RTLinux -- a hard real-time Linux add-on

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(Last updated: Nov. 19, 1999)

RTLinux is a hard real-time operating system that runs Linux as its lowest priority execution thread. The Linux thread is made completely preemptible so that realtime threads and interrupt handlers are never delayed by non-real-time operations. Real-time threads in RTLinux can communicate with Linux processes via shared memory or a file-like interface, so real-time applications can make use of all the powerful, non-real-time services of Linux. For example, it is easy to write a Perl script that displays data in Xwindows, responds to commands delivered over a network, and collects data from a real-time task.

RTLinux supports real-time interrupt handlers and realtime periodic tasks with interrupt latencies and scheduling jitter close to hardware limits. Worst case interrupt latency on a modest, reasonably configured x86 PC is under 15 microseconds from the moment the hardware interrupt is asserted. Better hardware configurations produce better timings. (This is a worst case scenario, not typical time. Time is measured from the moment the hardware asserts, not when the operating system scheduler runs.)

Realtime tasks in RTLinux can communicate with Linux processes via shared memory or a file-like interface. Thus, real-time applications can make use of all the powerful, non-real-time services of Linux, including:

Networking
Graphics
Windowing systems
Data analysis packages
Linux device drivers, and
Standard POSIX API

It is quite simple, for example, to create a Perl script that displays data in Xwindows, responds to commands delivered over a network, and collects data from a real-time task.

Three versions of RTLinux are available via ftp:

RTLinux V1 is an exceptionally stable system. It is in use both in delivered products and in laboratory environments. V1 RTLinux provides a simple API for starting, stopping, and scheduling realtime tasks.
RTLinux V2 offers a simplified version of the POSIX pthreads API, and will be compliant with the POSIX "Minimal Real-time" standard (in which the realtime component is considered a single, multithreaded POSIX process). The aim of the V2 kernel is to offer near-POSIX with no sacrifice of speed and simplicity, while proving hooks for add-ons for POSIX full compatibility. V2, released in November 1999, is an SMP capable x86 system.
RTLinux V3 Beta, released January 3, 2000, adds PowerPC support.

RTLinux, originally developed at the New Mexico Institute of Technology, is an open-source product. RTLinux-specific components are released under the GPL, and Linux components released under the standard Linux license. The source code is freely distributed. Non-GPL versions of the RTL components are available from FSMLabs.

RTLinux.com discusses commercial support and development.

Other articles about RTLinux:

Victor Yodaiken on the past, present, and future of RTLinux -- a comprehensive interview with the chief maintainer of RTLinux
The miniRTL project -- a miniature real-time Linux -- a brief introduction to miniRTL, which fits a full RTLinux-based standalone system within the storage capacity of a single floppy
Real Time Linux for Embedded Systems in the Internet Era -- a copy of the presentation given by Victor Yodaiken at the Embedded Linux Expo & Conference, June 2000
An Introduction to RTLinux -- one of the early whitepapers by Victor Yodaiken describing RTLinux, its architecture, and how it is used
The RTLinux Manifesto -- whitepaper by Victor Yodaiken which discusses the requirements of hard real-time systems and how RTLinux meets those needs
FSMLabs releases RTLinux version 3.0-pre8
New RTLinux release (V2.2) extends POSIX compliance

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