Scheduling Metrics

operating-systems scheduling

Definition

Scheduling Metrics

Scheduling metrics are quantitative measures used to evaluate and compare the performance of different scheduling strategies.

Key Metrics

Turnaround Time ($T_r$)

The total time a process spends in the system, from the moment of arrival to the moment of completion.

$$T_r=T_{completion}-T_{arrival}=\text{Wait Time}+\text{Service Time}$$

Wait Time ($w$)

The total time a process spends in the Ready Queue waiting for the CPU.

Service Time ($s$)

The actual amount of time the process is in the Running state executing on the processor.

Normalised Turnaround Time

The ratio of turnaround time to service time.

$$\text{Normalized }{T}_r=\frac{T_r}{s}$$

Significance: Indicates the relative delay experienced by a process. A value of 1.0 is the ideal minimum (no waiting). Higher values indicate more significant relative delays. It is particularly useful for comparing how fairly processes of different lengths are treated.

Response Ratio ($RR$)

Used specifically in the HRRN strategy to balance short processes and long-waiting ones.

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

Mechanism: As wait time $w$ increases, the ratio grows, eventually allowing even long processes to out-compete new short ones, thereby preventing starvation.