Performance Requirements

Performance requirements define how well or how rapidly the system must perform specific functions. Performance requirements encompass speed (database response times, for instance), throughput (transactions per second), capacity (concurrent usage loads), and timing (hard real-time demands). Stringent performance requirements seriously affect software design strategies and hardware choices, so define performance goals that are appropriate for the operating environment. All users want their applications to run instantly, but the real performance requirements will be different for the spell-check feature of a word processing program and a radar guidance system for a missile. Performance requirements should also address how the system’s performance will degrade in an overloaded situation, such as when a 911 emergency telephone system is flooded with calls. Following are some simple performance requirement examples:

PE-1. The temperature control cycle must execute completely in 80 milliseconds.

PE-2. The interpreter shall parse at least 5000 error-free statements per minute.

PE-3. Every Web page shall download in 15 seconds or less over a 50 KBps modem connection.

PE-4. Authorization of an ATM withdrawal request shall not take more than 10 seconds.