CODEWITHSUNDEEP
multithreadingreal-timeschedulingtbb
user3454602
user3454602

Reputation: 11

Multiple pre-emptive thread pools in TBB

We have a requirement to create a number of real time processing chains one running at n Hz and others running at x, y and z Hz in a single process. Where x, y and z are some multiple (not necessarily a simple multiple) of n. For example one chain running at 1Hz and others running at 3Hz and 4Hz. Each processing chain needs to make use of TBB to parallelize some of their computations etc. and so needs to have a pool of TBB worker threads that match the number of hardware processors, but the higher frequency chains need to pre-empt the lower frequency chains in order for the system to work. How can this be achieved using TBB?

It seems from the documentation that it is not possible to create competing pools of TBB workers that will pre-empt each other, since TBBs task groups etc. seem to share a single pool of real threads and there does not appear to be anyway to set the real system priority of a TBB worker thread, or am I missing something?

Upvotes: 1

Views: 457

Answers (1)

Anton
Anton

Reputation: 6537

First, TBB is not designed for real-time systems. Though, the requirement of a few Hz looks relaxed quite enough.

Second, TBB is not designed to serve as a thread pool. It suggests a user-level non-preemptive task scheduler instead of preemptive threads scheduling on OS-level. TBB supposes that tasks are finite and small enough to provide points where the scheduler can switch between them more efficiently than OS can switch execution contexts of threads. Thus except for some special cases, it does not make a sense to request more worker threads then HW provides (i.e. to oversubscribe) and so thread-level priorities do not make sense either with TBB.

Though if you are not convinced by the arguments above and want to try your design, there is a way using two community-preview features: task_arena for work isolation and task_scheduler_observer extensions to assign system priorities for worker thread entering an arena.

TBB task priority feature looks like a more native approach for TBB to the described requirements. Though, it is limited to only 3 levels of priorities, the priority of a task_group_context can be changed dynamically which can be used to reorder tasks on the fly.

Upvotes: 2

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