Limiting the Number of Threads Used by Parallel Frameworks

Run Your Program as a Serial Program

To run the current version of your program as a serial program, you need to limit the number of threads to 1. To run your program with a single thread:

• With Intel® TBB, in the main thread create a tbb::task_scheduler_init init(1); object for the lifetime of the program and run the executable again. For example:

     int main() {
       tbb::task_scheduler_init init(1);
       // ...rest of program...
  
       return 0;
     }
  

  The effect of task_scheduler_init applies separately to each user-created thread. So if the program creates threads elsewhere, you need to create a tbb::task_scheduler_init init(1); for that thread's lifetime as well. Use of certain Intel TBB features can prevent the program from running serially. For more information, see the Intel TBB documentation.

• With Intel® Cilk™ Plus, you can do one of the following:

  • Set the environment variable CILK_NWORKERS to 1 and run the executable again.

  • When using the Intel® C++ Compiler, set the compiler option -cilk-serialize (Linux* OS) or /Qcilk-serialize (Windows* OS) when building the target executable.

  • Before your program's first call to a function that performs a spawn (cilk_spawn or cilk_for), execute the cilkrts_set_param() function and specify nworkers as 1. For example:

  if (0!= cilkrts_set_param("nworkers","1"))
   {
      printf("Failed to set worker count\n");
      return 1;
   }
  

  Using cilkrts_set_param() overrides the value (if any) set by the CILK_NWORKERS environment variable. For more information, see the Intel Cilk Plus help.

• With OpenMP*, do one of the following:

  • Set the OpenMP* environment variable OMP_NUM_THREADS to 1 before you run the program.

  • Omit the compiler option that enables recognition of OpenMP pragmas and directives. On Windows* OS, omit /Qopenmp, and on Linux* OS omit -openmp.

For more information, see your compiler documentation.