12 Performance Requirements
12a Speed and Latency Requirements
Content
Specifies the amount of time available to complete specified tasks. These
requirements often refer to response times. They can also refer to the product’s abilityto operate at a speed suitable for the intended environment.
Motivation
Some products—usually real-time products—must be able to perform some of their
functionality within a given time slot. Failure to do so may mean catastrophic failure
(e.g., a ground-sensing radar in an airplane fails to detect an upcoming mountain) or
the product will not cope with the required volume of use (e.g., an automated ticketselling
machine).
Examples
Any interface between a user and the automated system shall have a maximum
response time of 2 seconds.
The response shall be fast enough to avoid interrupting the user’s flow of thought.
The product shall poll the sensor every 10 seconds.
The product shall download the new status parameters within 5 minutes of a change.
Fit Criterion
Fit criteria are needed when the description of the requirement is not quantified.
However, we find that most performance requirements are stated in quantified terms.
The exception is the second requirement shown above, for which the suggested fit
criterion is
The product shall respond in less than 1 second for 90 percent of the interrogations.
No response shall take longer than 2.5 seconds.
Considerations
There is a wide variation in the importance of different types of speed requirements.
If you are working on a missile guidance system, then speed is extremely important.
By contrast, an inventory control report that is run once every six months has very
little need for a lightning-fast response time.
Customize this section of the template to give examples of the speed requirements
that are important within your environment.