Introduction to Realtime Linux Jan Altenberg Linutronix GmbH Jan - - PowerPoint PPT Presentation

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Mar 06, 2023 194 likes •632 views

Introduction to Realtime Linux Jan Altenberg Linutronix GmbH Jan Altenberg Linutronix GmbH 1 Overview 3 Results: Which latencies can be achieved with the different Linutronix GmbH Jan Altenberg 4 Conclusion . . approaches? . . . 2

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Introduction to Realtime Linux

Jan Altenberg

Linutronix GmbH

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Overview

. . 1 What is Realtime? . . 2 Linux and Realtime . . 3 Results: Which latencies can be achieved with the different

approaches?

. . 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 time Missing 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 timeline

will 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 / automation multimedia systems aerospace fjnancial services …

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Requirements

Deterministic timing behaviour Preemption Priority Inheritance / Priority Ceiling

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Priority Inversion

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Approaches

Dual-Kernel In-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 Milano

Dual-Kernel approach Realtime in kernelspace Realtime in userspace very limited Design goal: Lowest latencies Supported platforms: x86, x86_64, and a couple of ARM platforms

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RTAI

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Xenomai

Founded 2001 Realtime in userspace Skins can emulate the API of different RTOSes Dual-Kernel approach Supported platforms: x86, x86_64, PowerPC, ARM, ia64

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Xenomai

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Known issues of dual-kernel approaches

Special API Special tools and libraries Microkernel needs to be ported for new HW and new Linux versions Bad scaling on big platforms

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Preempt RT

In-Kernel approach Founded by: Thomas Gleixner, Ingo Molnar Huge community Most of the features already made it into ''Mainline'' POSIX realtime Highly accepted in the community

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Preempt RT und Mainline

''Controlling a laser with Linux is crazy, but everyone in this room is crazy in his own way. So if you want to use Linux to control an industrial welding laser, I have no problem with your using Preempt RT'' - Linus Torvalds 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. Raw Spinlocks 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 hackbench IRQ tests at 10 kHz with the OSADL Latency Box Test duration 12h

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Load scenario: hackbench

Starts n groups of 20 clients and 20 servers Each client sends 100 messages to each server via a socket connection

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What has been measured?

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 a userspace 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 Linux Integration of PREEMPT_RT in Mainline Linux Simple usage of PREEMPT_RT Microkernels are hard to handle For the most common use-cases the Microkernels do NOT have better latencies FIQs offer fast latencies, but are hard to handle

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Questions?

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