Lock-free thread pool

Question

Is the following reasoning correct with respect to standard C++11 and/or also in a general context?

When implementing a thread pool one usually does not want to busy-loop those threads that have no work to do. Therefore, one needs some kind of std::condition_variable. Thus, one needs some kind of std::mutex. Thus, it is not possible to have a lock-free thread pool (that does not busy-loop).

Or am I missing some OS mechanism that would make it possible? Can you provide additional reasoning that suggests the non-existence of a lock-free non-busy-looping thread pool?

Answer 1

If you allow busy waiting, it is of course entirely possible. But without busy waiting, you need help from the OS - it doesn't have to be a mutex as such, it could of course be a read , poll , WaitForMultipleObject (in Windows) or other "wait for something" functionality provided by the OS.

I'm not 100% sure that it's IMPOSSIBLE to come up with something, but in general, a thread is either running, or it's held by the OS waiting for something. That "waiting for something in the OS" is, as far as I can see, always some sort of "lock".

However, the key here is this:

If locks in the case where there is "no work to do" is really a problem, then you probably should rethink your overall thread-pool usage, not try to remove the locks. Maybe larger packets of data needs to be put together by whatever "gives work to the threads".