Multilevel Feedback-Queue Scheduling Algorithm

Summary

This lecture introduces the multi-level feedback queue scheduling algorithm, an extension of multi-level queue scheduling. Unlike the latter, processes can move between queues based on CPU burst characteristics. Processes that use too much CPU time are moved to lower priority queues, while those waiting too long in lower queues may be moved up. The algorithm aims to prevent starvation and ensure fair CPU usage, with parameters like number of queues, scheduling algorithms per queue, and methods for upgrading or demoting processes between queues.

Takeaways

• 📋 Multi-level feedback queue scheduling allows processes to move between queues, unlike multi-level queue scheduling where processes remain in their assigned queue.
• 🕒 Processes are separated based on the characteristics of their CPU burst time, with processes using too much CPU time being moved to lower-priority queues.
• 🚀 Higher-priority queues are reserved for I/O-bound and interactive processes, which typically require shorter CPU bursts.
• ⏳ Processes in lower-priority queues that wait too long for CPU time can be moved to higher-priority queues to avoid starvation.
• 🔄 This method of moving processes between queues is a form of aging, preventing starvation by ensuring long-waiting processes eventually get CPU time.
• ⚖️ Scheduling within and between queues depends on different algorithms, such as round-robin with time quanta and first-come-first-serve for lower-priority queues.
• ⏱️ In the example, processes in the highest-priority queue (q0) are executed with an 8-millisecond time quantum, while lower queues have larger time quanta or different algorithms.
• 🔁 If a process does not finish within its time quantum, it is demoted to the next lower-priority queue, where it waits for the higher-priority queues to clear.
• ⚙️ The multi-level feedback queue scheduler uses parameters such as the number of queues, scheduling algorithms for each queue, and methods for upgrading or demoting processes.
• 🔄 Implementing multi-level feedback queue scheduling can be complex due to the need to manage scheduling algorithms, queue priorities, and process movements.

Q & A

What is the primary difference between multi-level queue scheduling and multi-level feedback queue scheduling?

The main difference is that in multi-level feedback queue scheduling, processes can move between queues based on their CPU burst characteristics, while in multi-level queue scheduling, processes are fixed in their assigned queues.

How are processes classified in multi-level feedback queue scheduling?

Processes are classified based on the characteristics of their CPU burst. Processes with longer CPU burst times may be moved to lower-priority queues, while those with shorter bursts may remain in higher-priority queues.

What happens to a process that uses too much CPU time in a high-priority queue?

If a process uses too much CPU time in a high-priority queue, it is moved to a lower-priority queue to give other processes a chance to execute.

How does the system handle input/output (I/O) bound and interactive processes in multi-level feedback queue scheduling?

I/O bound and interactive processes, which typically require less CPU time, are kept in higher-priority queues to ensure they get quicker access to the CPU.

What is aging, and how does it prevent starvation in multi-level feedback queue scheduling?

Aging refers to the process of moving a long-waiting process from a lower-priority queue to a higher-priority queue, preventing it from starving by ensuring it eventually gets CPU time.

Can you explain how time quantums work in the example provided for multi-level feedback queue scheduling?

In the example, each queue follows a different scheduling algorithm. For instance, queue 0 follows a round-robin approach with an 8-millisecond time quantum, queue 1 has a 16-millisecond quantum, and queue 2 follows a first-come, first-served algorithm.

What happens if a process does not complete execution within the time quantum in queue 0?

If a process in queue 0 does not complete its execution within the 8-millisecond time quantum, it is moved to the tail of queue 1, where it will wait for execution when queue 0 is empty.

How does scheduling occur among queues in multi-level feedback queue scheduling?

Scheduling among queues occurs based on their priority. Processes in higher-priority queues are executed first, and lower-priority queues are only considered when the higher-priority ones are empty.

What are some of the parameters that define a multi-level feedback queue scheduler?

The parameters include the number of queues, the scheduling algorithm for each queue, the method to determine when to upgrade or demote a process between queues, and the method to assign a process to a queue when it needs service.

Why is multi-level feedback queue scheduling considered more complex than multi-level queue scheduling?

It is more complex because it involves dynamic movement of processes between queues based on various factors, such as CPU burst time and wait time, which requires careful implementation and management of multiple scheduling algorithms and parameters.