Real Time Linux Operating Systems

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Real Time Linux Operating Systems

• Swaminathan Sivasubramanian and Sai Sudhir A
• swami_at_cs.vu.nl
• saisud_at_iastate.edu

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Linux as a Real-Time OS

• Traditional RT Systems used custom built systems
  which were not extensible I.e. tough to develop
  new applications
• However, as technology improved, generic
  real-time OS became acceptable
• In OS suited for extensible development Linux
  looks more appealing

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Why Linux?

• Linux (and its Real Time versions) are free!!
• Linux (and its Real Time versions) are Open
  Source!!
• Easy for developing RT applications

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Why not Linux?

• Linux didnt have any corporate support until now
• Linux, is a very good general purpose operating
  system, but not so for real-time OS (Why??)
• Because, the design motive of a conventional OS
  and RTOS is different

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Linux A Simplified View
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Linux as your real-time solution?

• Could increase priority for real-time tasks and
  assume they get scheduled
• Problem Linux optimizes average case whereas an
  RTOS should work under worst case assumptions

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Example Pseudo-code

• begin
• lock process into memory (This will prevent it
  being out swapped to disk) set the process
  scheduling policy to high priority
• do n loops
• read start time sleep for 100
  milliseconds read end time calculate
  deviation and store result
• end loop
• output average and worst-case deviations
• end

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Example Pseudo-code (contd..)

• Measure difference between actual sleep time and
  expected sleep time
• On an idle system it is approx. 100 microseconds
• When I/O activity also takes place it now becomes
  30 ms
• Not suitable for hard RT tasks

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Linux conflicts with RT constraints

• Coarse grained synchronization long intervals
  when a task has exclusive use of data thereby
  holding up RT tasks
• Linux will batch all operations for efficient use
  of H/W e.g. frees a set of pages instead of a
  single page delaying all processes

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Linux conflicts with RT constraints (contd..)

• Linux doesn't preempt low-priority task during
  system call, queues higher priority tasks
• Linux reorders requests from multiple processes
  to make use of H/W efficiently e.g. to minimize
  disk-head movement
• Linux will make high priority tasks wait for low
  priority to release resources

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Real Time Linux approaches

• Modify the current Linux kernel to guarantee RT
  constraints
• Used by KURT
• Make the standard Linux kernel run as a task of
  the real-time kernel
• Used by RT-Linux

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Modifying Linux kernel

• Advantages
• Most problems, such as interrupt handling,
  already solved
• Less initial labor
• Disadvantages
• No guaranteed performance
• RT tasks dont always have precedence over non-RT
  tasks.

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Running Linux as a process of a second RT kernel

• Advantages
• Can make hard real time guarantees
• Easy to implement a new scheduler
• Disadvantages
• Initial port difficult, must know a tremendous
  amount about underlying hardware
• Running a small real-time executive is not a
  substitute for a full fledged RTOS

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KURT Overview

• Developed at University of Kansas
• Soft real-time system
• Refines the temporal granularity of Linux
• Motivation RT tasks may need a time resolution
  on the order of microseconds, while non-RT tasks
  may need a resolution of only milliseconds

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KURT Overview (continued)

• Result Timer interrupts are programmed to
  service earliest scheduled event (results in
  aperiodic timer interrupts)
• Not suitable for hard real-time systems
• KURT cant guarantee priority of RT tasks over
  non-RT tasks
• An RT task can be blocked by a non-RT task (eg
  during disk I/O) leading to priority inversion
• Suitable for soft RT systems

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RT-Linux Overview

• Open source Linux project
• Supports x86, PowerPC, Alpha
• Patch of the regular Linux kernel (simply install
  the patch and recompile the kernel)
• Provides an RT API for developers
• Runs Linux kernel as lowest priority process

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RT-Linux Task Structure
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RT-Linux Interrupt Dispatcher
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Linux Modules

• Code that can be dynamically linked to the kernel
• Modules run in kernel space and hence have access
  to kernel data structures
• Kernel maintains kernel symbol table listing its
  resources so that loaded modules can use kernel
  resources
• insmod, rmmod, lsmod

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RT-Linux Overview (continued)

• RT tasks are coded as modules
• Modules are inserted and removed at users
  discretion
• Extremely good at handling periodic tasks
• Communicates with non-RT kernel and other RT
  tasks via FIFO queues that are pinned in memory
• Tools are provided for graphical analysis of RT
  execution

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Problems with RT-Linux

• Currently no support for aperiodic tasks
• Not very useful for complex RT systems
• Currently limited to simple problems

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Important References

• RT-Linux http//www.rtlinux.org