C++ Multithreading optimization

Question

I am new to threading in C++ but I've done enough reading to at least get what I'm working on to compile. As of yet it hasn't improved performance at all. Right now I just have it creating the number of threads as there are loops but I can imagine that can pretty quickly cause the system to thrash. Is there a better alternative to brute force controlling the number of threads? I also intend to run this on the WestGrid computing system where I can specify the number of processors to use. What is the best way to set the number of threads to optimize for the number of processors.

void ExecuteCRTProcess(const long &numberOfRows, ZZ* rij, const ZZ &powRoh, const int &rowLength, long* PublicKey, ZZ* rQ0, const ZZ &Q0, ZZ* primes, const ZZ &productOfPrimes, ZZ* resultsArray, const bool IsItPrimeArray, long* caseStudy, long* caseStudyTracker, const ZZ &X0, const long &Roh){
int rc;
pthread_t threads[numberOfRows];
struct parameters ThreadParameters[numberOfRows];

for(int i = 0; i< numberOfRows ; i++){
    FillR(rij,powRoh,rowLength); // fill up the vector rij with random numbers between 0 powRoh
    MultiplyVectorByTwo(rij, rowLength, i, IsItPrimeArray); //Multiply rij vector by 2. If calculating Xi' also add 1 to appropriate values.
    ThreadParameters[i].rij = rij;
    ThreadParameters[i].rQ0 = rQ0;
    ThreadParameters[i].primes = primes;
    ThreadParameters[i].rowLength = rowLength;
    ThreadParameters[i].Q0 = Q0;
    ThreadParameters[i].i = i;
    ThreadParameters[i].X0 = X0;
    rc = pthread_create(&threads[i],NULL,CRTNew,(void *)&ThreadParameters[i]);
    if(rc){
        cout << "Error: unable to create thread, " << rc << endl;
        exit(-1);
    }
    for(long j = 0; j< rowLength; j++){
        cout << (resultsArray[i] % primes[j]) << " ";
    }
    cout << endl;*/

}
for(int i = 0; i< numberOfRows; i++){
    pthread_join(threads[i], NULL);
    resultsArray[i] = ThreadParameters[i].result;
    }
}

The threads created run this function

void* CRTNew(void *threadArg){
    struct parameters *local_data;
    local_data = (struct parameters *) threadArg;
    ZZ a, p, A, P, crt;
    long Z, Public;
    a = local_data->rQ0[local_data->i];
    p = local_data->Q0;
    A = local_data->rij[0];
    P = local_data->primes[0];
    for(int i = 1; i<=local_data->rowLength; i++){
       A = A%P;
       Z = CRT(a, p, A, P);
       A = local_data->rij[i]; P = local_data->primes[i];
       if(i == local_data->rowLength) Public = Z;
    }
   if(a < 0) crt = a+p;
   else crt = a%p;
   local_data->result = crt%local_data->X0;
   pthread_exit(NULL);

}

Answer 1

'What is the best way to set the number of threads to optimize for the number of processors';

1) Create [num of cores] threads, (or maybe a few more), at app startup.

2) Never create any more threads

3) Never let the threads terminate.

4) Have them wait for work tasks on a producer-consumer thread, in the manner of a pool.

Alternatively, use a thread pool class or equivalent parallel language feature that already works.