What is round-robin scheduling?

Question

In a multitasking operating system context, sometimes you hear the term round-robin scheduling. What does it refer to?
What other kind of scheduling is there?

Answer 1

The answers here and even the Wikipedia article describe round-robin scheduling to inherently include periodic timeslicing. While this is very common, I believe that Round-Robin scheduling and timeslicing are not exactly the same thing. Certainly, for timeslicing to make sense, round-robin schedling is implied when rotating to each task, however you can do round-robin scheduling without having timeslicing. That is, each task at the same priority in the round-robin rotation can be allowed to run until they reach a resource block condition and only then having the next task in the rotation run. In other words, when equal priority tasks exist, the reschedling points are not time pre-emptive.

The above idea is actually realized specifically in the case of Wind River's VxWorks kernel. Within their priority scheme, tasks of each priority run round robin but do not timeslice without specifically enabling that feature in the kernel. The reason for this flexibility is to avoid the overhead of timeslicing tasks that are already known to run into a block within a well bounded time.

Therefore, while timeslicing based scheduling implies round-robin scheduling, round-robin scheduling does not require equal time based timeslicing.