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Small-footprint and Real-time Linux-es

Small-footprint and Real-time Linux-es. Athanasios Stathopoulos Vladimir Bychkovskiy. Operating System (OS) goals. Generic operating systems goals provide high level abstraction simplicity of use fairness in scheduling application independence some quality of service (QoS)

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Small-footprint and Real-time Linux-es

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  1. Small-footprint and Real-time Linux-es Athanasios Stathopoulos Vladimir Bychkovskiy

  2. Operating System (OS) goals • Generic operating systems goals • provide high level abstraction • simplicity of use • fairness in scheduling • application independence • some quality of service (QoS) • Real-Time and Embedded OS goals • very low latency (“real-time”) response • determinism • small-footprint

  3. What is out there? • Open Source • Microcontroller (no MMU) OSes: • uClinux - small-footprint Linux • QoS extensions for desktop: • Linux-SRT and QLinux • soft real-time kernel extension • target: media applications • Embedded PC • RTLinux, RTAI • hard real time OS • fully compatible with GNU/Linux • Commercial: • vxWorks, QNX, HardHat Linux, WinCE, etc

  4. uClinux details • Provides • Common Linux API • Full TCP/IP stack • Popular filesystems support • Supported platforms • Originally developed for Motorolla 68K • Large number of platforms currently supported • Special features • Kernel size < 512kb • Limited multitasking • Designed for portability

  5. Linux-SRT Single QoS scheduler guarantees a CPU % for an application. Special features: real-time X server that prioritizes rendering based on scheduling parameters Supported platforms: same as main stream linux Summary: User-oriented QLinux CPU % guaranteed to groups using hierarchical scedulers Special features: additional schedulers for network, disk devices Supported platforms: same as main stream linux Summary: Research-oriented Soft Real-Time Linuxes details

  6. When is Hard Real-Time needed? • Hard vs. Soft Real-Time • Hard RT always guarantees exact timing • Hard Real-Time applications • Closed-loop control systems • Stimulus/response systems • Examples • Robotics, Animatronics • Industrial equipment • Neural, fuzzy and adaptive systems • However: • Hard Real-Time extensions can degrade OS performance of non time-critical applications

  7. RTLinux: The Real Time Linux • Goals • maintain compatibility with GNU/Linux • handle tasks with hard-real time constrains • simplicity and reliability of the RT • tradeoff: ease of use • Approach • real-time microkernel • Linux kernel is running as lowest priority task in the real-time schedule • real-time applications are written as microkernel modules • microkernel is transparent to regular application

  8. RTLinux: Microkernel • Microkernel • IS a thin layer of abstraction • PROVIDES an interface for hardware interrupts and API for real-time tasks • MEDIATES requests to/from hardware Picture from www.embedded.com website.

  9. RTLinux: Microkernel (cont.) • Components • Processor clock control • Real-Time scheduler • POSIX-like interface to device drivers • Real-Time FIFOs • Shared memory buffers • POSIX-like IPC • Blocking mutexes • Semaphores • Source-level debugger support • Serial port interface

  10. RTLinux: Microkernel Details • Scheduler • Preemptive, priority based • Hardware context switch is not used • Reason: saves too much state, not fast enough • Stack is used instead • RT kernel handles hardware interrupts • Linux thread does not directly receive them • When Linux wants to disable interrupts • The microkernel stops passing them to the Linux thread • However, real-time tasks can receive them

  11. RTLinux: Real-Time Tasks • RT kernel modules • Take advantage of microkernel’s RT API • Can provide a device-like interface (RTFIFO) for non real-time tasks • Can share devices with non real-time tasks • RT tasks always get serviced first • Example: Dual Data Acquisition Picture from www.embedded.com website.

  12. RTLinux vs. RTAI • RTAI is based on RTLinux • However, design philosophy differs • RTAI: Focused on adding new features • Easier to use • Caveat: performance hit • RTLinux: RTOS must be very fast • Any additional functionality must be optional • Microkernel must remain “micro” • Platform support • RTLinux: x86, Alpha, PPC, Linux v. 1.3-2.4 • RTAI: x86, Linux v. 2.2

  13. Priority Inversion and RTLinux • Unlike most RTOS’s RTLinux does not attempt to solve the priority inversion problem. • “I think that priority inheritance is for people who want to build complex critical real-time systems that sometimes work.”, Victor Yodaiken, RTLinux author. • Author suggests: “… use well known, time-tested methods such as flip buffers and message queues (or RTfifos).” • Another proposed solution is to disable microkernel thread switching while semaphores are held • This approach is similar to priority ceiling • However: System will give up multitasking capabilities inside critical sections.

  14. References • uClinux Home page, http://www.uclinux.org • SRT-Linux Home page, http://www.uk.research.att.com/~dmi/linux-srt/ • QLinux Home page, http://www.cs.umass.edu/~lass/software/qlinux/ • Ivchenko, A. “Real-Time Linux”, Embedded System Programming, May 2001 • RTLinux Home page, http://www.rtlinux.org • RTLinux FAQ, http://www.rtlinux.org/documents/faq.html • “RTLinux: An interview with Victor Yodaiken”, http://www.acm.org/crossroads/xrds6-1/yodaiken.html • RTAI, http://opensource.lineo.com/rtai.html • Sha, L.; Rajkumar, R.; Lehoczky, J.P. Priority inheritance protocols: an approach to real-time synchronization. IEEE Transactions on Computers, vol.39, (no.9), Sept. 1990. p.1175-85. http://ielimg.ihs.com/iel1/12/2066/00057058.pdf

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