2.2.3.2 Scheduling Policy Attribute

The scheduling policy attribute describes how the thread is scheduled for execution relative to the other threads in the program. A thread has one of the following scheduling policies:

• SCHED_FIFO (first-in/first-out (FIFO))-The highest priority thread runs until it blocks. If there is more than one thread with the same priority and that priority is the highest among other threads, the first thread to begin running continues until it blocks. If a thread with this policy becomes ready, and it has a higher priority than the currently running thread, then it preempts the current thread and begins running immediately.

• SCHED_RR (round-robin (RR))-The highest priority thread runs until it blocks; however, threads of equal priority, if that priority is the highest among other threads, are timesliced. (Timeslicing is a mechanism that ensures that every thread is allowed time to execute by preempting running threads at fixed intervals.) If a thread with this policy becomes ready, and it has a higher priority than the currently running thread, then it preempts the current thread and begins running immediately.

• SCHED_FG_NP (also known as SCHED_OTHER) (Default)-All threads are timesliced. Under this policy, all threads receive some scheduling regardless of priority. Therefore, no thread is completely denied execution time. Nevertheless, higher priority threads receive more execution time than lower priority threads. Threads with the default scheduling policy can be denied execution time by FIFO or RR threads.

• SCHED_BG_NP (Background)-Like the default (throughput) scheduling policy, this policy ensures that all threads, regardless of priority, receive some scheduling. However, background threads can be denied execution time by FIFO or RR threads, and receive less execution time than default policy threads.

Note that only SCHED_FIFO and SCHED_RR are portable. The name SCHED_OTHER is also portable, but its behavior may vary. For example, on some platforms it could be identical to SCHED_ FIFO or SCHED_RR. The other policies are DECthreads extensions. You can use either of the following methods to set the scheduling policy attribute:

• Set the scheduling policy attribute in the attributes object, which establishes the scheduling policy of a new thread when it is created. To do this, call the pthread_attr_ setschedpolicy routine. This allows the creator of a thread to establish the created thread's initial scheduling policy. Note that this value is used only if the attributes object is set so that the created thread does not inherit its priority from the creating thread. Inheriting priority is the default behavior.

• Change the scheduling policy of an existing thread (and, at the same time, the scheduling parameters) by calling the pthread_setschedparam routine. This has no effect on an attributes object.

Section 2.7 describes and shows the effect of the scheduling policy on thread scheduling.