TBB thread pool unexpectedly increasing

Question

We have a piece of code that utilises TBB to spawn tasks to perform some processing this is done using the following TBB code to initialise the TBB thread pool:

tbb::task_scheduler_init(8);

Then for each task we want to spawn we use the following code (where MainTask is derived from the tbb::task class):

task = new (tbb::task::allocate_root()) MainTask(theAction, theOutputData);
tbb::task::enqueue(*task);

When we run our code we start off with a thread pool that is the same as the number of cores (in our case 8 threads) as expected but as the program executes and spawns new TBB tasks, as described above, the number of threads at some random points suddenly increase. After 40 minutes of program execution the thread count increases from 8 to 15 between.

Why is this happening? Shouldn’t TBB keep the number of worker threads fixed to equal the number of cores?

Answer 1

As I said in another answer to you: Don't worry :-)

TBB does great job preventing actual over-subscription - only 8 threads will be active in your program at the same time. Though for various reasons, it needs more threads than hardware resources sometimes. One example is tbb::task_arena with no master slots reserved and another recent addition is tbb::global_control class which allows to change the number of active threads in the pool dynamically. Unfortunately, the way how TBB implements it leaves some space for the data race. It happens when some threads are on its way back to thread-pool to get some sleep while a new work arrives and requests all the 8 threads to start processing immediately; but that these threads in the intermediate state are not accounted in the thread-pool yet and new threads created instead.

TBB reduced the window for this data race as much as possible but to close it completely, a synchronization needed on the hot path which will affect general performance. Thus the decision was made to allow the data race and get less obstacles on the hot path.

But again, don't worry, there is no resource leak because TBB has hard limit for the maximum number of threads it can create this way. Depending on platform, this number varies somewhere from 2x to 4x (though it's internal implementation specifics which keep changing).

Though, I'm surprised that it goes that far with 15 threads created and I understand your concerns. TBB team will appreciate if you share a reproducer with them. You can contribute the reproducer through either TBB Forum or OSS site.