Real-time

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

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.