[cryptonector]

Priority inheritance isn't always trivial to implement. The protocols by which a high-priority thread passes its higher priority to a lower priority thread need to be synchronous, otherwise both could end up executing concurrently at high priority. Or you could make the protocol asynchronous but the scheduler fair enough, dividing the time between the two threads, or perhaps have the threads swap priority while the originally-lower-priority thread works on behalf of the other.

With asynchronous designs you also have a problem identifying when the lower-priority thread is done working on behalf of the higher-priority thread. You can work around this by only letting the worker inherit the higher priority for a small amount of time, but this is guess work: after all, the worker itself could be using async I/O and processing many different clients' requests over any time span, so how can the scheduler know when it's working on behalf of one client or another?

If you trust the lower-priority threads then just let them manage their priorities according to those of the clients they manage. Now you need something like SCM_RIGHTS/SCM_CREDS to pass priority tokens/info around.

Asynchrony is essential, so I suspect that automatic priority inheritance can't quite be had.