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Multilevel Queue Scheduling | MLQ CPU Scheduling

Multilevel Queue Scheduling

You use your computer for a lot of tasks at one time. You play coding games on PrepBytes, communicate on WhatsApp, and listen to music. Have you ever questioned how all of these processes can run simultaneously? You can debate the merits of a multi-core CPU, but what if you only had one CPU at your disposal? We employ scheduling methods to achieve this. One such CPU scheduling algorithm is multilevel queue scheduling, in which the tasks that the CPU must carry out are separated into several groups according to various characteristics. In this article, we’ll learn about multilevel queue scheduling and go through benefits and drawbacks of multilevel queue scheduling.

What is Multilevel Queue Scheduling?

Each algorithm supports a particular operation, however, in a broader system, some processes necessitate priority algorithm scheduling. While some processes (interactive processes) seek to remain in the system, other processes are background processes whose execution can be postponed.

Different systems may have different numbers of ready queue algorithms both between and within queues. For such maintenance, a round-robin procedure with different time quantum is often used. There are different kinds of scheduling algorithms made for situations where it is simple to group the operations. Different scheduling algorithms are needed for two varied types of processes since they have different response times and resource demands. There is a distinction between the foreground (interactive) and background (batch) processes. Processes in the background are given precedence over foreground processes.

The multilevel queue scheduling method has divided the ready queue into seven separate queues. Depending on their priority, such as memory capacity, process priority, or kind, these processes are each given a queue. Each queue’s scheduling process is unique. The foreground process uses some queues, while the background process uses others. The background queue can be planned using an FCFS technique, while the foreground queue can be scheduled using a round-robin method.

Example of Multilevel Queue Scheduling

To further understand how this scheduling operates, let’s look at an example of a multilevel queue-scheduling algorithm with five queues:

  • System process
  • Interactive processes
  • Interactive editing processes
  • Batch processes
  • Student processes

Every queue would take precedence over queues with lower priorities. Until the high-priority queues are empty, no process may proceed. No other process may run in the scenario mentioned above until the system, interactive, and editing process queues are empty. A batch process will be preempted if an interactive editing process joins the ready queue while one is already running.
The following are descriptions of the procedures utilized in the example above:

System Process
The OS has its own process to run, known as the System Process.

Interactive Process
The System Process, a process that the OS has to run, is known as such.

Batch Process
An operating system function called batch processing gathers applications and data into a batch before processing begins.

Student Process
The system process is consistently given top priority, whereas the student procedures are consistently given bottom priority.

Advantages of Multilevel Queue Scheduling:

Here are some advantages of using a multilevel queue scheduling algorithm:

**1. Improved performance:** By dividing the processes into different queues based on their priority or type, the operating system can allocate resources more efficiently. This means that the system can handle more processes at the same time and execute them more quickly, resulting in improved performance.

**2. Better resource allocation:** Multilevel queue scheduling helps in better resource allocation as it assigns different priorities to different processes based on their requirements. This ensures that high-priority processes get the resources they need first, reducing the likelihood of resource starvation.

**3. Fairness:** The multilevel queue scheduling algorithm ensures that all processes are treated fairly, regardless of their priority or type. Each process is allocated CPU time in a predefined manner, and the system ensures that each queue is serviced at a specific rate.

**4. Flexibility:** Multilevel queue scheduling provides flexibility in scheduling processes of different types. For instance, the foreground processes can be given a higher priority than the background processes, which can be run at a lower priority.

**5. Improved responsiveness:** The multilevel queue scheduling algorithm ensures that high-priority processes are executed first, which can improve system responsiveness. This is particularly important for real-time applications that require immediate processing of data.

Disadvantages of Multilevel Queue Scheduling:

While multilevel queue scheduling has several advantages, there are also some drawbacks of using this algorithm. Here are some of them:

**1. Complex implementation:** Multilevel queue scheduling is a complex algorithm to implement. It requires a significant amount of programming effort and system resources to set up multiple queues and manage the priority and scheduling of processes in each queue.

**2. High Overhead:** The overhead associated with maintaining multiple queues can be significant, especially in systems with a large number of processes. The system has to constantly monitor and update the queues, which can slow down the system’s performance.

**3. Inefficient use of resources:** In some cases, multilevel queue scheduling may result in inefficient use of resources. For instance, if a process in a low-priority queue needs more resources, it may have to wait for an extended period before it gets the resources it needs.

**4. Starvation:** Starvation is another potential issue with multilevel queue scheduling. If a queue has a large number of low-priority processes, they may never get the resources they need, resulting in starvation.

5. Limited scalability: Multilevel queue scheduling is not highly scalable. As the number of queues and processes increases, the system’s overhead and complexity also increase, making it difficult to manage.

Let’s explain it with an example:

Let’s take an explain of MLQ which shows how MQL Scheduling works

As we saw in the above table Queue 1 has higher priority then Queue and the Round Robin is used in Queue 1 ( Time Quantum=2) while first come first serve is used in Queue 2.

Lets see the GANT CHART:

Both the queues are procesed from the Start. Therefore, queues (P1, P2) runs first (because of greater priority) in a Round Robin way 1 finished after 7 units.

The process in Queue 2 (Process P3) starts running (as there is no process in queue 1) but while it is executing, P4 enter the queue 1 and interrupts P3, and then P3 takes the CPU and completes the execution.

Multilevel Feedback Scheduling

Although every algorithm supports a different activity, some operations necessitate scheduling via a priority algorithm in a broader system. This style of organization benefits from minimal scheduling but is rigid; there is a separate queue for foreground or background processes, but they do not switch between queues or change their foreground or background nature.

Operations that need I/O and are interactive are given priority in this technique. It has a distinct CPU burst time and is a distinct process. It makes it possible for a process to change queues. A process will be moved to the lowest priority queue if it uses up too much processing time. A process may be moved to a higher priority queue if it waits in a lower priority queue for an excessive amount of time. Starvation is avoided with this kind of aging.

The following are the multilevel feedback queue scheduler’s parameters:

  • The system’s scheduling method for each queue.
  • the system’s queue number
  • the process for deciding when to demote a queue to a lower-priority queue.
  • This process chooses the time at which it is upgraded to a higher priority queue.
  • The procedure for choosing which processes will be added to the queue and when they will need service

Frequently Asked Questions

Here are some frequently asked questions on multilevel queue scheduling.

1. What is Multilevel Queue Scheduling?
CPU scheduling known as "multilevel queue scheduling" divides the processes that are in the ready state into various groups, each of which has its own scheduling requirements.

2. What do you mean by CPU scheduling?
The process of CPU scheduling chooses which process will control the CPU while the other processes are put on hold.

3. What are the advantages of multilevel queue scheduling?
We can use several scheduling techniques using multilevel queue scheduling, and it has minimal scheduling overhead.

4. What are the disadvantages of multilevel queue scheduling?
It is rigid, and there is a danger that lower-priority processes could go hungry.

5. What is Starvation?
When low-priority processes are blocked for an extended period of time while high-priority processes continue to run, this is known as starvation.

In this post, we went into great detail on multilevel queue scheduling in operating systems. We hope that this blog post has given you a better understanding of multilevel queue scheduling, including what it is, how it operates, and its benefits and drawbacks. The articles listed below can help you improve your understanding of operating systems.

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