Multilevel Feedback queue scheduling (MFQ)

 

Multilevel Feedback queue scheduling (MFQ)

·         Multilevel feedback queue scheduling is an enhancement of multi-levelqueue scheduling. In this scheme, processes can move between the different queue

·         The various processes are separates in different queue on the basis of their CPU Burst Char characteristics

·         If a process consumes a lot of CPU time , it is placed into a lower priority queue. Thus I/O bound and interactive process are placed in the higher priority queue and CPU bound pricesses are in lower priority

·         If a processes waits too long in a lower priority queue it is moved higher priority queue. Such an aging prevents starvation.

·         The top priority queue is given smallest CPU time Quantum

·         If the quantum expires before the process voluntarily relinquishes the CPU, the process is placed at the back of the next lower queue.

·         In many multi-level feedback schemes, the quantum gives too the process as it moves to each lower level queue become larger

·         Whenever any new process arrives un the top queue, the running process in FCFS queue is preempted as it has lower priority.

·         Any process that spends an amount of elpsed time exceeding the aging interval will be promoted to the next higher priority queue.

·         These parameters makes system both flexible and complex. These parameters are:

1.       The number of queues.

2.       The scheduling algorithm for each queue

3.       The time slice associated with each queue

4.       The condition that will couse a process to move downward into a lower priority queue

5.       The condition that will couse and process to move upward into the hiher priority queue.

6.       The mechanism for detremaining which queue a new process should enter into

7.       Whether arrival of a process into high level queurrrrre will preempt a process from a lower level queue

 Example – 

Consider a system which has a CPU bound process, which requires the burst time of 40 seconds.The multilevel Feed Back Queue scheduling algorithm is used and the queue time quantum ‘2’ seconds and in each level it is incremented by ‘5’ seconds.Then how many times the process will be interrupted and on which queue the process will terminate the execution? 

Solution – 
Process P needs 40 Seconds for total execution. 
At Queue 1 it is executed for 2 seconds and then interrupted and shifted to queue 2. 
At Queue 2 it is executed for 7 seconds and then interrupted and shifted to queue 3. 
At Queue 3 it is executed for 12 seconds and then interrupted and shifted to queue 4. 
At Queue 4 it is executed for 17 seconds and then interrupted and shifted to queue 5. 
At Queue 5 it executes for 2 seconds and then it completes. 
Hence the process is interrupted 4 times and completes on queue 5. 

Advantages:

1. It is more flexible.
2. It allows different processes to move between different queues.
3. It prevents starvation by moving a process that waits too long for lower priority queue to the higher priority queue.

Disadvantages:

1. For the selection of the best scheduler, it require some other means to select the values.
2. It produces more CPU overheads.
3. It is most complex algorithm.