I. A Scheduler For Closely Co-Operating Processes.

We've recently switched from a polling scheduler (each process provides
a predicate to determine whether it is runnable) to an event-driven
scheduler.  The new scheduler has several interesting features (1)
support for co-operating processes that can have complicated,
interconnected, scheduling requirements, (2) a priority promotion scheme
that has no overhead in the contention free case, (3) a very efficient,
and robust, timer mechanism that allows arbitrary user code to execute
asynchronously, and (4) fully replaceable, user-supplied, policy modules
to allow the scheduler to support widely different policies on different
computers.  Forced compatibility with the existing scheduler
suggested new scheduling primitives.  We think these hybrids may be
generally useful; they combine the flexibility of the polling scheduler
with the efficiency of the event-driven process mechanism.