Multi-Class Scheduling (SCHED_CLASS)

The multi-class scheduler option allows multiple scheduling classes to exist simultaneously. Each active thread is subject to a single scheduling class at any one time, but can change class dynamically.

The multi-class scheduler provides the following scheduling policies:

• Priority-based round-robin, with a fixed time quantum (CLASS_RR).

• Real-time scheduling (CLASS_RT)

Round Robin Scheduling (CLASS_RR)

The optional CLASS_RR scheduling class is available only within the SCHED_CLASS scheduler. It is similar to SCHED_FIFO but implements a priority-based, preemptive policy with round-robin time slicing based on a configurable time quantum. Priority of threads may vary from K_RR_PRIOMAX (0, highest priority) to K_RR_PRIOMIN (255, lowest priority). CLASS_RR uses the full range of priorities (256) of the SCHED_CLASS scheduler.

The SCHED_RR policy is similar to the SCHED_FIFO policy, except that an elected thread is given a fixed time quantum. If the thread is still running at quantum expiration, it is placed at the end of its priority ready queue, and then may be preempted by threads of equal priority. The thread's quantum is reset when the thread becomes ready after being blocked. It is not reset when the thread is elected again after a preemption. The time quantum is the same for all priority levels and all threads. It is defined by the K_RR_QUANTUM value (100 milliseconds).

For details, see the ROUND_ROBIN(5FEA) man page.

Real-Time Scheduling (CLASS_RT)

The CLASS_RT scheduling class is available only within the SCHED_CLASS scheduler. It implements the same policy as the real-time class of UNIX SVR4.0. Refer to the man page of UNIX SVR4.0 for a complete description.

The real-time scheduling policy is essentially a round-robin policy, with per-thread time quanta. The priority of a thread may vary between K_RT_PRIOMAX (159, highest priority) and K_RT_PRIOMIN (100, lowest priority).

The order of priorities is inverted compared to the CLASS_FIFO priorities. CLASS_RT uses only a sub-range of the SCHED_CLASS priorities. This sub-range corresponds to the range [96, 155] of CLASS_FIFO and CLASS_RR.

The CLASS_RT manages scheduling using configurable priority default time quanta.

SCHED API

The SCHED feature API is summarized in the following table: