10 new rank= 6
array1=7  array2= 24 comm_size2=3 new rank= 0
size=4414736  size2= 0 comm_size2=3 new rank= 2
size=4414736  size2= 0 comm_size2=3 new rank= 1
size=4414736  size2= 0 comm_size2=10 new rank= 3
array1=2  array2= 6 comm_size2=3 new rank= 1
array1=1  array2= 6 comm_size2=3 new rank= 0
size=4414736  size2= 0 comm_size2=1 new rank= 0
array1=3  array2= 6 comm_size2=3 new rank= 2
size=4414736  size2= 0 comm_size2=10 new rank= 1
array1=0  array2= 0 comm_size2=1 new rank= 0
size=4414736  size2= 0 comm_size2=3 new rank= 1
size=4414736  size2= 0 comm_size2=3 new rank= 0
size=4414736  size2= 0 comm_size2=10 new rank= 0
size=4414736  size2= 0 comm_size2=10 new rank= 8
array1=5  array2= 15 comm_size2=3 new rank= 1
array1=6  array2= 15 comm_size2=3 new rank= 2
array1=4  array2= 15 comm_size2=3 new rank= 0
size=4414736  size2= 0 comm_size2=10 new rank= 4
size=4414736  size2= 0 comm_size2=10 new rank= 9
array1=19  array2= 145 comm_size2=327670 new rank= 200
array1=15  array2= 145 comm_size2=327670 new rank= 200
array1=17  array2= 145 comm_size2=327670 new rank= 200
array1=18  array2= 145 comm_size2=327670 new rank= 200
array1=14  array2= 145 comm_size2=327670 new rank= 200
array1=13  array2= 145 comm_size2=327670 new rank= 200
array1=12  array2= 145 comm_size2=327670 new rank= 200
array1=16  array2= 145 comm_size2=327670 new rank= 200
array1=11  array2= 145 comm_size2=327670 new rank= 200
array1=10  array2= 145 comm_size2=327670 new rank= 200

Any ideas of why it is happening?

**Codeplug** · 11-08-2010

'group_size' and 'group_size2' are uninitialized variables, so their values mean nothing.

You are splitting your processes into "buckets":

Code:

[rank] = bucket X
--------------------
[0]     = bucket 0
[1-3]   = bucket 1
[4-6]   = bucket 2
[7-9]   = bucket 3
[10-20] = bucket 4
[21-N]  = bucket 5

With 10 processes, you'll have 4 groups (buckets) containing 1, 3, 3, and 3 processes.
With 20 processes, you'll have 5 groups containing 1, 3, 3, 3, and 10 processes.

The call to MPI_Allreduce is incorrect. The third parameter is the number of elements in the send buffer (first parameter), which should be 1 in this case.

gg

**Chris_1980** · 11-08-2010

Dear codeplug,

Many thanks for your reply. You helped me a lot. I couldnt spot this

Best,
Chris

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