Chapter 5. Priorities and Policies

All Linux threads have one of the following scheduling policies:
  • SCHED_OTHER or SCHED_NORMAL: The default policy
  • SCHED_BATCH: Similar to SCHED_OTHER, but with a throughput orientation
  • SCHED_IDLE: A lower priority than SCHED_OTHER
  • SCHED_FIFO: A first in/first out realtime policy
  • SCHED_RR: A round-robin realtime policy
The policies that are critical to Red Hat Enterprise Linux for Real Time are SCHED_OTHER, SCHED_FIFO, and SCHED_RR.
SCHED_OTHER or SCHED_NORMAL is the default scheduling policy for Linux threads. It has a dynamic priority that is changed by the system based on the characteristics of the thread. Another thing that effects the priority of SCHED_OTHER threads is their nice value. The nice value is a number between -20 (highest priority) and 19 (lowest priority). By default, SCHED_OTHER threads have a nice value of 0. Adjusting the nice value will change the way the thread is handled.
Threads with a SCHED_FIFO policy will run ahead of SCHED_OTHER tasks. Instead of using nice values, SCHED_FIFO uses a fixed priority between 1 (lowest) and 99 (highest). A SCHED_FIFO thread with a priority of 1 will always be scheduled ahead of any SCHED_OTHER thread.
The SCHED_RR policy is very similar to the SCHED_FIFO policy. In the SCHED_RR policy, threads of equal priority are scheduled in a round-robin fashion. Generally, SCHED_FIFO is preferred over SCHED_RR.
SCHED_FIFO and SCHED_RR threads will run until one of the following events occurs:
  • The thread goes to sleep or begins waiting for an event
  • A higher-priority realtime thread becomes ready to run
If one of these events does not occur, the threads will run indefinitely on that processor, and lower-priority threads will not be given a chance to run. This can result in system service threads failing to run, and operations such as memory swapping and filesystem data flushing not occurring as expected.