Highest Response Ratio Next

operating-systems scheduling

Definition

Highest Response Ratio Next (HRRN)

Highest Response Ratio Next (HRRN) is a scheduling strategy that selects the process with the highest response ratio, defined as:
$RR=\frac{w+s}{s}$
where $w$ is the time spent waiting in the Ready Queue and $s$ is the estimated service time.

Characteristics

• Selection Function: The process with the maximum $RR$ is chosen.
• Decision Mode: Non-preemptive.

Advantages

• Fairness: Unlike SPN, HRRN prevents starvation. As a process waits, its numerator $(w+s)$ increases, eventually ensuring that even a long process $(larges)$ will have the highest ratio.
• Efficiency: Favours shorter processes (smaller $s$) while gradually increasing the priority of longer-waiting processes.
• Overhead: Like SPN, it requires the estimation of service times.