• Due to its advantages, Linux and open-source software are more and more commonly used in embedded applications
  • However, some applications also have real-time constraints
  • They, at the same time, want to
    • Get all the nice advantages of Linux: hardware support, components re-use, low cost, etc.
    • Get their real-time constraints met

Embedded Linux and real time

  • Linux is an operating system part of the large Unix family
  • It was originally designed as a time-sharing system
    • The main goal was to get the best throughput from the available hardware, by making the best possible usage of resources (CPU, memory, I/O)
    • Time determinism was not taken into account
  • On the opposite, real-time constraints imply time determinism, even at the expense of lower global throughput
  • Best throughput and time determinism are contradictory requirements

Linux and real-time approaches

  • Over time, two major approaches have been taken to bring real-time requirements into Linux
  • Approach 1
    • Improve the Linux kernel itself so that it matches real-time requirements, by providing bounded latencies, real-time APIs, etc.
    • Approach taken by the mainline Linux kernel and the PREEMPT_RT project.
  • Approach 2
    • Add a layer below the Linux kernel that will handle all the real-time requirements, so that the behaviour of Linux doesn't affect real-time tasks.
    • Approach taken by RTLinux, RTAI and Xenomai

An alternative approach is to use specific hardware to run real-time work on:

  • Dedicating a CPU core to a real-time OS or to a real-time application, using some kind of hypervizor.
  • Running real-time work on an FPGA
  • Running real-time work on a dedicated microcontroller. For example, the TI AM335x CPU (used in the Beaglebone Black) has a "Programmable Real-Time Unit and Industrial Communication Subsystem (PRU-ICSS)", which can be used for real-time processing.

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