Overhead.

The user-mode implementation uses an additional thread to actually perform the receive and invoke the dispatcher(s). The thread will then signal the thread(s) waiting on the flow handle(s) to which the packets were dispatched. This involves an additional context switch. However, to be frank, we believe that user-mode implementations without OS support for the dispatch will remain experimental prototypes; so we do not consider this additional overhead a problem. With OS support, the interrupt handler will directly awake the Java thread to which the packet was dispatched.