What was believed to be impossible, is reality now: Reliable realtime determinism with Linux

Schramberg, (PresseBox) - .
- How to obtain data of the realtime capability of a stateoftheart computer system?
To determine whether a given system (hardware, firmware and software) is deterministic with respect to a given response deadline is much more difficult now than it was in earlier days. Until 10 to 20 years ago, the worstcase latency of a system could be determined theoretically: Using the socalled path analysis, the longest critical section that ever could be encountered in a system was determined, and the processor cycles required by the instructions of this section were then multiplied with the cycle duration. This procedure, unfortunately, is no longer possible, since in modern highperformance processors there is no way to obtain the computing time of a given section of instructions. Most of the time, it is extremely fast - up to 1 million times faster than 30 years ago, but very rarely it may be very slow - probably even slower than it was back then.

The only alternative to determine whether a system is suitable for a given realtime project is to empirically determine the worstcase preemption latency. However, this sounds easier than it is, since the extremely high number of possible interferences in a stateoftheart computer system require a very large number of single recordings under as many different conditions as possible. This was one of the reasons to establish the OSADL QA Farm where today more than 50 different Linux systems undergo continuous longterm recordings of a large number of variables. Among others, the worstcase system latency in idle state and under average load and peakload conditions of the various subsystems is determined repeatedly. All Linux kernels are equipped with the realtime patches developed and maintained by the Linux RT community led by kernel developer Thomas Gleixner.

The OSADL QA Farm celebrates its oneyear anniversary and presents longterm latency plots
Several systems of the OSADL QA Farm now run for more than a year under continuous monitoring conditions. With 100 million wakeup cycles used to determine the worstcase latency twice per day, 730 recordings in a year's time are based on a total of 73 billion cycles. This is equivalent to an industrial control system running 365 days at 500 μs cycle interval.

The six 3D graphs in Figures 1a to 1f represent a subsample of the more than 50 systems currently under test in the OSADL QA Farm. Each graph consists of repeated latency plots put before one another with the time scale running from back to front. A latency plot displays the number of samples within a given latency class (resolution 1 μs). The logarithmic frequency values at the yscale ensure that even a single outlier would be displayed. The total absence of any outlier in all the very different systems clearly demonstrates that the perfect determinism of the mainline Linux realtime kernel is a generic feature; it is not restricted to any particular architecture. In an industrial system that requires asynchronous external events to be processed in user space within 200 μs, for example, this deadline never would be reached by any of the systems throughout an entire year.

OSADL keeps growing
The OSADL QA Farm and the many other activities of the organization are made possible by the contributions of its members that are gratefully acknowledged. It should be emphasized in this context that some OSADL services, such as most of the QA data, are made available to nonmembers as well. In addition to the 33 members a year ago, five new members have joined OSADL since then and are helping OSADL to further facilitate the use of Open Source software for industrial production and in industrial products. OSADL extends a warm welcome to its new member companies

- Komax AG, Dierikon, Switzerland,
- BELIMO Automation AG, Hinwil, Switzerland,
- Beijing Shenzhou Aerospace Software Technology Co., Ltd., Beijing, China,
- Fr. Sauter AG, Basle, Switzerland, and
- KUKA Laboratories GmbH, Augsburg, Germany who have been added to our member poster (Figure 2).

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