c - How to get all ranks in MPI to do a send a value to rank 0 which then does a blocking receive on all of them? -

say have n processes:

they calculation, , send result rank 0. want happen:

rank 0 wait till has result of ranks , add them up.

how do this? also, want avoid following:

for eg. 4 processes p0, p1, p2, p3,

p1 -> p0 p2 -> p0 p3 -> p0 

in meanwhile p1 has finished calculation , p1->p0 happens again.

i want p0 make addition 3 processes in 1 cycle before doing next one.

can suggest mpi function this? know mpi_gather i'm not sure if blocking

i've thought of this:

#include <mpi.h> #include <stdio.h>  int main() { int pross, rank,p_count = 0; int count = 10; mpi_init(&argc,&argv); mpi_comm_size(mpi_comm_world,&pross); mpi_comm_rank(mpi_comm_world,&rank);  int * num = malloc((pross-1)*sizeof(int));          if(rank !=0)         {             mpi_send(&count,1,mpi_int,0,1,mpi_comm_world);         }         else         {                         mpi_gather(&count, 1,mpi_int,num, 1, mpi_int, 0,mpi_comm_world);             for(ii = 0; ii < pross-1;ii++ ){printf("\n num %d \n",num[ii]); p_count += num[ii]; } } mpi_finalize(); } 

i'm getting error:

  *** process received signal ***   signal: segmentation fault (11)   signal code: address not mapped (1)   failing @ address: (nil)   [ 0] /lib/x86_64-linux-gnu/libpthread.so.0(+0x11630)[0x7fb3e3bc3630]   [ 1] /lib/x86_64-linux-gnu/libc.so.6(+0x90925)[0x7fb3e387b925]   [ 2] /usr/lib/libopen-pal.so.13(+0x30177)[0x7fb3e3302177]   [ 3] /usr/lib/libmpi.so.12(ompi_datatype_sndrcv+0x54c)[0x7fb3e3e1e3ec]   [ 4] /usr/lib/openmpi/lib/openmpi/mca_coll_tuned.so(ompi_coll_tuned_gather_intra_basic_linear+0x143)[0x7fb3d53d9063]   [ 5] /usr/lib/libmpi.so.12(pmpi_gather+0x1ba)[0x7fb3e3e29a3a]   [ 6]  sosuks(+0xe83)[0x55ee72119e83]   [ 7] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf1)[0x7fb3e380b3f1]   [ 8]  sosuks(+0xb5a)[0x55ee72119b5a]   *** end of error message *** 

also, tried:

#include <mpi.h> #include <stdio.h>  int main() { int pross, rank,p_count = 0; int count = 10; mpi_init(&argc,&argv); mpi_comm_size(mpi_comm_world,&pross); mpi_comm_rank(mpi_comm_world,&rank);  int * num = malloc((pross-1)*sizeof(int));          if(rank !=0)         {             mpi_send(&count,1,mpi_int,0,1,mpi_comm_world);         }         else         {                         mpi_gather(&count, 1,mpi_int,num, 1, mpi_int, 0,mpi_comm_world);             for(ii = 0; ii < pross-1;ii++ ){printf("\n num %d \n",num[ii]); p_count += num[ii]; } } mpi_finalize(); } 

i'm getting error here:

  *** process received signal ***   signal: segmentation fault (11)   signal code: address not mapped (1)   failing @ address: 0x560600000002   [ 0] /lib/x86_64-linux-gnu/libpthread.so.0(+0x11630)[0x7fefc8c11630]   [ 1] mdscisuks(+0xeac)[0x5606c1263eac]   [ 2] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf1)[0x7fefc88593f1]   [ 3] mdscisuks(+0xb4a)[0x5606c1263b4a]   *** end of error message *** 

for second attempt, thing note here send , recv successful root receive 2 messages ranks reason. segmentation fault seen due there being 2 elements in num, dont see why num receives twice.

i'm calling code as

mpiexec -n 6 ./sosuks 

can tell me better / correct way implement idea ?

update:

apart answer below found mistake in implementation above wanted share:

#include <mpi.h> #include <stdio.h>  int main() { int pross, rank,p_count = 0; int count = 10; mpi_init(&argc,&argv); mpi_comm_size(mpi_comm_world,&pross); mpi_comm_rank(mpi_comm_world,&rank); mpi_status status; int * num = malloc((pross-1)*sizeof(int));          if(rank !=0)         {             mpi_send(&count,1,mpi_int,0,1,mpi_comm_world);         }         else         {         int var,lick = 0;         for(lick = 1; lick < pross; u++)         {         int fetihs;         mpi_recv(&fetihs,1,mpi_int,lick,1,mpi_comm_world,&status);                    var += fetihs;         }      // things var } mpi_finalize(); } 

in case, sneftel pointed out, need mpi_reduce. also, don't need explicit synchronization before cycle completes.

#include <mpi.h> #include <stdio.h> #include <stdlib.h>  int main(int argc, char* argv[]) {     int pross, rank, p_count, count = 10;      mpi_init(&argc,&argv);     mpi_comm_size(mpi_comm_world, &pross);     mpi_comm_rank(mpi_comm_world, &rank);      int* num = malloc((pross-1)*sizeof(int));      // master not send data itself.     // workers send data master.      (int i=0; i<3; ++i)     {         // prove no further sync needed.         // same answer in each cycle.         p_count = 0;           if (rank == 0)         {             // has not effect since master uses p_count both              // send , receive buffers due mpi_in_place.             count = 500;              mpi_reduce(mpi_in_place, &p_count, 1, mpi_int, mpi_sum, 0, mpi_comm_world);         }         else         {             // slave p_count irrelevant.             mpi_reduce(&count, null, 1, mpi_int, mpi_sum, 0, mpi_comm_world);         }          if (rank == 0)         {             printf("p_count = %i\n", p_count);         }          // slaves send data master before cycle completes.         // no need explicit sync such mpi_barrier.         // mpi_barrier(mpi_comm_world); // no need.     }      mpi_finalize(); } 

in code above count in slaves reduced p_count in master. note mpi_in_place , 2 mpi_reduce calls. can same functionality setting count = 0 , calling mpi_reduce ranks without mpi_in_place.

for (int i=0; i<3; ++i) {     p_count = 0;         if (rank == 0) count = 0;      mpi_reduce(&count, &p_count, 1, mpi_int, mpi_sum, 0, mpi_comm_world);            }