For the general case, you can probably make your code a lot simpler
with MPI_COMM_SPLIT. Perhaps something like the following (off the
top of my head -- be sure to check this for correctness!):
// easy case -- even number of procs
if (size % 2 == 0) {
MPI_Comm_split(MPI_COMM_WORLD, rank / 2, 0, &comm1);
MPI_Comm_split(MPI_COMM_WORLD, ((rank + 1) % size) / 2, 0,
&comm2);
if (rank % 2 == 0) {
comm_right = comm1;
comm_left = comm2;
} else {
comm_right = comm2;
comm_left = comm1;
}
}
// odd number of procs
else {
// first exclude the last proc
color = (rank == size - 1) ? MPI_UNDEFINED : rank / 2;
MPI_Comm_split(MPI_COMM_WORLD, color, 0, &comm1);
// now exclude the first proc
color = (rank == 0) ? MPI_UNDEFINED : (rank + 1) / 2;
MPI_Comm_split(MPI_COMM_WORLD, color, 0, &comm2);
if (rank % 2 == 0) {
comm_right = comm1;
comm_left = comm2;
} else {
comm_right = comm2;
comm_left = comm1;
}
// now make the comm between 0 and the last proc
color = (rank == 0 || rank == size - 1) ? 0 : MPI_UNDEFINED;
MPI_Comm_split(MPI_COMM_WORLD, color, 0, &comm1);
if (rank == 0) {
comm_left = comm1;
} else if (rank == size - 1) {
comm_right = comm1;
}
}
You could probably reduce this logic down a bit, but you get the idea.
On Aug 18, 2005, at 8:41 AM, Jim Lasc wrote:
> Hi,
> I tried to do the following:
> make a ring where there is a communicator between every two nodes
> (processes) of the ring.
> The code I made is added below. Every MPI_Comm_create returns
> allways MPI_SUCCESS,
> but I skipped all the error checking to make the code more readable.
> The "problem"-part is for if you have a unpair number of nodes.
> 0 -com1- 1 -com2- 3 -com1- 4 -com2- 5 -COM3- 0
> (otherwise 0 has two com1's, but it will be clair in the code...)
>
> I know that I could allso use the following technique to set up the
> communicators:
> * comm 0-1
> * 1 sends msg to 2
> * comm 1-2
> * 2 sends msg to 3
> * ...
> But I prefer to use my method.
>
> Now the problem is:
> There's allways one link which doesn't works (though
> MPI_Comm_create didn't gave an error).
> It's allways either the link 0-n (n is highest MPI-rank in
> MPI_COMM_WORLD), 0-1 or n-(n-1).
> Which link it is depends of the order of the three "blocks" ( FIRST
> - SECOND -PROBLEM),
> which can be swapped; strange enough it's the block in the middle
> which determines the bad link.
>
> My question:
> Does anyone knows what is wrong with the code below?
> Are there other methods to attain the same (not the msg-technique I
> described above) ?
>
> Thanks in advance for any ideas and suggestions.
>
> Jim.
>
> #include <iostream>
> #include <windows.h>
> #include "mpi.h"
>
> #pragma comment(lib,"mpi.lib")
>
> static MPI_Comm* pCommL=NULL;
> static MPI_Comm* pCommR=NULL;
> int* pIntL=(int*)malloc(sizeof(int));
> int* pIntR=(int*)malloc(sizeof(int));
>
>
> void SendLeft(void *buf, int count, MPI_Datatype datatype,int tag);
>
> int main( int argc, char **argv)
> {
> int size, rank,newrank;
> MPI_Init(&argc,&argv);
> MPI_Comm_size( MPI_COMM_WORLD, &size);
> MPI_Comm_rank( MPI_COMM_WORLD, &rank);
>
> MPI_Comm c_buur1, c_buur2, c_buur3;
> MPI_Group g_buren, world;
>
> int tempgroup[2];
> bool problem=(rank==size-1 && rank%2==0);
> bool problem2=(rank==0 && size%2==1);
>
> /***************FIRST***************/
>
> if(!problem){
> if(rank%2==0){
> tempgroup[0]=(rank+1+size)%size;
> tempgroup[1]=rank;}
> else{
> tempgroup[0]=rank;
> tempgroup[1]=(rank-1+size)%size;
> }
>
> MPI_Comm_group(MPI_COMM_WORLD, &world);
> MPI_Group_incl(world, 2, tempgroup, &g_buren);
> MPI_Comm_create(MPI_COMM_WORLD, g_buren, &c_buur1);
> MPI_Group_free(&g_buren);
>
> if(rank%2==0){
> pCommL=&c_buur1;
> *pIntL=0;
> }
> else{
> pCommR=&c_buur1;
> *pIntR=1;
> }
> }
> /***************SECOND***************/
> if(!problem2){
> if(rank%2==0){
> tempgroup[0]=(rank-1+size)%size;
> tempgroup[1]=rank;
> }
> else{
> tempgroup[0]=rank;
> tempgroup[1]=(rank+1+size)%size;
> }
>
> MPI_Comm_group(MPI_COMM_WORLD, &world);
> MPI_Group_incl(world, 2, tempgroup, &g_buren);
> MPI_Comm_create(MPI_COMM_WORLD, g_buren, &c_buur2);
> MPI_Group_free(&g_buren);
>
> if(rank%2==0){
> pCommR=&c_buur2;
> *pIntR=0;
> }
> else{
> pCommL=&c_buur2;
> *pIntL=1;
> }
> }
> /***************PROBLEMS...***************/
> if(problem || problem2){
> tempgroup[0]=0;
> tempgroup[1]=size-1;
> MPI_Comm_group(MPI_COMM_WORLD, &world);
> MPI_Group_incl(world, 2, tempgroup, &g_buren);
> MPI_Comm_create(MPI_COMM_WORLD, g_buren, &c_buur3);
> MPI_Group_free(&g_buren);
> if(rank==0){
> pCommR=&c_buur3;
> *pIntR=1;
> }
> else{
> pCommL=&c_buur3;
> *pIntL=0;
> }
> }
> /
> **********************************************************************
> */
> int aa=1;
> MPI_Request request;
> MPI_Irecv(&aa, 1, MPI_INT, MPI_ANY_SOURCE,22, *pCommR, &request);
> if(rank==0) SendLeft(&aa, 1, MPI_INT,22);
>
> int ontvangen=0;
>
> while(!ontvangen){
> MPI_Status status;
> MPI_Test(&request, &ontvangen, &status);
> if(ontvangen) SendLeft(&aa, 1, MPI_INT,22);
> }
> printf("\n%d RECEIVED",rank);fflush(stdout);
>
> MPI_Finalize();
> return 0;
> }
>
>
> void SendLeft(void *buf, int count, MPI_Datatype datatype,int tag){
> MPI_Send(buf, count, datatype, *pIntL, tag,*pCommL);
> }
> _______________________________________________
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