Round-robin scheduling

Updated: May 06, 2022

In round-robin (SCHED_RR) scheduling, a thread selected to run continues executing until it:

Figure 1. Round-robin scheduling. Thread A ran until it consumed its timeslice; the next READY thread (Thread B) now runs.

A timeslice is the unit of time assigned to every process. Once it consumes its timeslice, a thread is put at the end of its queue in the ready queue and the next READY thread at the same priority level is given control.

A timeslice is calculated as:

4 × ticksize

If your processor speed is greater than 40 MHz, then the ticksize defaults to 1 millisecond; otherwise, it defaults to 10 milliseconds. So, the default timeslice is either 4 milliseconds (the default for most CPUs) or 40 milliseconds (the default for slower hardware).

Apart from time-slicing, the round-robin scheduling method is identical to FIFO scheduling.