I have the following C/C++ code using OpenMP:
int nProcessors=omp_get_max_threads();
if(argv[4]!=NULL){
printf("argv[4]: %s\n",argv[4]);
nProcessors=atoi(argv[4]);
printf("nProcessors: %d\n",nProcessors);
}
omp_set_num_threads(nProcessors);
printf("omp_get_num_threads(): %d\n",omp_get_num_threads());
exit(0);
As you can see, I'm trying to set the number of processors to use based on an argument passed on the command line.
However, I'm getting the following output:
argv[4]: 2 //OK
nProcessors: 2 //OK
omp_get_num_threads(): 1 //WTF?!
Why isn't omp_get_num_threads()
returning 2?!!!
As has been pointed out, I'm calling omp_get_num_threads()
in a serial region, hence the function returns 1
.
However, I have the following parallel code:
#pragma omp parallel for private(i,j,tid,_hash) firstprivate(firstTime) reduction(+:nChunksDetected)
for(i=0;i<fileLen-CHUNKSIZE;i++){
tid=omp_get_thread_num();
printf("%d\n",tid);
int nThreads=omp_get_num_threads();
printf("%d\n",nThreads);
...
which outputs:
0 //tid
1 //nThreads - this should be 2!
0
1
0
1
0
1
...
The omp_get_num_threads()
call returns 1 in the serial section of the code. See Link
So you need to have parallel code to get the correct value, here how your code should look like:
#include <iostream>
#include <omp.h>
int main (int argc, const char * argv[])
{
int nProcessors = omp_get_max_threads();
std::cout<<nProcessors<<std::endl;
omp_set_num_threads(nProcessors);
std::cout<<omp_get_num_threads()<<std::endl;
#pragma omp parallel for
for(int i = 0; i < 5; i++){
int tid = omp_get_thread_num();
std::cout<<tid<<"\t tid"<<std::endl;
int nThreads = omp_get_num_threads();
std::cout<<nThreads<<"\t nThreads"<<std::endl;
}
exit(0);
}
This code produces:
2
1
0 tid
2 nThreads
0 tid
2 nThreads
0 tid
2 nThreads
1 tid
2 nThreads
1 tid
2 nThreads
It seems that you have either open mp not enabled or your loop is not in the form that can be parallized by openmp