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Multilevel feedback queue scheduler for Tock

Based on the MLFQ rules described in “Operating Systems: Three Easy Pieces” by Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau.

This scheduler can be summarized by the following rules:

  • Rule 1: If Priority(A) > Priority(B), and both are ready, A runs (B doesn’t).
  • Rule 2: If Priority(A) = Priority(B), A & B run in round-robin fashion using the time slice (quantum length) of the given queue.
  • Rule 3: When a job enters the system, it is placed at the highest priority (the topmost queue).
  • Rule 4: Once a job uses up its time allotment at a given level (regardless of how many times it has given up the CPU), its priority is reduced (i.e., it moves down one queue).
  • Rule 5: After some time period S, move all the jobs in the system to the topmost queue.


Nodes store per-process state