Multilevel Queue Scheduling

• Another class of scheduling algorithms has been created for situations in which processes are easily classified into different groups.
• A common division is made between foreground (interactive) processes and background (batch) processes.
  • These two types of processes have different response-time requirements and so may have different scheduling needs.
  • In addition, foreground processes may have priority (externally defined) over background processes.
• A multilevel queue scheduling algorithm partitions the ready queue into several separate queues (see Fig. 10).

  Figure 10: Multilevel queue scheduling.

  
• The processes are permanently assigned to one queue, generally based on some property of the process, such as memory size, process priority, or process type.
• Each queue has absolute priority over lower-priority queues and also each queue has its own scheduling algorithm. The foreground queue might be scheduled by an RR algorithm, while the background queue is scheduled by an FCFS algorithm.
• In addition, there must be scheduling among the queues, which is commonly implemented as fixed-priority pre-emptive scheduling. For example, the foreground queue may have absolute priority over the background queue.