Книга: Embedded Linux Primer: A Practical, Real-World Approach

17.1.2. Hard Real Time

17.1.2. Hard Real Time

Hard real time is characterized by the results of a missed deadline. In a hard real-time system, if a deadline is missed, the results are often catastrophic. Of course, catastrophic is a relative term. If your embedded device is controlling the fuel flow to a jet aircraft engine, missing a deadline to respond to pilot input or a change in operational characteristics can lead to disastrous results.

Note that the duration of the deadline has no bearing on the real-time characteristic. Servicing the tick on an atomic clock is such an example. As long as the tick is processed within the 1-second window before the next tick, the data remains valid. Missing the processing on a tick might throw off our global positioning systems by feet or even miles!

With this in mind, we draw on a commonly used set of definitions for soft and hard real time. For soft real-time systems, the value of a computation or result is diminished if a deadline is missed. For hard real-time systems, if a single deadline is missed, the system is considered to have failed, and might have catastrophic consequences.

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