Parallelizing many nested for loops in openMP c++

问题

Hi i am new to c++ and i made a code which runs but it is slow because of many nested for loops i want to speed it up by openmp anyone who can guide me. i tried to use '#pragma omp parallel' before ip loop and inside this loop i used '#pragma omp parallel for' before it loop but it does not works

    #pragma omp parallel
    for(int ip=0; ip !=nparticle; ip++){
        inf14>>r>>xp>>yp>>zp;
        zp/=sqrt(gamma2);
        counter++;
        double para[7]={0,0,Vz,x0-xp,y0-yp,z0-zp,0};
        if(ip>=0 && ip<=43){
             #pragma omp parallel for
             for(int it=0;it<NT;it++){  
             para[6]=PosT[it];
                for(int ix=0;ix<NumX;ix++){
                    para[3]=PosX[ix]-xp;
                    for(int iy=0;iy<NumY;iy++){
                        para[4]=PosY[iy]-yp;
                        for(int iz=0;iz<NumZ;iz++){
                            para[5]=PosZ[iz]-zp;
                            int position=it*NumX*NumY*NumZ+ix*NumY*NumZ+iy*NumZ+iz;
                            rotation(para,&Field[3*position]);
                            MagX[position] +=chg*Field[3*position];
                            MagY[position] +=chg*Field[3*position+1];
                            MagZ[position] +=chg*Field[3*position+2];
                        }   
                    }
                }
            }   
        }
    }enter code here

and my rotation function also has infinite integration for loop as given below

for(int i=1;;i++){
    gsl_integration_qag(&F, 10*i, 10*i+10, 1.0e-8, 1.0e-8, 100, 2, w, &temp, &error);
    result+=temp;
    if(abs(temp/result)<ACCURACY){
        break;
    }
}

i am using gsl libraries as well. so how to speed up this process or how to make openmp?

回答1:

If you don't have inter-loop dependences, you can use the collapse keyword to parallelize multiple loops altoghether. Example:

void scale( int N, int M, float A[N][M], float B[N][M], float alpha ) {
  #pragma omp for collapse(2)
  for( int i = 0; i < N; i++ ) {
    for( int j = 0; j < M; j++ ) {
      A[i][j] = alpha * B[i][j];
    }
  }
}

I suggest you to check out the OpenMP C/C++ cheat sheet (PDF), which contain all the specifications for loop parallelization.

回答2:

Do not set parallel pragmas inside another parallel pragma. You might overhead the machine creating more threads than it can handle. I would establish the parallelization in the outter loop (if it is big enough):

#pragma omp parallel for
    for(int ip=0; ip !=nparticle; ip++)

Also make sure you do not have any race condition between threads (e.g. RAW).

Advice: if you do not get a great speed-up, a good practice is iterating by chunks and not only by one increment. For instance:

int num_threads = 1;
#pragma omp parallel
{
#pragma omp single
    {
        num_threads = omp_get_num_threads();
    }
}
int chunkSize = 20; //Define your own chunk here
for (int position = 0; position < total; position+=(chunkSize*num_threads)) {
    int endOfChunk = position + (chunkSize*num_threads);
    #pragma omp parallel for
    for(int ip = position; ip < endOfChunk ; ip += chunkSize) {
        //Code
    }
}

来源：https://stackoverflow.com/questions/49872940/parallelizing-many-nested-for-loops-in-openmp-c

标签

c++

parallel-processing

openmp

nested-loops