Selfish Round Robin CPU Scheduling

Selfish Round Robin (SRR) is a CPU scheduling algorithm that modifies traditional round robin by introducing dynamic priority adjustments. Unlike standard round robin where all processes get equal treatment, SRR allows processes to "selfishly" increase their priority based on execution time, creating a more adaptive scheduling approach.

The traditional round-robin scheduling algorithm is preemptive, giving each process a fixed time slice. After the quantum expires, the process moves to the end of the ready queue. SRR enhances this by maintaining two separate queues and allowing priority manipulation to favor longer-running processes.

How Selfish Round Robin Works

SRR maintains two distinct process lists:

• New Queue Contains newly arrived processes that must wait

• Accepted Queue Contains processes eligible for CPU execution using round robin

New processes cannot immediately join the Accepted queue. They must wait until accepted processes complete their service or meet certain criteria. Within the Accepted queue, processes receive CPU time using traditional round robin with equal time slices.

Dynamic Priority Adjustment

Processes can dynamically alter their priority levels during execution. When a process requests higher priority, the operating system redistributes priority levels among all processes. This creates a competitive environment where:

• Longer-running processes can maintain CPU access by increasing priority

• Other processes must compete by adjusting their own priorities

• Priority changes affect the order of CPU allocation within the Accepted queue

Example Execution

Consider three processes P1 (high priority), P2 (medium priority), and P3 (low priority):

Advantages

• Improved throughput Processes utilize CPU time more efficiently

• Adaptive scheduling Priority adjustments based on actual CPU usage

• Reduced starvation for long-running processes compared to strict priority schemes

Disadvantages

• Potential for abuse Processes can monopolize CPU by constantly increasing priority

• Increased waiting time New processes must wait for Accepted queue entry

• Not suitable for real-time systems Unpredictable scheduling due to dynamic priorities

• Higher response time Short processes may wait behind long-running high-priority ones

Conclusion

Selfish Round Robin scheduling balances fairness with efficiency by allowing dynamic priority adjustments while maintaining round robin principles. It works well for systems where longer processes need sustained CPU access, but the potential for priority manipulation makes it unsuitable for time-critical applications.