The authors propose a number of assignment and scheduling policies, ranging from asymmetric handling of operating system (OS) processing on designated OS processors, to the symmetric scheduling of OS processing on any available processor. These policies are evaluated under a variety of load conditions. Small systems, with up to three processors, are studied experimentally using the Mach operating system on a VAX-11/784. A discrete event simulator is used to study larger and more varied system configurations. For asymmetric systems, the results show that the OS preempt policy, which gives OS processing preemptive priority over application processing, provides the best performance in almost all situations.