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# Highest Response Ratio Next (HRRN) Scheduling

Last Updated on May 2, 2024 by Abhishek Sharma

In the realm of operating systems, scheduling algorithms play a crucial role in managing the execution of processes efficiently. One such algorithm, known as Highest Response Ratio Next (HRRN) Scheduling, aims to maximize the overall throughput and minimize the average response time of processes. In this article, we delve into the intricacies of HRRN Scheduling, its key concepts, advantages, and implementation.

## What is HRRN Scheduling?

HRRN Scheduling is a non-preemptive scheduling algorithm that selects the next process to execute based on its response ratio, which is calculated as the ratio of the waiting time of a process to its expected run time. The idea behind HRRN Scheduling is to give preference to processes that have been waiting longer and have a smaller expected run time, thus aiming to reduce the average waiting time and improve overall system performance.

### Basic Concepts of HRRN Scheduling

Below are the concepts of HRRN Scheduling:

• Response Ratio Calculation: The response ratio of a process is calculated using the formula: Response Ratio = (Waiting Time + Expected Run Time) / Expected Run Time. The waiting time is the time a process has spent waiting in the ready queue, and the expected run time is the estimated time the process requires to complete its execution.
• Process Selection: When a scheduling decision needs to be made, the process with the highest response ratio is selected for execution. This ensures that processes with a higher priority (i.e., longer waiting time and shorter expected run time) are given preference.
• Non-Preemptive Nature: HRRN Scheduling is a non-preemptive algorithm, meaning that once a process is selected for execution, it continues to run until it completes its execution or blocks.
• Starvation Prevention: HRRN Scheduling helps prevent starvation by ensuring that processes with longer waiting times are given priority. Even processes with shorter expected run times have a chance of being selected if they have been waiting long enough.

• Improved Response Time: HRRN Scheduling aims to minimize the average response time of processes by giving priority to those that have been waiting longer.
• Fairness: The algorithm ensures fairness by considering both the waiting time and expected run time of processes when making scheduling decisions.
• High Throughput: By prioritizing processes with shorter expected run times, HRRN Scheduling can lead to higher overall throughput and system efficiency.
• Easy to Implement: HRRN Scheduling is relatively simple to implement compared to other scheduling algorithms, as it only requires calculating the response ratio for each process.

### Implementation of HRRN Scheduling

Implementing HRRN Scheduling involves several key steps:

• Initialization: Initialize the waiting time and expected run time for each process.
• Calculate Response Ratios: Calculate the response ratio for each process using the formula mentioned earlier.
• Process Selection: Select the process with the highest response ratio for execution.
• Execute Process: Execute the selected process until it completes its execution or blocks.
• Update Waiting Times: Update the waiting times of all other processes in the ready queue.
• Repeat: Repeat the process selection and execution steps until all processes have completed their execution.

Example Scenario
Consider a system with three processes: A, B, and C. Process A has been waiting for 5 milliseconds with an expected run time of 10 milliseconds, process B has been waiting for 3 milliseconds with an expected run time of 6 milliseconds, and process C has been waiting for 7 milliseconds with an expected run time of 14 milliseconds.

The response ratios for each process are calculated as follows:

• Process A: (5 + 10) / 10 = 1.5
• Process B: (3 + 6) / 6 = 1.5
• Process C: (7 + 14) / 14 = 1.5

In this scenario, all processes have the same response ratio, so the next process to execute can be selected arbitrarily or based on a predefined order.

Conclusion
Highest Response Ratio Next (HRRN) Scheduling is a scheduling algorithm that aims to improve system performance by giving priority to processes with higher response ratios, which indicates a higher priority based on waiting time and expected run time. By considering both factors, HRRN Scheduling offers improved response times, fairness, and high throughput, making it a valuable scheduling algorithm in operating system design.

## FAQs about Highest Response Ratio Next (HRRN) Scheduling

Below are some of the FAQs related to Highest Response Ratio Next (HRRN) Scheduling:

1. How does HRRN Scheduling differ from other scheduling algorithms?
HRRN Scheduling differs from other scheduling algorithms in that it considers both the waiting time and expected run time of processes when making scheduling decisions. This allows it to prioritize processes that have been waiting longer and have shorter expected run times.

2. How is the response ratio calculated in HRRN Scheduling?
The response ratio of a process in HRRN Scheduling is calculated using the formula: Response Ratio = (Waiting Time + Expected Run Time) / Expected Run Time. The waiting time is the time a process has spent waiting in the ready queue, and the expected run time is the estimated time the process requires to complete its execution.

3. Can HRRN Scheduling lead to starvation?
HRRN Scheduling is designed to prevent starvation by giving priority to processes that have been waiting longer. Even processes with shorter expected run times have a chance of being selected if they have been waiting long enough, thus ensuring fairness in process scheduling.

4. How does HRRN Scheduling handle processes with varying execution times?
HRRN Scheduling does not prioritize processes based solely on their expected run times. Instead, it considers both the waiting time and expected run time of processes, allowing it to adapt to varying execution times and prioritize processes accordingly.

5. Is HRRN Scheduling suitable for real-time systems?
HRRN Scheduling may not be suitable for real-time systems where strict deadlines must be met, as it does not guarantee that processes will be scheduled based on their deadlines. However, it can be used in systems where high throughput and fairness are more important than meeting strict deadlines.