Scheduling

Linux

Since version 2.5 of the kernel, Linux has used a multilevel feedback queue with priority levels ranging from 0-140. 0-99 are reserved for real-time tasks and 100-140 are considered nice task levels. For real-time tasks, the time quantum for switching processes is approximately 200 ms and 10 ms for nice tasks. The scheduler will run through the queue of all ready processes, letting the highest ones go first and run through their time slice, and afterwards they will be placed in an expired queue. Then when the active queue is empty the expired queue will then be the active and vice versa. From versions 2.6 to 2.6.23, the kernel used an O(1) scheduler. In version 2.6.23, they replaced this method with the Completely Fair Scheduler that uses Red Black trees instead of queues.