title: Multiple pre-emptive thread pools in TBB
body: <p>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?</p>
  
  <p>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?</p>