Highest Response Ratio Next

operating-systems scheduling

Definition

Highest Response Ratio Next (HRRN)

Highest Response Ratio Next (HRRN) selects the process with the highest response ratio:

$$RR=\frac{w+s}{s}$$

where $w$ is the waiting time and $s$ is the estimated service time.

Properties

Selection

The process with maximum $RR$ is chosen.

Non-Preemptive

Once running, a process continues until it terminates or blocks for I/O.

Starvation Freedom

Unlike SPN, HRRN prevents starvation. As $w$ increases, even long processes eventually gain the highest ratio.

Efficiency

Favours shorter processes (small $s$) while gradually increasing priority of longer-waiting processes.

Overhead

Requires estimation of service times, similar to SPN.