This technical whitepaper by Kevin Dankwardt goes into considerable detail on some of the alternative approaches to adding real-time capabilities to Linux. Dankwardt covers both “internal” Linux enhancements (such as scheduling improvements) and external enhancements (such as RTLinux and RTAI), and provides a comparison summary that indicates that no single product offers an all-inclusive set of real-time… capabilities. Dankwardt writes . . .

“Tolstoy opens Anna Karenina with a famous quote: 'Happy families are all alike; every unhappy family is unhappy in its own way.' Similarly: reliable real-time systems are all alike; every unreliable real-time system is unreliable in its own way. In a real-time system, if you miss as much as one deadline, you've under performed. There are a multitude of ways to go wrong, and only one way to do things right.”

“Lately, the question of whether (and how) Linux can be made to serve the needs of real-time applications has been the subject of much debate, in a discussion made complicated by a multitude of definitions for real-time. We see the terms 'hard', 'firm', and 'soft' real-time being used. These, along with 'guarantee', 'deterministic', 'preemptible', 'fully preemptible', and 'latency', often pepper the discussions.”

“In this article, we review the methods available to serve the needs of applications that involve performance deadlines. We shall see that although precise requirements vary from application to application, there are a number of fundamental issues that are almost always of concern. Since multiple vendors provide differing approaches, it is natural that no one solution will serve all applications best. Through this article, we hope to assist developers in evaluating the value of various alternatives relative to their specific applications. Our objective is to provide insight into the various choices vendors have made in creating their unique approaches. What good is a preemptible kernel? What advantage is there in having my application running in kernel-space? What disadvantage?”

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