Fair-share CPU scheduling

Introduction

Fair-share CPU scheduling is a scheduling algorithm used in operating systems that aims to allocate CPU resources fairly among different user or process groups. The fair-share scheduler assigns a weight to each process or group based on its historical usage and allocates CPU resources based on these weights, ensuring that no group is starved of resources for an extended period of time. This allows for better resource utilization and provides equal opportunities for all groups to access the CPU. Fair-share scheduling is commonly used in multi-user systems and virtualized environments where multiple users or virtual machines share a single physical CPU.

Fair-share scheduling Algorithms

There are several fair-share scheduling algorithms that can be used in operating systems, including −

• Weighted Fair Queueing (QFQ) − This algorithm assigns a weight to each process or flow based on its traffic characteristics and ensures that each flow receives its fair share of the CPU time.

• Generalized Processor Sharing (GPS) − GPS is a more general version of WFQ and provides more precise control over the allocation of CPU resources. It is based on the concept of fluid modeling and assigns a virtual processor to each process or flow.

• Stochastic Fairness Queueing (SFQ) − This algorithm divides the available bandwidth into equal-sized time slots and assigns each flow to a particular time slot based on its hash value. This ensures that each flow receives a fair share of the CPU time.

• Fair Queueing (FQ) − FQ assigns a queue to each flow and serves the queues in a round-robin manner, ensuring that each queue receives its fair share of the CPU time.

• Virtual Round Robin (VRR) − VRR assigns a virtual time slice to each process or flow and serves them in a round-robin manner, ensuring that each process or flow receives its fair share of the CPU time.

Implementation of Fair-share Scheduling

Fair-share scheduling can be implemented in different ways, including the kernel or in user-level software.

• Kernel-level implementation − The implementation of fair-share scheduling in the kernel involves the following steps −

• Identify process groups − The kernel identifies process groups based on the ownership of processes. Each group is assigned a weight based on its historical CPU usage.

• Calculate fair-share − The kernel calculates the fair-share of CPU resources for each process group based on its weight.

• Allocate CPU resources − The kernel allocates CPU resources to each process group based on its fair-share. If a process group has not used its fair-share, it is allowed to accumulate unused resources for later use.

• Adjust weights − The kernel periodically adjusts the weights of each process group based on its recent CPU usage to ensure fair resource allocation.

• User-level Implementation − The following steps are required to implement the fair-share scheduling in the user-level −

• Identify process groups − Group processes together based on their resource requirements or ownership. Each group is treated as a unit for resource allocation.

• Assign weights − Assign a weight to each process group based on its resource requirements or historical usage. The weight represents the group's entitlement to CPU resources.

• Calculate fair-share − Calculate the fair-share of CPU resources for each process group based on its weight. The fair-share represents the group's share of the total available CPU resources.

• Allocate CPU resources − Allocate CPU resources to each process group based on its fair-share. If a process group has not used its fair-share, it is allowed to accumulate unused resources for later use.

• Monitor and adjust weights − Monitor the CPU usage of each process group and adjust their weights periodically based on their usage. This ensures that each group receives its fair share of CPU resources over time.

• Implement in code − Implement the fair-share CPU scheduling algorithm in user-level software using programming languages or libraries. The code should be able to group processes, assign weights, calculate fair-shares, and allocate resources based on the fair-share.

Advantages of Fair-share CPU Scheduling

Below are some advantages that this algorithm provides −

• Fairness − Fair-share scheduling ensures that CPU resources are allocated fairly among different process groups, regardless of their priorities or resource requirements.

• Resource utilization − Fair-share scheduling optimizes resource utilization by allocating CPU resources based on historical usage and entitlements, rather than on a fixed priority scheme.

• Prevention of starvation − Fair-share scheduling prevents resource starvation by allocating CPU resources based on a process group's entitlements and unused resources.

• Predictability − : Fair-share scheduling provides predictable CPU resource allocation to each process group, which can improve system stability and performance.

• Scalability − Fair-share scheduling is scalable and can be applied to systems with a large number of processes or process groups, ensuring that each group receives its fair share of CPU resources.

• Flexibility − Fair-share scheduling allows users to adjust the weight of each process group based on their resource requirements and usage patterns, providing flexibility in resource allocation.

• Improved responsiveness − Fair-share scheduling can improve system responsiveness by ensuring that CPU resources are allocated fairly among different process groups, reducing the likelihood of CPU-intensive processes monopolizing the resources.

Limitations of Fair-share CPU scheduling

Overall, while fair-share CPU scheduling can provide a balanced approach to resource allocation, it may not be suitable for all systems or applications and requires careful configuration and management to ensure proper operation.

• Overhead − Fair-share CPU scheduling can be computationally expensive and require additional overhead to track CPU usage and calculate fair − share weights.

• Complexity − Fair-share scheduling is more complex than other scheduling algorithms and requires additional configuration and management to ensure proper operation.

• Inflexibility − Fair-share scheduling may not be suitable for systems with hard real-time requirements or for applications that require a fixed allocation of CPU resources.

• Dependency on user-level implementation − User-level implementation of fair-share scheduling may not be as efficient as kernel-level implementation and may introduce additional overhead and complexity.

• Difficulty in tuning − Tuning the fair-share weights can be difficult, as it requires understanding the resource requirements and usage patterns of each process group.

• Limited effectiveness − Fair-share scheduling may not be effective in systems with a large number of short-lived processes or in systems with highly variable resource requirements.

Integration with other scheduling algorithms

Fair-share scheduling can be integrated with other scheduling algorithms to improve the overall efficiency and fairness of the system. Here are some ways in which fair-share scheduling can be integrated with other scheduling algorithms −

• Multi-level Feedback Queue − Fair-share scheduling can be integrated with a multi-level feedback queue (MLFQ) algorithm to provide a more balanced approach to resource allocation. The MLFQ algorithm can be used to allocate CPU resources based on priority, while fair-share scheduling can be used to ensure that each process group receives its fair share of CPU resources.

• Priority-based scheduling − Fair-share scheduling can be integrated with a priority-based scheduling algorithm to provide a more balanced approach to resource allocation. The priority-based algorithm can be used to allocate CPU resources based on priority, while fair-share scheduling can be used to ensure that each process group receives its fair share of CPU resources.

• Real-time scheduling − Fair-share scheduling can be integrated with a real-time scheduling algorithm to provide a more balanced approach to resource allocation. The real-time scheduling algorithm can be used to allocate CPU resources based on deadlines, while fair-share scheduling can be used to ensure that each process group receives its fair share of CPU resources.

Calculation of Fair-share

The calculation of fair-share in fair-share CPU scheduling is based on the concept of entitlement and usage. The fair-share weight of a process group is calculated as follows −

• Determine the historical usage of the process group over a recent period of time (e.g. last minute, last hour, etc.).

• Calculate the entitlement of the process group based on the total CPU resources available and the number of process groups.

• Calculate the fair-share weight of the process group as the ratio of historical usage to entitlement.

For example, suppose there are three process groups in the system with historical CPU usage as follows −

• Group A: 10 seconds

• Group B: 20 seconds

• Group C: 30 seconds

Assuming that the total CPU time available is 60 seconds, the entitlement of each process group is 20 seconds (i.e. 60 / 3). The fair-share weight of each process group can be calculated as follows −

• Group A: 10 / 20 = 0.5

• Group B: 20 / 20 = 1

• Group C: 30 / 20 = 1.5

So, Group A is entitled to use 50% of the CPU resources, Group B is entitled to use 100% of the CPU resources, and Group C is entitled to use 150% of the CPU resources. The fair-share scheduler will allocate CPU resources to each process group based on their fair-share weights, ensuring that each process group receives its fair share of CPU resources over time.

Conclusion

Overall, fair-share CPU scheduling is a useful technique for ensuring that CPU resources are allocated fairly and efficiently among different process groups, and it can be a valuable addition to any operating system scheduler.