Number of threads used by Intel TBB

Question

How does Intel TBB choose the number of threads to used for a parallel section?

Is there some kind of specification available?

Answer 1

As of TBB Version 2.2 the task scheduler will be automatically initialized and on runtime take care of the numbers of threads to use, if you manually want to change that number, you can use one of the following methods:

When you create the scheduler, you can specify the number of threads as

tbb::task_scheduler_init init(nthread);

else you can use

tbb::task_scheduler_init init(tbb::task_scheduler_init::automatic);

In this case, tbb scheduler creates as many threads as your CPU cores

Answer 2

Letting TBB decide the number of threads in the pool is the recommended option - it will usually create as many worker threads as there are logical CPUs on the machine - see Class reference for tbb::task_scheduler_init.

It's not easy to find out how many worker threads exist or are executing tasks at any given time - this is a deliberate design choice. From Intel's TBB Parallel Programming Course:

How do I know how many threads are available?

Do not ask!

• Not even the scheduler knows how many threads really are available
• There may be other processes running on the machine
• Routine may be nested inside other parallel routines