Scheduling policy for threads

SCHED_FIFO

After a thread with this policy is scheduled, it runs to completion unless it is blocked, it voluntarily yields control of the processor, or a higher-priority thread becomes dispatchable. Only fixed-priority threads can have a SCHED_FIFO scheduling policy.

SCHED_RR

When a SCHED_RR thread has control at the end of the time slice, it moves to the tail of the queue of dispatchable threads of its priority. Only fixed-priority threads can have a SCHED_RR scheduling policy.

SCHED_OTHER

This policy is defined by POSIX Standard 1003.4a as implementation-defined. The recalculation of the running thread's priority value at each clock interrupt means that a thread may lose control because its priority value has risen above that of another dispatchable thread.

SCHED_FIFO2

The policy is the same as for SCHED_FIFO, except that it allows a thread which has slept for only a short amount of time to be put at the head of its run queue when it is awakened. This time period is the affinity limit (tunable with schedo -o affinity_lim).

SCHED_FIFO3

A thread whose scheduling policy is set to SCHED_FIFO3 is always put at the head of a run queue. To prevent a thread belonging to SCHED_FIFO2 scheduling policy from being put ahead of SCHED_FIFO3, the run queue parameters are changed when a SCHED_FIFO3 thread is enqueued, so that no thread belonging to SCHED_FIFO2 will satisfy the criterion that enables it to join the head of the run queue.

SCHED_FIFO4

A higher priority SCHED_FIFO4 scheduling class thread does not preempt the currently running low priority thread as long as their priorities differ by a value of 1. The default behavior is the preemption of the currently running low priority thread on a given CPU by a high priority thread that becomes eligible to run on the same processor.

The scheduling policies are set with the thread_setsched() system call and are only effective for the calling thread. However, a thread can be set to the SCHED_RR scheduling policy by issuing a setpri() call specifying the process ID; the caller of setpri() and the target of setpri() do not have to match.

Only processes that have root authority can issue the setpri() system call. Only threads that have root authority can change the scheduling policy to any of the SCHED_FIFO options or SCHED_RR. If the scheduling policy is SCHED_OTHER, the priority parameter is ignored by the thread_setsched() subroutine.

Threads are primarily of interest for applications that currently consist of several asynchronous processes. These applications might impose a lighter load on the system if converted to a multithreaded structure.