Get Your (RTOS Task and ISR) Priorities Straight!

Get your priorities straight!  Or suffer the consequence of missed deadlines.  Of course, I’m talking here about the relative priorities of your real-time tasks and interrupt service routines.  In my travels around the embedded design community, I have learned that most real-time systems are designed with ad hoc priorities.

Unfortunately, mis-prioritized systems often “appear” to work fine without discernibly missing critical deadlines in testing.  The worst-case workload may have never yet happened in the field or there is sufficient CPU to accidentally succeed despite the lack of proper planning.  This has lead to a generation of embedded software developers being unaware of the proper technique.  There is simply too little feedback from non-reproducible deadline misses in the field to the team that designed it—unless a death and a lawsuit forces an investigation.

Truth be told: There is a science to the process of assigning relative priorities.  That science is associated with the “rate monotonic algorithm,” which provides a formulaic way to assign task priorities based on facts.  It is also associated with the “rate monotonic analysis,” which helps you prove that your correctly-prioritized tasks and ISRs will find sufficient available CPU bandwidth between them during extreme busy workloads called “transient overload.”   It’s too bad most engineers don’t know how to use these tools.

I’ve written on the why and how of RMA before and recommend you start with my article Introduction to Rate-Monotonic Scheduling and then read my blog post 3 Things Every Programmer Should Know About RMA.

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