Multilevel Queue (MLQ) CPU Scheduling

It may happen that processes in the ready queue can be divided into different classes where each class has its own scheduling needs. For example, a common division is a foreground (interactive) process and a background (batch) process. These two classes have different scheduling needs. For this kind of situation Multilevel Queue Scheduling is used. 

Now, let us see how it works. 

Advantages of Multilevel Queue CPU Scheduling:

• The processes are permanently assigned to the queue, so it has advantage of low scheduling overhead.

Disadvantages of Multilevel Queue CPU Scheduling:

• Some processes may starve for CPU if some higher priority queues are never becoming empty.
• It is inflexible in nature.

Ready Queue is divided into separate queues for each class of processes. For example, let us take three different types of processes System processes, Interactive processes, and Batch Processes. All three processes have their own queue. Now, look at the below figure. 

The Description of the processes in the above diagram is as follows:

• System Processes: The CPU itself has its own process to run which is generally termed as System Process.
• Interactive Processes: An Interactive Process is a type of process in which there should be same type of interaction.
• Batch Processes: Batch processing is generally a technique in the Operating system that collects the programs and data together in the form of the batch before the processing starts.

All three different type of processes has their own queue. Each queue has its own Scheduling algorithm. For example, queue 1 and queue 2 uses Round Robin while queue 3 can use FCFS to schedule their processes. 

Scheduling among the queues: What will happen if all the queues have some processes? Which process should get the CPU? To determine this Scheduling among the queues is necessary. There are two ways to do so – 

1. Fixed priority preemptive scheduling method – Each queue has absolute priority over the lower priority queue. Let us consider following priority order queue 1 > queue 2 > queue 3.According to this algorithm, no process in the batch queue(queue 3) can run unless queues 1 and 2 are empty. If any batch process (queue 3) is running and any system (queue 1) or Interactive process(queue 2) entered the ready queue the batch process is preempted.
2. Time slicing – In this method, each queue gets a certain portion of CPU time and can use it to schedule its own processes. For instance, queue 1 takes 50 percent of CPU time queue 2 takes 30 percent and queue 3 gets 20 percent of CPU time.

Example Problem :
Consider below table of four processes under Multilevel queue scheduling. Queue number denotes the queue of the process. 

Priority of queue 1 is greater than queue 2. queue 1 uses Round Robin (Time Quantum = 2) and queue 2 uses FCFS. 

Below is the gantt chart of the problem : 

Working:

• At starting, both queues have process so process in queue 1 (P1, P2) runs first (because of higher priority) in the round robin fashion and completes after 7 units
• Then process in queue 2 (P3) starts running (as there is no process in queue 1) but while it is running P4 comes in queue 1 and interrupts P3 and start running for 5 second and 
• After its completion P3 takes the CPU and completes its execution. 

This article is contributed by Ashish Sharma. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. 

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