A New Metric: Response Time

Thus, if we knew job lengths, and that jobs only used the CPU, and our only metric was turnaround time, STCF would be a great policy. In fact, for a number of early batch computing systems, these types of scheduling algorithms made some sense. However, the introduction of time-shared machines changed all that. Now users would sit at a terminal and demand interactive performance from the system as well. And thus, a new metric was born: response time.

Defining response time

We define response time as the time from when the job arrives in a system to the first time it is scheduled.Some define it slightly differently, e.g., to also include the time until the job produces some kind of “response”; our definition is the best-case version of this, essentially assuming that the job produces a response instantaneously.

More formally:

$T$_$response$ $=$ $T$_$firstrun$ $−$ $T$_$arrival$

For example, if we had the schedule above (with A arriving at time 0, and B and C at time 10), the response time of each job is as follows: 0 for job A, 0 for B, and 10 for C (average: 3.33).