On realtime operating systems: How to compare performance ?

To compare realtime operating systems one must often rely on specifications given by the manufacturers. These published data are often measured by so-called benchmarks. But with the help of benchmarks it is not possible to collect sufficient information to determine the system behaviour for worst cases. Therefore it must be clarified which latency times exist and how they arise. Of great significance is the additional latency time of the operating system, which is compulsory for the design of the application. In this paper it is investigated whether and how long a given task, activated by an external interrupt, is delayed until task completion. For that all latency times which can occur are discussed, specifically the worst cases. We are taking a closer look at realtime operating systems which encourage so-called kernel-level threads. The advantage of these kernel-level threads systems in contrast to traditional systems are discussed referring to performance. At the moment we are testing two systems, LynxOS and HP-RT. We want to measure the worst-case latency times and therefore illustrate how this is done.