engineering

Mixed criticality

Mixed criticality is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Mixed criticality rather than just read about it. In short: A mixed criticality system is a system containing computer hardware and software that can execute several applications of different criticality, such as safety-critical and non-safety critical, or of different safety integrity level (SIL). Different criticality applications are engineered to different levels of assurance, with high criticality applications being the most costly to design and verify.

Key takeaways

Mixed criticality belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Mixed criticality to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Mixed criticality from memory before moving on to harder problems.

Reference excerpt

A mixed criticality system is a system containing computer hardware and software that can execute several applications of different criticality, such as safety-critical and non-safety critical, or of different safety integrity level (SIL). Different criticality applications are engineered to different levels of assurance, with high criticality applications being the most costly to design and verify. These kinds of systems are typically embedded in a machine such as an aircraft whose safety must be ensured.

Principle Traditional safety-critical systems had to be tested and certified in their entirety to show that they were safe to use. However, many such systems are composed of a mixture of safety-critical and non-critical parts, as for example when an aircraft contains a passenger entertainment system that is isolated from the safety-critical flight systems. Some issues to address in mixed criticality systems include real-time behaviour, memory isolation, data and control coupling. Computer scientists have developed techniques for handling systems which thus have mixed criticality, but there are many challenges remaining especially for multi-core hardware.

Priority and criticality Basically, most errors are currently committed when making confusion between priority attribution and criticality management. As priority defines an order between different tasks or messages to be transmitted inside a system, criticality defines classes of messages which can have different parameters depending on the current use case. For example, in case of car crash avoidance or obstacle anticipation, camera sensors can suddenly emit messages more often, and so create an overload in the system. That is when we need to make Mixed-Criticality operate : to select messages to absolutely guarantee on the system in these overload cases.

Research projects EU funded research projects on mixed criticality include:

MultiPARTES DREAMS PROXIMA CONTREX SAFURE CERTAINTY VIRTICAL T-CREST PROARTIS ACROSS (Artemis) EMC2 (Artemis) RECOMP Artemis ARAMIS (in German) and ARAMIS II IMPReSS UK EPSRC funded research projects on mixed criticality include:

MCC Several research projects have decided to present their research results at the EU-funded Mixed-Criticality Forum

Workshops and seminars Workshops and seminars on Mixed Criticality Systems include:

1st International Workshop on Mixed Criticality Systems (WMC 2013) 2nd International Workshop on Mixed Criticality Systems (WMC 2014) 3rd International Workshop on Mixed Criticality Systems (WMC 2015) 4th International Workshop on Mixed Criticality Systems (WMC 2015) Dagstuhl Seminar on Mixed Criticality on Multicore/Manycore Platforms (2015) Dagstuhl Seminar on Mixed Criticality on Multicore/Manycore Platforms (2017)

References

External links Karlsruhe Institute of Technology: Mixed Criticality in Safety-Critical Systems Washington University in St Louis: A Research Agenda for Mixed-Criticality Systems

Illustrations

Worked examples

Example 1 — a first encounter with Mixed criticality

Start with the simplest possible case. Write down what Mixed criticality claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Mixed criticality before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Mixed criticality ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Mixed criticality

In research: Mixed criticality appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Mixed criticality in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Mixed criticality is common in secondary-school and first-year university syllabi. It links to neighbouring topics Safety engineering, Software engineering, so understanding it makes those chapters shorter.
In everyday life: Look for Mixed criticality outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Mixed criticality in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mixed criticality means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Mixed criticality out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mixed criticality in simple terms?

Why does Mixed criticality matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Mixed criticality?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Mixed criticality.