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. In the current version (2007.2), plugins for the following scheduling policies are included:

• Partitioned EDF (P-EDF)
• Partitioned EDF with synchronization support (PSN-EDF)
• Global EDF (G-EDF)
• Global EDF with synchronization support (GSN-EDF)
• Global non-preemptive EDF (G-NP-EDF)
• Global Feedback-Controlled EDF (FC-EDF)
• EDF for heterogeneous task systems (EDF-HSB)
• PFAIR (both staggered and aligned quanta are supported)

The latest public release of LITMUS^RT occurred on 10/29/2007.

The LITMUS^RT development effort is being supported by grants from Intel 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:

• Aaron D. Block
• Björn B. Brandenburg
• John M. Calandrino
• UmaMaheswari Devi (graduated Dec. 2006)
• Hennadiy Leontyev

• 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, November 2007, to appear. Postscript. PDF.

• A. Block, B. Brandenburg, J. Anderson, and S. Quint, "Feedback-Controlled Adaptive Multiprocessor Real-Time Systems", in submission, October 2007. Postscript. PDF.

• 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?", in submission, October 2007. Postscript, PDF.

  Extended version, including all graphs: Postscript, PDF.

• 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, July 2007, to appear. Postscript, PDF.

• 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 latest version of LITMUS^RT is 2007.2 and was released on 10/29/2007. It consists of our Linux kernel modifications in the form of a patch against Linux 2.6.20, liblitmus, the user-space API for real-time tasks, and libso, a library for shared objects that provides synchronization primitives suitable for real-time tasks.

• litmus-rt-2007.2.patch (328 KB)
  Applies against Linux 2.6.20 (see Section Install below).
• liblitmus-2007.2.tgz (11 KB)
• libso-2007.2.tgz (16 KB)

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. Further, some resources (e.g. semaphores) that should be dynamically allocated are allocated statically in the current version.

Old releases:

• 2007.1 (May 2007)
  Based on Linux 2.6.20.
  litmus-rt-2007.1.patch (266 KB)
  liblitmus-2007.1.tgz (6.8 KB)
  libso-2007.1.tgz (16 KB)
  

The current release of LITMUS^RT, version 2007.2, 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 two user-space libraries that provide the LITMUS^RT real-time API. Note, that the current implementation only works on the Intel x86 architecture.

Patching the Kernel

The extension to the Linux kernel is released as a patch against Linux 2.6.20. To install the LITMUS^RT kernel, first download the Linux kernel 2.6.20 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 2.6.20
wget http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.20.tar.bz2
tar xjf linux-2.6.20.tar.bz2
wget http://www.cs.unc.edu/~anderson/litmus-rt/download/litmus-rt-2007.2.patch
mv linux-2.6.20 litmus-rt
# apply the LITMUS RT patch
cd litmus-rt
patch -p1 < ../litmus-rt-2007.2.patch
# create a working kernel configuration with HZ=1000
make gconfig
# compile the kernel
make bzImage
make modules
# proceed to install kernel, build initrd, etc.
...

After the kernel has been compiled and installed, the boot-loader must be configured to load the desired real-time scheduler plugin at boot time (run time selection of the real-time scheduler is currently not supported). The real-time scheduler plugin is selected with the rtsched kernel parameter.

rtsched= {linux, pfair, part_edf, global_edf, 
          global_edf_np, edf_hsb, gsn_edf, psn_edf, adaptive}

For example, on our test machine, we use the following GRUB entry:

title  Kernel 2.6.20-LITMUSRT
root   (hd0,5)
kernel /boot/vmlinuz-2.6.20-LITMUSRT root=/dev/sda6 ro vga=791 rtsched=gsn_edf
initrd /boot/kernel-2.6.20-LITMUSRT.img

Libraries

The two user-space libraries, liblitmus and libso, provide their own makefiles and thus are easy to compile. The kernel interface for real-time tasks is provided by liblitmus, which can be compiled as follows:

cd $DIR
wget http://www.cs.unc.edu/~anderson/litmus-rt/download/liblitmus-2007.2.tgz
tar xzf liblitmus-2007.2.tgz
cd liblitmus 
make

The second library, libso, provides shared object and synchronization support for real-time tasks. It depends on liblitmus and must be installed in "parallel" (they must have the same parent directory). The following commands will obtain and compile libso, including the test programs:

cd $DIR
wget http://www.cs.unc.edu/~anderson/litmus-rt/download/libso-2007.2.tgz
tar xzf libso-2007.2.tgz
cd libso
make
make tests

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

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

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