Linux SCHED_OTHER, SCHED_FIFO and SCHED_RR – differences

Can someone explain the differences between SCHED_OTHER, SCHED_FIFO and SCHED_RR?

Thanks

Answer

SCHED_FIFO and SCHED_RR are so called “real-time” policies. They implement the fixed-priority real-time scheduling specified by the POSIX standard. Tasks with these policies preempt every other task, which can thus easily go into starvation (if they don’t release the CPU).

The difference between SCHED_FIFO and SCHED_RR is that among tasks with the same priority, SCHED_RR performs a round-robin with a certain timeslice; SCHED_FIFO, instead, needs the task to explicitly yield the processor.

SCHED_OTHER is the common round-robin time-sharing scheduling policy that schedules a task for a certain timeslice depending on the other tasks running in the system.

Update: since Linux 3.14, there is an additional policy called SCHED_DEADLINE. This policy implements the Constant Bandwidth Server (CBS) algorithm on top of Earliest Deadline First queues. Each task under this policy is assigned a deadline, and the earliest-deadline task is executed. The best resource describing this algorithm is Deadline scheduling in the Linux kernel.

Update 2: since Linux 4.13, SCHED_DEADLINE has replaced CBS with the Greedy Reclamation of Unused Bandwidth (GRUB) algorithm.