Tuning MLFQ And Other Issues

Parameterizing the scheduler

A few other issues arise with MLFQ scheduling. One big question is how to parameterize such a scheduler. For example, how many queues should there be? How big should the time slice be per queue? How often should priority be boosted in order to avoid starvation and account for changes in behavior? There are no easy answers to these questions, and thus only some experience with workloads and subsequent tuning of the scheduler will lead to a satisfactory balance.

For example, most MLFQ variants allow for varying time-slice length across different queues. The high-priority queues are usually given short time slices; they consist of interactive jobs, after all, and thus quickly alternating between them makes sense (e.g., 10 or fewer milliseconds). The low-priority queues, in contrast, contain long-running jobs that are CPU-bound; hence, longer time slices work well (e.g., 100s of ms).

The figure below shows an example in which two jobs run for 20 ms at the highest queue (with a 10-ms time slice), 40 ms in the middle (20-ms time slice), and with a 40-ms time slice at the lowest.