A Technical History of the SEI

processor utilization. The sample applications allowed them to make the critical decision to build
upon RMS instead of EDF. The result of their work was rate monotonic analysis (RMA).
RMA is the application of generalized rate monotonic scheduling [Sha 1984]. It provides the theoretic
basis to bring engineering analysis to the design of real-time applications. It requires much
less information than the timeline approach and makes it much easier to accommodate integration
and evolution of complex real-time systems. RMA also provides the theoretic basis to bring engineering
analysis to real-time computing standards, such as languages, operating systems, middleware,
and hardware bus arbitration. Experience in applying RMA to real systems motivated the
SEI and collaborators to evolve new analytic tools.
The Consequence: Engineering
Replaces Art
An important factor in RMA is the ability to minimize
priority inversion, where a high-priority task is blocked
by a lower priority task. It helps system designers predict
whether task deadlines will be met before costly implementation.
This important factor has been instrumental
in enabling RMA to influence a host of hardware and
software standards.
Today, RMA is a basic component in real-time computing
textbooks and taught in many universities, such as
CMU and University of Illinois Urbana-Champaign. A
companion RMA handbook provides the definitive
guide for practitioners [Klein 1993]. RMA is also the
only real-time scheduling technology approved by the
Federal Aviation Administration for Level A avionic
software in networked control applications with distributed
computers, sensors, and actuators. In other practical
applications, the F-16 was the first Air Force aircraft
that utilized generalized rate monotonic scheduling. In
2000, Lockheed Martin included RMS scheduling in the
F-35 design baseline, as it had become an established,
foundational engineering practice.
The View from Others
The navigation payload software
for the next block of Global Positioning
System upgrade recently
completed testing. ... This design
would have been difficult or impossible
prior to the development
of rate monotonic theory.
– L. Doyle, and J. Elzey ITT,
Aerospace Communication
Division (p.1) [Doyle 1993]
Through the development of Rate
Monotonic Scheduling, we now
have a system that will allow
[Space Station] Freedom’s computers
to budget their time, to
choose between a variety of tasks,
and decide not only which one to
do first but how much time to
spend in the process.
– Aaron Cohen, Deputy
Administrator of NASA, in
an October 1992 lecture
(p.3) [Cohen 1992]
New Challenges: A fundamental assumption of realtime
scheduling theories, including RMA, is that the
worst-case execution time of a task is the same whether it runs alone or with other tasks. Processor
cache memory invalidates this assumption. Current multicore architectures exacerbate this
problem, because software running in one core could cause severe delays in other cores via the
interference of shared last-level cache among cores. Just as RMA has changed many hardware
and software standards in the past, RMA offers promise that this multicore design problem will
also be fixed in the future. Currently, the University of Illinois at Urbana-Champaign is collaborating
with SEI and industry to address this new challenge.
CMU/SEI-2016-SR-027 | SOFTWARE ENGINEERING INSTITUTE | CARNEGIE MELLON UNIVERSITY 37
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