Multi Level Queue Scheduling (MLQ)

 

Multi Level Queue Scheduling (MLQ)

·         Multilevel queue scheduling classifies the processes according to their types for example, a multilevel queue scheduling algorithm makes a common.

·         In this scheduling ready queue is divided into various queue that are called sub queues. A subqueue is a distinct operational queue

·         The process are permanently assigned to subqueues, generally based on some property of the process such as memory size,priority or process type

·         Each subqueue has its process sucheduling algorithm. For example interactive process at the foreground may use round robin scheduling while batch jobs at the background may use the FCFS method

·         For example, consider a system with four different queues

1.       System processes

2.       Interactive processes

3.       End-user processes

4.       Interactive processes

·         In this example, each queue has absolute priority absolute over low priority queues. No process in a batch queue could run unless the queue for system process and interactive processes were all empty.

·         If an interactive process entered the ready queue while a batch process was running the batch process would be preempted. For example,solaries 2 operating system uses this form of algorithm

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 : 

 

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. 

Advantages:

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

Disadvantages:

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