The LITMUS^RT project is a soft real-time extension of the Linux kernel with focus on multiprocessor real-time scheduling and synchronization. The Linux kernel is modified to support the sporadic task model and modular scheduler plugins. Both partitioned and global scheduling is supported.

The current version of LITMUS^RT is 2008.2 and is based on Linux 2.6.24. It was released on 12/01/2008 and includes plugins for the following scheduling policies:

• Partitioned EDF with synchronization support (PSN-EDF)
• Global EDF with synchronization support (GSN-EDF)
• Clustered EDF (C-EDF)
• PFAIR (both staggered and aligned quanta are supported)

Earlier versions (2007.1 — 2007.3), which are based on Linux 2.6.20 and support additional scheduling policies, are discussed on a separate page dedicated to the LITMUS^RT 2007 series.

The first version of LITMUS^RT, which was implemented in Spring 2006, is based on Linux 2.6.9.

The LITMUS^RT development effort is being supported by grants from, SUN Corp., Intel Corp., IBM Corp., The National Science Foundation (grant CCR 0615197), and The U.S. Army Research Office (grant W911NF-06-1-0425).

The LITMUS^RT project is led by Dr. James H. Anderson.

The implementation effort is carried out by students of the Real-Time Systems Group at the University of North Carolina at Chapel Hill:

• Björn B. Brandenburg (current maintainer)
• John M. Calandrino

(Additional contributors contributed to earlier versions of LITMUS^RT.)

1. B. Brandenburg and J. Anderson “Reader-Writer Synchronization for Shared-Memory Multiprocessor Real-Time Systems”, Proceedings of the 21st Euromicro Conference on Real-Time Systems, pp. 184-193, July 2009. Postscript. PDF. Long version with blocking terms: Postscript. PDF.

2. J. Calandrino and J. Anderson “On the Design and Implementation of a Cache-Aware Multicore Real-Time Scheduler”, Proceedings of the 21st Euromicro Conference on Real-Time Systems, pp. 194-204, July 2009. Postscript. PDF.

3. B. Brandenburg and J. Anderson, “A Comparison of the M-PCP, D-PCP, and FMLP on LITMUS^RT”, Proceedings of the 12th International Conference on Principles of Distributed Systems, pp. 105-12, December 2008. Postscript. PDF.

4. B. Brandenburg, J. Calandrino, and J. Anderson, “On the Scalability of Real-Time Scheduling Algorithms on Multicore Platforms: A Case Study”, Proceedings of the 29th IEEE Real-Time Systems Symposium, pp. 157-169, December 2008. Postscript. PDF.

5. B. Brandenburg and J. Anderson, “An Implementation of the PCP, SRP, D-PCP, M-PCP, and FMLP Real-Time Synchronization Protocols in LITMUS^RT”, Proceedings of the 14th IEEE International Conference on Embedded and Real-Time Computing Systems and Applications, pp. 185-194, August 2008. Postscript. PDF.

   Note: The work described in this paper took part in a branch that is currently not part of the main distribution. For reference, we provide the branch as a separate download:

   • litmus-rt-RTCSA08.patch
   • liblitmus-RTCSA08.tgz
   • SHA256 check sums

   Please don't use this version for active development. If you are interested in this work, it would be best to first port the desired features to a current version of LTIMUS^RT and merge them into the main distribution.

6. A. Block, B. Brandenburg, J. Anderson, and S. Quint, “An Adaptive Framework for Multiprocessor Real-Time Systems”, Proceedings of the 20th Euromicro Conference on Real-Time Systems, pp. 23-33, July 2008. Postscript. PDF.

7. B. Brandenburg, J. Calandrino, A. Block, H. Leontyev, and J. Anderson, “Real-Time Synchronization on Multiprocessors: To Block or Not to Block, to Suspend or Spin?”, Proceedings of the 14th IEEE Real-Time and Embedded Technology and Applications Symposium, pp. 342-353, April 2008. Postscript. PDF.

   Extended version, including all graphs: Postscript, PDF.

8. B. Brandenburg, A. Block, J. Calandrino, U. Devi, H. Leontyev, and J. Anderson, "LITMUS^RT: A Status Report", Proceedings of the 9th Real-Time Linux Workshop, pp. 107-123, November 2007. Postscript. PDF.

9. B. Brandenburg and J. Anderson, "Integrating Hard/Soft Real-Time Tasks and Best-Effort Jobs on Multiprocessors", Proceedings of the 19th Euromicro Conference on Real-Time Systems, pp. 61-70, July 2007. Postscript, PDF.

10. J. Calandrino, H. Leontyev, A. Block, U. Devi, and J. Anderson, "LITMUS^RT: A Testbed for Empirically Comparing Real-Time Multiprocessor Schedulers ", Proceedings of the 27th IEEE Real-Time Systems Symposium, pp. 111-123, December 2006. Postscript, PDF.

The source code of LITMUS^RT is made available as open source under the terms of the GNU General Public License (GPL).

The current release of LITMUS^RT is 2008.2. It consists of our Linux kernel modifications in the form of a patch against Linux 2.6.24 and liblitmus, the user-space API for real-time tasks, as well as ft_tools, a collection of tools used for tracing with Feather-Trace (which is part of the LITMUS^RT patch).

LITMUS^RT 2008.2

LITMUS^RT 2008.1

Please note that the current implementation is a prototype with certain limitations. Most notably, it is not secure in a multiuser context, i.e., real-time system calls do not require superuser privileges.

Older releases: LITMUS^RT 2007 series.

The current release of LITMUS^RT consists of an extension of the Linux kernel that adds support for the sporadic task model, a scheduler plugin infrastructure, and some scheduler plugins, as well as a user-space library that provides the LITMUS^RT real-time API. Note that the current implementation only works on the Intel x86-32 and sparc64 architectures.

Patching the Kernel

The extension to the Linux kernel is released as a patch against Linux 2.6.24. To install the LITMUS^RT kernel, first download the Linux kernel 2.6.24 and untar it in a directory of your choice (hereafter referred to as $DIR). Second, apply the LITMUS^RT patch (see Section Download) and configure, compile, and install the kernel as usual. The patch is -p1 applicable. To summarize, the LITMUS^RT kernel can be obtained, patched, and compiled with the following commands:

cd $DIR
# get Linux 2.6.24
wget http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.24.tar.bz2
tar xjf linux-2.6.24.tar.bz2
wget http://www.cs.unc.edu/~anderson/litmus-rt/download/2008.2/litmus-rt-2008.2.patch
mv linux-2.6.24 litmus2008
# apply the LITMUS RT patch
cd litmus2008
patch -p1 < ../litmus-rt-2008.2.patch
# create a working kernel configuration
#  - select HZ=1000
#  - enable in-kernel preemptions
#  - disable NO_HZ
#  - don't use power management options like frequency scaling
#  - disable support for group scheduling
make menuconfig
# compile the kernel
make bzImage
make modules
# proceed to install kernel, build initrd, etc.
...

When configuring the kernel, note that there is a menu (at the very end of the list) with LITMUS^RT-specific configuration options. For reference, we provide a configuration that is known to work under QEMU.

Libraries

The user-space library for real-time tasks, liblitmus, depends on the LITMUS^RT kernel kernel and provides its own makefile. In order to compile liblitmus, you need to adjust the variable KERNEL_DIR in the Makfile to point to your copy of the kernel.

cd $DIR
wget http://www.cs.unc.edu/~anderson/litmus-rt/download/2008.2/liblitmus-2008.2.tgz
tar xzf liblitmus-2008.2.tgz
cd liblitmus 
# change KERNEL_DIR in Makefile to point to the kernel source
make

Please refer to the documentation on how to use the LITMUS^RT real-time API as provided by liblitmus.

Unfortunately, most of the documentation has yet to be written. To get an overview of the architecture of the kernel extension, we recommend reading the paper “LITMUS^RT: A Status Report”.

Real-Time Scheduling Policies

The kernel contains the following real-time scheduling policy implementations:

• PFAIR, an implementation of the PD² algorithm,
• PSN-EDF, a partitioned EDF (P-EDF) implementation with support for the real-time synchronization protocol FMLP,
• GSN-EDF, a global EDF (G-EDF) implementation with support for the real-time synchronization protocol FMLP,
• C-EDF (Clustered EDF), a hybrid of G-EDF and P-EDF, and
• Linux, a placeholder policy that disables all real-time functionality added by the LITMUS^RT patch.

Only one policy can be active at any time. Initially (i.e., during boot), the "Linux" policy is active. You can use the tool showsched (part of liblitmus) to display the name of the currently active policy.

Changing the Active Policy

You can use the tool setsched (part of liblitmus) to select a new plugin at run time.

Only root can change the active policy, and only when there are no real-time tasks present.

Writing Real-Time Tasks

The user space library that provides the LITMUS^RT API, liblitmus, contains two example real-time tasks (base_task.c and base_mt_task.c) that both illustrate how to use the API and provide a skeleton for real-time task development. To get started with development, please take a look these example programs.

Tracing Overheads and Scheduling Decisions

LITMUS^RT provides numerous tracing facilities that are discussed in-depth in the tutorial Tracing with LITMUS^RT.

Please contact bbb[AT]cs.unc.edu if you have any questions.

Linux is a registered trademark of Linus Torvalds.
The LITMUS^RT logo was designed by Jasper McChesney of Break for Sense Design.
Web design by Björn Brandenburg.