Exploring Priority-Based Round Robin Scheduling

Key Highlights of Priority-Based Round Robin

Hybrid Approach: Priority-Based Round Robin combines the fairness of Round Robin with the importance handling of Priority Scheduling.
Time Quantum and Priorities: Processes are scheduled based on both their assigned priority level and a fixed time slice (quantum).
Mitigating Starvation: This hybrid method aims to reduce the risk of lower-priority processes being indefinitely delayed, a common issue in pure Priority Scheduling.

CPU scheduling is a fundamental concept in operating systems, determining which process gets access to the CPU and for how long. Two common scheduling algorithms are Priority Scheduling and Round Robin. While each has its advantages, they also have limitations. Priority Scheduling can lead to starvation of low-priority processes, while pure Round Robin treats all processes equally, potentially neglecting urgent tasks. To address these issues, a hybrid approach known as Priority-Based Round Robin scheduling has been developed. This method seeks to combine the best features of both algorithms, providing a more balanced and efficient scheduling solution.

Understanding the Core Concepts

What is Priority Scheduling?

Priority Scheduling is a CPU scheduling algorithm that assigns a priority level to each process. The CPU is allocated to the process with the highest priority. If two processes have the same priority, they are typically scheduled using a tie-breaking rule, such as First-Come, First-Served (FCFS). Priority Scheduling can be either preemptive or non-preemptive. In preemptive priority scheduling, if a new process arrives in the ready queue with a higher priority than the currently executing process, the CPU is immediately allocated to the new process. In non-preemptive priority scheduling, the currently executing process completes its burst time before the CPU is allocated to another process.

An advantage of Priority Scheduling is that it allows for the execution of important tasks quickly. However, a significant disadvantage is the potential for starvation, where low-priority processes may never get to execute if there is a continuous stream of high-priority processes.

Here's an image illustrating the concept of CPU scheduling:

Illustration of CPU Scheduling

What is Round Robin Scheduling?

Round Robin (RR) scheduling is a preemptive scheduling algorithm designed for time-sharing systems. It assigns a fixed unit of time, called a time quantum (or time slice), to each process in a cyclic manner. Processes are placed in a circular queue, and the CPU scheduler iterates through the queue, allocating the CPU to each process for the duration of the time quantum. If a process does not complete within its time quantum, it is preempted and moved to the end of the ready queue, and the CPU is allocated to the next process in the queue.

The primary advantage of Round Robin scheduling is fairness; it ensures that every process gets a chance to execute and prevents starvation. However, the performance of RR scheduling is highly dependent on the size of the time quantum. A very small quantum can lead to excessive context switching overhead, while a very large quantum can make RR scheduling behave similarly to FCFS.

Below is a video explaining the Round Robin algorithm in more detail:

Explanation of the Round Robin Algorithm

Combining Priority and Round Robin

The Need for a Hybrid Approach

Both Priority Scheduling and Round Robin have their strengths and weaknesses. In many real-world scenarios, processes have varying degrees of importance, making a pure Round Robin approach less than ideal. Conversely, relying solely on Priority Scheduling can lead to the indefinite postponement of less critical tasks. This is where the concept of Priority-Based Round Robin scheduling becomes valuable. By integrating priority levels into the Round Robin framework, the scheduler can prioritize important processes while still ensuring that lower-priority processes eventually get a chance to execute.

How Priority-Based Round Robin Works

Priority-Based Round Robin scheduling typically involves maintaining multiple ready queues, one for each priority level. The scheduler first serves processes in the highest-priority queue. Within each priority queue, processes are scheduled using the Round Robin algorithm with a defined time quantum. When the highest-priority queue is empty, the scheduler moves to the next lower-priority queue and applies Round Robin scheduling there. This continues until all processes are executed.

This approach allows high-priority processes to be executed quickly, similar to pure Priority Scheduling. However, because Round Robin is used within each priority level, processes of the same priority share the CPU fairly. Furthermore, by eventually moving to lower-priority queues, this method reduces the risk of starvation for less critical processes, although it doesn't entirely eliminate it if high-priority processes are constantly arriving.

Illustrating with Five Processes

Scenario Setup

Let's consider a scenario with five processes: P1, P2, P3, P4, and P5. Each process has an arrival time, a burst time (the total CPU time required for execution), and a priority level. A lower priority number indicates a higher priority (e.g., priority 1 is higher than priority 2). We will also assume a time quantum for the Round Robin scheduling within each priority level.

Here is a table representing the processes and their attributes:

Process	Arrival Time	Burst Time	Priority
P1	0	10	2
P2	1	5	1
P3	3	8	3
P4	5	6	2
P5	7	4	1

Attributes of the Five Processes

For this example, let's assume a time quantum of 3 units.

Execution Flow with Priority-Based Round Robin (Time Quantum = 3)

Time 0:

P1 arrives (Priority 2). It is the only process, so it starts executing.

Time 1:

P2 arrives (Priority 1). Since P2 has a higher priority than P1, P1 is preempted, and P2 starts executing.

Time 3:

P3 arrives (Priority 3). P2 is still executing. P2's priority (1) is higher than P3's priority (3), so P2 continues. P2 has executed for 2 time units (from time 1 to time 3).

Time 4:

P2 completes its burst time (5 units). P2 finishes. The ready queue now contains P1 (remaining burst time 10 - 3 = 7) and P3 (burst time 8). P1 has a higher priority (2) than P3 (3), so P1 resumes execution.

Time 5:

P4 arrives (Priority 2). P1 is executing. P1 and P4 have the same priority (2). P1 has executed for 1 time unit since resuming (from time 4 to time 5).

Time 7:

P5 arrives (Priority 1). P1 is executing. P5 has the highest priority (1). P1 is preempted (having executed for 1 + 3 = 4 units in total so far), and P5 starts executing.

Time 11:

P5 completes its burst time (4 units). P5 finishes. The ready queue now contains P1 (remaining burst time 7 - 3 = 4), P4 (burst time 6), and P3 (burst time 8). The highest priority processes in the queue are P1 and P4, both with priority 2. They will be scheduled using Round Robin.

Time 11 - 14:

P1 executes for 3 time units (its time quantum). Remaining burst time for P1 is 4 - 3 = 1. P1 is moved to the end of the priority 2 queue.

Time 14 - 17:

P4 executes for 3 time units (its time quantum). Remaining burst time for P4 is 6 - 3 = 3. P4 is moved to the end of the priority 2 queue.

Time 17 - 18:

P1 resumes execution as it's the next in the priority 2 queue. It completes its remaining burst time of 1 unit. P1 finishes.

Time 18 - 21:

P4 resumes execution. It executes for 3 time units (its time quantum). Remaining burst time for P4 is 3 - 3 = 0. P4 finishes.

Time 21 - 29:

The priority 2 queue is now empty. The scheduler moves to the priority 3 queue, which contains P3 (burst time 8). P3 executes and completes its burst time.

The execution order based on this scenario would be: P1 (briefly) -> P2 -> P1 -> P4 -> P1 -> P4 -> P3.

This example demonstrates how higher-priority processes (P2 and P5) are given preference, while processes with the same priority (P1 and P4) are scheduled in a round-robin fashion. Lower-priority processes (P3) are executed after higher-priority ones have finished or yielded the CPU.

An image representing a Gantt chart for a scheduling algorithm might look like this:

Example of a Gantt Chart (Illustrative)

Advantages and Disadvantages

Advantages of Priority-Based Round Robin

Improved Responsiveness for High-Priority Tasks: Critical processes are handled promptly due to their higher priority.
Reduced Starvation (compared to pure Priority Scheduling): By using Round Robin within each priority level and eventually moving to lower levels, processes are less likely to be indefinitely delayed.
Fairness within Priority Levels: Processes of the same importance share the CPU fairly due to the Round Robin mechanism.

Disadvantages of Priority-Based Round Robin

Complexity: Implementing and managing multiple priority queues adds complexity to the scheduler.
Difficulty in Determining Priorities: Assigning appropriate priority levels to processes can be challenging and may require system-specific knowledge.
Potential for Starvation (though reduced): If there is a constant influx of high-priority processes, lower-priority processes may still experience significant delays. Techniques like aging (gradually increasing the priority of processes that have been waiting for a long time) can help mitigate this.

Comparison with Other Scheduling Algorithms

It's helpful to see how Priority-Based Round Robin compares to other common scheduling algorithms:

Feature	Priority Scheduling	Round Robin	Priority-Based Round Robin
Preemptive	Can be (Preemptive or Non-preemptive)	Yes	Yes
Fairness	Low (can lead to starvation)	High (equal time slices)	Moderate (fairness within priority levels)
Starvation	High risk	Low risk	Reduced risk (compared to pure Priority)
Complexity	Moderate	Simple	Higher
Responsiveness for High Priority	High	Moderate (depends on quantum)	High

Comparison of Scheduling Algorithms

FAQ

What is a time quantum in Round Robin scheduling?

A time quantum is a fixed, small unit of time that the CPU scheduler allocates to each process in a Round Robin system. Once a process has executed for its time quantum, it is preempted and moved to the end of the ready queue.

What is starvation in the context of CPU scheduling?

Starvation occurs when a process is perpetually denied access to the CPU because other processes with higher priorities or shorter burst times are always scheduled ahead of it.

How does aging help prevent starvation in Priority Scheduling?

Aging is a technique where the priority of a waiting process is gradually increased over time. This ensures that even low-priority processes will eventually reach a priority level high enough to be scheduled for execution, preventing indefinite starvation.

Is Priority-Based Round Robin always the best scheduling algorithm?

No, the best scheduling algorithm depends on the specific requirements of the system and the workload. Priority-Based Round Robin offers a good balance for systems that require both responsiveness for important tasks and a degree of fairness for all processes, but its complexity might be a drawback in simpler systems.