I'm using Intel C/C++ 64-bit compiler 12.0.3.132 with Visual Studio 2010.

I want to change the number of workers (i.e., the number of cores that will be utilized) dynamically by calling__cilkrts_set_param. In OpenMP, it can be set by either by calling omp_set_num_threads or setting environment variable OMP_NUM_THREADS. I think the following call should change the number of workers: __cilkrts_set_param("nworkers", nthreads); However, it's not working well. Its behaviors are hard to predict. I wrote a very simple program that dynamically changes the number of workers after a lot of struggling.

#include 
#include 

void cilk_test(const char* nthreads)
{
  __cilkrts_set_param("nworkers", nthreads);
  std::cout << "1st try: nthreads: " << nthreads
    << "; cilk: " << __cilkrts_get_nworkers()
    << std::endl;

  __cilkrts_end_cilk();
  __cilkrts_init();

  __cilkrts_set_param("nworkers", nthreads);
  std::cout << "2nd try: nthreads: " << nthreads
    << "; cilk: " << __cilkrts_get_nworkers()
    << std::endl;

  cilk_for (int i = 0; i < 10; ++i)
  {
    for (volatile int64_t k = 0; k < 500000000LL; ++k);
  }
}

int main(int argc, char* argv[])
{
  cilk_test("4");
  cilk_test("6");
  cilk_test("8");
  cilk_test("2");
  return 0;
}

The code looks weired, but this is needed to reset the number of workers. Here is the output on my Core i7 2600 (Sandy Bridge): 4 cores with 8 threads by hyperthreading. 1st try: nthreads: 4; cilk: 4 2nd try: nthreads: 4; cilk: 4 1st try: nthreads: 6; cilk: 4 2nd try: nthreads: 6; cilk: 6 1st try: nthreads: 8; cilk: 6 2nd try: nthreads: 8; cilk: 8 1st try: nthreads: 2; cilk: 8 2nd try: nthreads: 2; cilk: 2 It shows that the very first time __cilkrts_set_param is working. However, later calls will not immediately change the runtime. I needed to retry again. Unfortunately, I also found that even such hack was not successful on two socket machines: 2 processors of Xeon E5656 (Westmere): total 12 cores with 12 threads (hyperthreading is disabled). So far, I can't find a way to dynamically set the number of workers on this two socket machine. First try: nthreads: 4; cilk: 4 Second try: nthreads: 4; cilk: 4 First try: nthreads: 6; cilk: 4 Second try: nthreads: 6; cilk: 4 First try: nthreads: 8; cilk: 4 Second try: nthreads: 8; cilk: 4 First try: nthreads: 2; cilk: 4 Second try: nthreads: 2; cilk: 4 In this machine, after setting the number of workers at first, no further reseting was successful.

How can I successfully and easily change the number of workers dynamically?