TBB: How to set different number of threads in different parts of code?

Question

I have a tbb::task_scheduler_init instance init which is initialized with some certain number of threads in a part of code which I don't have control, in later parts of code I want to run with different number of threads. How can I do it through this instance init ?

Is there a better way than doing the following?

init.terminate();
init.initialize(my_preferred_number_of_threads);

/*
 run some code
*/

init.terminate();
init.initialize(original_number_of_threads); // restore the original tbb scheduler

Answer 1

You can use tbb::task_arena for your needs.

Taken from tbb::task_arena documentation

tbb::task_scheduler_init def_init; // Use the default number of threads.
tbb::task_arena limited(2);        // No more than 2 threads in this arena.
tbb::task_group tg;

limited.execute([&]{ // Use at most 2 threads for this job.
  tg.run([]{ // run in task group
    tbb::parallel_for(1, N, unscalable_work());
  });
});

// Run another job concurrently with the loop above.
// It can use up to the default number of threads.
tbb::parallel_for(1, M, scalable_work());

// Wait for completion of the task group in the limited arena.
limited.execute([&]{ tg.wait(); });