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Understanding Predictability in Real-Time Systems

Posted by Guest Blogger on Sep 28, 2015 8:22:14 PM

The following is a guest post by veteran developer E. Douglas Jensen. Doug is internationally recognized as one of the original pioneers and leading visionaries of distributed real-time and time-critical computer systems. His principal interest is advancing and applying the principles and best practices in the field of time-critical embedded systems for control applications at all levels of an enterprise. He has four decades of performing system architecture, detailed design, and software/hardware implementation on the system's entire life cycle-from initial requirements to deployment to product improvements.You can follow Doug on Facebook.

NOTE: We are looking for more guest contributors to our blog. If you have an interesting perspective on embedded development or tips and techniques you want to share with your fellow developers, please contact Stephen Martin (

Predictable is not the same as deterministic

Virtually everyone's personal and professional lives--and a great many physical, chemical, etc. processes--have numerous actions with a property that I term timeliness quality of service (QoS).

This timeliness QoS property is an integral part of the logic of the actions, not (for example) a performance metric of the action's logic, such as "how fast" an action takes place.

An action's timeliness QoS property has two keystone elements:

  • the action's completion time constraint (a deadline is a familiar, simple special case)
  • the predictability of the action's completion time with respect to the time constraint (always meeting all deadlines is a familiar, simple special case).
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