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Introduction to Realtime Linux
Jan Altenberg
Linutronix GmbH
Jan Altenberg Linutronix GmbH 1
Overview
..1 What is Realtime?
..2 Linux and Realtime
..3 Results: Which latencies can be achieved with the differentapproaches?
..4 Conclusion
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What is Realtime?
Fast execution time?
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What is Realtime?
Performance?
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What is Realtime?
It's all about DETERMINISM!
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What is Realtime?
Correctness means execution at the correct timeMissing the timeslot will lead to an error condition
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Realtime
Remember!Missing the timeslot will lead to an error condition
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Realtime
Missing the timelinewill cause a damage to your machine or even a person might get hurt:
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What about ''Softrealtime''?
…PLEASE PLEEAAASSEEE forget about this word!!! :)
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Who is using it?
industry / automationmultimedia systemsaerospacefinancial services…
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Requirements
Deterministic timing behaviourPreemptionPriority Inheritance / Priority Ceiling
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Priority Inversion
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Approaches
Dual-KernelIn-Kernel / Single Kernel
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Dual-Kernel
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Single-Kernel
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RTAI
Prof. Paolo Mantegazza, University of MilanoDual-Kernel approachRealtime in kernelspaceRealtime in userspace very limitedDesign goal: Lowest latenciesSupported platforms: x86, x86_64, and a couple of ARM platforms
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RTAI
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Xenomai
Founded 2001Realtime in userspaceSkins can emulate the API of different RTOSesDual-Kernel approachSupported platforms: x86, x86_64, PowerPC, ARM, ia64
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Xenomai
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Known issues of dual-kernel approaches
Special APISpecial tools and librariesMicrokernel needs to be ported for new HW and new Linux versionsBad scaling on big platforms
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Preempt RT
In-Kernel approachFounded by: Thomas Gleixner, Ingo MolnarHuge communityMost of the features already made it into ''Mainline''POSIX realtimeHighly accepted in the community
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Preempt RT undMainline
''Controlling a laser with Linux is crazy, but everyone in this room is crazyin his own way. So if you want to use Linux to control an industrialwelding laser, I have no problem with your using Preempt RT'' - LinusTorvalds auf dem Kernel Summit 2006
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How Preempt RT brings Realtime to Linux?
Remember once again…Preemption is the most important requirement for a Realtime System
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How Preempt RT brings Realtime to Linux?
Locking Primitives: Spinlocks are replaced by RT Mutexes. RawSpinlocks behave like the original Spinlocks.Interrupt Handlers run in a kernel thread
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Preempt RT
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Latency Measurements on a Cortex A9 platform
ARM Cortex A9 SOC (Altera Cyclone V)System load: 100% CPU load with hackbenchIRQ tests at 10 kHz with the OSADL Latency BoxTest duration 12h
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Load scenario: hackbench
Starts n groups of 20 clients and 20 serversEach client sends 100 messages to each server via a socketconnection
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What has beenmeasured?
Latency and Jitter
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Latency measurement
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Userspace Latency: The most important usecase
The most important usecase is the latency of a userspace task. Usually auserspace task needs to be synced with an external event.
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Xenomai: latency userspace task
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PREEMPT_RT: latency userspace task
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PREEMPT_RT: latency userspace task (isolated CPU)
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Latency userspace task - comparison
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Latency within the Kernel
…or how to compare apples with pears!! ;-)
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Latency: Kernel - Xenomai
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Latency: Kernel - PREEMPT_RT
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Latency: Kernel - PREEMPT_RT (isolated CPU)
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Latency: Kernel - PREEMPT_RT with FIQ (fast interrupt)
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Latency: Kernel - Comparison
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Conclusion
PREEMPT_RT became the de-facto standard for Realtime LinuxIntegration of PREEMPT_RT in Mainline LinuxSimple usage of PREEMPT_RTMicrokernels are hard to handleFor the most common use-cases the Microkernels do NOT havebetter latenciesFIQs offer fast latencies, but are hard to handle
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Questions?
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