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<title>LITMUS RT: Linux Testbed for Multiprocessor Scheduling in Real-Time Systems</title>
</head>
<body>
<div class="logobox">
<img src="inc/litmusrt.png" alt="LITMUS^RT: Linux Testbed for Multiprocessor Scheduling in Real-Time Systems" />
<p class="authors">
<a href="http://www.cs.unc.edu/~anderson/">Dr. James H. Anderson &
Students</a>,
<a href="http://www.unc.edu">The University of North Carolina at Chapel Hill</a>
</p>
</div>
<div class="alertbox">
<h3 class="notopmargin">Interested in kernel hacking and real-time systems? We're hiring!</h3>
<p>
<a href="http://www.cs.unc.edu/rt">UNC's real-time group</a> is actively recruiting <strong>PhD-seeking graduate students</strong> with a strong background in Linux kernel development. Applicants must be highly-motivated self-starters and eager to complete a <a href="http://www.cs.unc.edu/Admin/AcademicPrograms/Doctoral/DoctoralReqOfficial.html">rigorous PhD program in computer science</a>. Having contributed to Linux (or other open source systems-level projects) and/or maintained an open source project is definitively a plus. (Extra bonus points for submitting a high-quality patch to LITMUS<sup>RT</sup>.)
</p>
<p class="nobottommargin">
Have a look at our group's <a href="http://www.cs.unc.edu/%7Eanderson/papers.html">recent publications</a> to get a feel for the kind of work that we are doing. Sounds good? Great! Next step: check out the graduate school's <a href="http://www.cs.unc.edu/Admissions/">application requirements</a> and, if you meet the requirements, contact <a href="http://www.cs.unc.edu/~anderson">Prof. Anderson</a>.
<br/><strong>Fun, challenging problems are waiting!</strong>
</p>
</div>
<div class="nav">
<p>
<a href="#about">about</a> -
<a href="#support">support</a> -
<a href="#collaborators">collaborators</a> -
<a href="#publications">publications</a> -
<a href="#download">download</a> -
<a href="#install">installation</a> -
<a href="#doc">documentation</a> -
<a href="#development">development</a>
</p>
</div>
<h2 id="about">About</h2>
<div class="box">
<p class="nomargin">
The LITMUS<sup>RT</sup> 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.
</p>
<p>Please have a look at <a href="slides/rtns2010-keynote.pptx">Prof. Anderson's RTNS'10 keynote address</a> slides for a recent overview of our motivation, the project, and recent results.
</p>
<h3>Goals</h3>
<p class="notopmargin">
The primary purpose of the LITMUS<sup>RT</sup> project is to <strong>provide a useful experimental platform for applied real-time systems research</strong>. In that regard, LITMUS<sup>RT</sup> provides abstractions and interfaces within the kernel that simplify the prototyping of multiprocessor real-time scheduling and synchronization algorithms (compared to modifying a "vanilla" Linux kernel). As a secondary goal, LITMUS<sup>RT</sup> serves as a <strong>proof of concept</strong>, showing that algorithms such as PFAIR can be implemented on current hardware. Finally, we hope that parts of LITMUS<sup>RT</sup> and the "lessons learned" may find value as blueprints/sources of inspiration for other (both commercial and open source) implementation efforts.
</p>
<h3>Non-Goals</h3>
<p class="notopmargin">
LITMUS<sup>RT</sup> is not a production-quality system, and we have currently no plans to turn it into one. LITMUS<sup>RT</sup> is not "stable," <em>i.e.</em>, interfaces and implementations may change without warning between releases. POSIX-compliance is not a goal; the LITMUS<sup>RT</sup>-API offers alternate system call interfaces. While we aim to follow Linux-coding guidelines, LITMUS<sup>RT</sup> is not targeted at being merged into mainline Linux. Rather, we hope that some of the ideas protoyped in LITMUS<sup>RT</sup> may eventually find adoption in Linux.
</p>
<h3>Current Version</h3>
<p class="notopmargin">
The current version of LITMUS<sup>RT</sup> is <strong>2011.1</strong> and is based on Linux 2.6.36.
It was released on 01/26/2011 and includes plugins for the following
scheduling policies:
</p>
<ul>
<li> Partitioned EDF with synchronization support (PSN-EDF)</li>
<li> Global EDF with synchronization support (GSN-EDF)</li>
<li> Clustered EDF (C-EDF) </li>
<li> PFAIR (both staggered and aligned quanta are supported)</li>
</ul>
<p>
Please refer to the <a href="#download">download</a> and <a href="#install">installation</a> sections for details.
</p>
<p>Earlier versions, which supported additional scheduling policies, are discussed on separate pages dedicated to the <a href="litmus2010.html">LITMUS<sup>RT</sup> 2010</a> (based on Linux 2.6.34), <a href="litmus2008.html">LITMUS<sup>RT</sup> 2008</a> (based on Linux 2.6.24), and <a href="litmus2007.html">LITMUS<sup>RT</sup> 2007 </a> (based on Linux 2.6.20) series.
</p>
<p class="nobottommargin">
The first version of LITMUS<sup>RT</sup> (implemented in Spring 2006)
was based on Linux 2.6.9.
</p>
<h3> Development Plans </h3>
There are plans to port LITMUS<sup>RT</sup> to the PowerPC platform. Please contact us for details.
</div>
<h2 id="support">Support</h2>
<div class="box">
<p class="nomargin">
The LITMUS<sup>RT</sup> development effort is being supported by grants from
AT&T, IBM, and Northrop Grumman Corps.; the National Science Foundation (grants CNS 0834270 and CNS 0834132); the U.S.
Army Research Office (grant W911NF-09-1-0535); and the Air Force Office of Scientific Research (grant FA 9550-09-1-0549).
</p>
</div>
<h2 id="collaborators">Collaborators</h2>
<div class="box">
<p class="notopmargin"> The LITMUS<sup>RT</sup> project is led by <a
href="http://www.cs.unc.edu/~anderson/">Dr. James H. Anderson</a>.
</p>
<p>
The developers of the current version are:
</p>
<ul>
<li>
<a href="http://www.cs.unc.edu/~bbb/">Björn B. Brandenburg</a> (UNC Chapel Hill, maintainer)
</li>
<li> <a href="http://www.sprg.uniroma2.it/home/bastoni/">Andrea Bastoni</a> (University of Rome "Tor Vergata")
</li>
<li> <a href="http://www.cs.unc.edu/~gelliott">Glenn Elliot</a> (UNC Chapel Hill)
</li>
<li> <a href="http://www.cs.unc.edu/~cjk">Christopher Kenna</a> (UNC Chapel Hill)
</li>
</ul>
<p >
Additional collaborators contributed to the previous <a href="litmus2010.html#collaborators">LITMUS<sup>RT</sup> 2010</a>,
<a href="litmus2008.html#collaborators">LITMUS<sup>RT</sup> 2008</a>, and <a href="litmus2007.html#collaborators">LITMUS<sup>RT</sup> 2007</a> versions.
</p>
<p class="nobottommargin">
We always welcome new contributors; please see <a href="#development">Development</a> below for information on how to get
access to our source code repository.
</p>
</div>
<h2 id="publications">Publications</h2>
<div class="box">
<ol class="nomargin">
<li><p>
A. Bastoni, B. Brandenburg and J. Anderson,
“Is Semi-Partitioned Scheduling Practical?”,
<cite>in submission</cite>, January 2011.
<a href="http://www.cs.unc.edu/~bbb/papers/ecrts11a.pdf">PDF</a>.
Longer version with all graphs: <a href="http://www.cs.unc.edu/~bbb/papers/ecrts11a-long.pdf">PDF</a> </p>
<p> For reference, all evaluated plugins as well as the required userspace tools are provided as part of the following patches (against version 2010.2).
</p>
<ul>
<li>
<a href="download/ECRTS11/litmus-rt-semi-part.patch">litmus-rt-semi-part.patch</a>
</li>
<li>
<a href="download/ECRTS11/liblitmus-semi-part.patch">liblitmus-semi-part.patch</a>
</li>
</ul>
</li>
<li><p>
A. Bastoni, B. Brandenburg and J. Anderson,
“An Empirical Comparison of Global, Partitioned, and Clustered Multiprocessor Real-Time Schedulers”,
<cite>Proceedings of the 31th IEEE Real-Time Systems Symposium</cite>, pp. 14-24, December 2010.
<a href="http://www.cs.unc.edu/~anderson/papers/rtss10c.pdf">PDF</a>.
Longer version with all graphs: <a href="http://www.cs.unc.edu/~anderson/papers/rtss10clong.pdf">PDF</a> </p>
</li>
<li><p>J. Anderson, “Real-Time Multiprocessor Scheduling: Connecting Theory and Practice ”, keynote address, <cite>18th International Conference on Real-Time and Network Systems</cite>, November 4, 2010.
<a href="slides/rtns2010-keynote.pptx">PPTX</a>.
</p>
</li>
<li><p>
G. Elliott and J. Anderson,
“Real-Time Multiprocessor Systems with GPUs”,
<cite>Proceedings of the 18th International Conference on Real-Time and Network Systems</cite>, pp. 197-206, November 2010.
<a href="http://www.cs.unc.edu/~anderson/papers/rtns10.pdf">PDF</a>.
Longer version with appendices: <a href="http://www.cs.unc.edu/~anderson/papers/rtns10extended.pdf">PDF</a> </p>
</li>
<li>
<p>
A.Bastoni, B. Brandenburg and J. Anderson
“Cache-Related Preemption and Migration Delays: Empirical Approximation and Impact on Schedulability”,
<cite>Proceedings of the Sixth International Workshop on Operating Systems Platforms for Embedded Real-Time Applications</cite>, pp. 33-44, July 2010.
<a href="http://www.cs.unc.edu/~anderson/papers/ospert10.pdf">PDF</a>.
</p>
</li>
<li><p>
B. Brandenburg and J. Anderson,
“On the Implementation of Global Real-Time
Schedulers”, <cite>Proceedings of the 30th IEEE Real-Time Systems Symposium</cite>, pp. 214-224, December 2009.
<a href="http://www.cs.unc.edu/~anderson/papers/rtss09a.pdf">PDF</a>.
Longer version with all graphs:
<a href="http://www.cs.unc.edu/~anderson/papers/rtss09a_long.pdf">PDF</a>.
</p>
<p> For reference, all evaluated plugins are provided as part of the following patch (against version 2008.3).
</p>
<ul>
<li>
<a href="download/RTSS09/litmus-rt-RTSS09.patch">litmus-rt-RTSS09.patch</a>
</li>
</ul>
</li>
<li>
<p>
B. Brandenburg and J. Anderson
“Reader-Writer Synchronization for Shared-Memory Multiprocessor Real-Time Systems”,
<cite>Proceedings of the 21st Euromicro Conference on Real-Time Systems</cite>, pp. 184-193, July 2009.
<a href="http://www.cs.unc.edu/~anderson/papers/ecrts09b.pdf">PDF</a>.
Long version with blocking terms:
<a href="http://www.cs.unc.edu/~anderson/papers/ecrts09b-long.pdf">PDF</a>.
</p>
</li>
<li>
<p>
J. Calandrino and J. Anderson
“On the Design and Implementation of a Cache-Aware Multicore Real-Time Scheduler”,
<cite>Proceedings of the 21st Euromicro Conference on Real-Time Systems</cite>, pp. 194-204, July 2009.
<a href="http://www.cs.unc.edu/~anderson/papers/ecrts09c.pdf">PDF</a>.
</p>
</li>
<li>
<p>
M. Mollison, B. Brandenburg, and J. Anderson
“Towards Unit Testing Real-Time Schedulers in LITMUS<sup>RT</sup>”,
<cite>Proceedings of the Fifth International Workshop on Operating Systems Platforms for Embedded Real-Time Applications</cite>, pp. 33-39, July 2009.
<a href="http://www.cs.unc.edu/~anderson/papers/ospert09.pdf">PDF</a>.
</p>
</li>
<li>
<p>
B. Brandenburg and J. Anderson,
“A Comparison of the M-PCP, D-PCP, and FMLP on LITMUS<sup>RT</sup>”,
<cite>Proceedings of the 12th International Conference on Principles of Distributed Systems</cite>, pp. 105-124, December 2008.
<a href="http://www.cs.unc.edu/~anderson/papers/opodis08.pdf">PDF</a>.
</p>
</li>
<li>
<p>
B. Brandenburg, J. Calandrino, and J. Anderson,
“On the Scalability of Real-Time Scheduling Algorithms on Multicore Platforms: A Case Study”,
<cite>Proceedings of the 29th IEEE Real-Time Systems Symposium</cite>,
pp. 157-169, December 2008.
<a href="http://www.cs.unc.edu/~anderson/papers/rtss08b.pdf">PDF</a>.
</p>
</li>
<li>
<p>
B. Brandenburg and J. Anderson,
“An Implementation of the PCP, SRP, D-PCP, M-PCP,
and FMLP Real-Time Synchronization Protocols in LITMUS<sup>RT</sup>”,
<cite>Proceedings of the 14th IEEE International Conference on Embedded and Real-Time Computing Systems and Applications</cite>, pp. 185-194, August 2008.
<a href="http://www.cs.unc.edu/~anderson/papers/rtcsa08.pdf">PDF</a>.
</p>
<p><strong>Note:</strong> 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:
</p>
<ul>
<li>
<a href="download/RTCSA08/litmus-rt-RTCSA08.patch">litmus-rt-RTCSA08.patch</a>
</li>
<li>
<a href="download/RTCSA08/liblitmus-RTCSA08.tgz">liblitmus-RTCSA08.tgz</a>
</li>
<li><a href="download/RTCSA08/SHA256SUMS">SHA256 check sums</a>
</li>
</ul>
<p>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<sup>RT</sup> and merge them into the main distribution.
</p>
</li>
<li>
<p>
A. Block, B. Brandenburg, J. Anderson,
and S. Quint, “An Adaptive Framework for Multiprocessor Real-Time Systems”,
<cite>Proceedings of the 20th Euromicro Conference on Real-Time Systems</cite>, pp. 23-33, July 2008.
<a href="http://www.cs.unc.edu/~anderson/papers/ecrts08b.pdf">PDF</a>.
</p>
</li>
<li>
<p>
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?”, <cite> Proceedings of the 14th IEEE Real-Time and Embedded
Technology and Applications Symposium</cite>, pp. 342-353, April 2008.
<a href="http://www.cs.unc.edu/~anderson/papers/rtas08.pdf">PDF</a>.
</p>
<p>
Extended version, including all graphs:
<a href="http://www.cs.unc.edu/~anderson/papers/rtas08along.pdf">PDF</a>.
</p>
</li>
<li>
<p>
B. Brandenburg, A. Block, J. Calandrino, U. Devi, H. Leontyev, and J. Anderson,
"LITMUS<sup>RT</sup>: A Status Report", <cite> Proceedings of the 9th
Real-Time Linux Workshop</cite>, pp. 107-123, November 2007.
<a href="http://www.cs.unc.edu/~anderson/papers/rtlws07.pdf">PDF</a>.
</p>
</li>
<li>
<p>
B. Brandenburg and J. Anderson, "Integrating Hard/Soft Real-Time Tasks
and Best-Effort Jobs on Multiprocessors", <cite> Proceedings of the 19th Euromicro
Conference on Real-Time Systems</cite>, pp. 61-70, July 2007.
<a href="http://www.cs.unc.edu/~anderson/papers/ecrts07b.pdf">PDF</a>.
</p>
</li>
<li>
<p>
J. Calandrino, H. Leontyev, A. Block, U. Devi, and J. Anderson,
"LITMUS<sup>RT</sup>: A Testbed for Empirically Comparing Real-Time
Multiprocessor Schedulers ", <cite>Proceedings of the 27th IEEE Real-Time Systems
Symposium</cite>, pp. 111-123, December 2006.
<a href="http://www.cs.unc.edu/~anderson/papers/rtss06b.pdf">PDF</a>.
</p>
</li>
</ol>
</div>
<h2 id="download">Download</h2>
<div class="box">
<p class="notopmargin">
The source code of LITMUS<sup>RT</sup> is made available as open source
under the terms of the <a href="http://www.gnu.org/licenses/gpl.txt">GNU
General Public License (GPL)</a>. LITMUS<sup>RT</sup> is released as a patch against Linux. Additionally,
it is also available as a <span class="src">git</span> repository (see <a href="#development">Development</a> below).
</p>
<p>
The current release of LITMUS<sup>RT</sup> is 2010.2.
It consists of our Linux kernel modifications in the form of
a patch against Linux 2.6.34 and
<span class="src">liblitmus</span>, the user-space API for real-time
tasks, as well as <span class="src">ft_tools</span>, a collection of tools
used for tracing with <a href="http://www.cs.unc.edu/~bbb/feathertrace/">Feather-Trace</a> (which is part of the LITMUS<sup>RT</sup> patch).
</p>
<h3 class="relname">LITMUS<sup>RT</sup> 2011.1</h3>
<div class="release">
<p>
Based on Linux 2.6.36. Released in January 2011.
</p>
<h4>Files:</h4>
<ul class="notopmargin">
<li>
<a href="download/2011.1/litmus-rt-2011.1.patch">litmus-rt-2011.1.patch</a>
</li>
<li>
<a href="download/2011.1/liblitmus-2011.1.tgz">liblitmus-2011.1.tgz</a>
</li>
<li>
<a href="download/2011.1/ft_tools-2011.1.tgz">ft_tools-2011.1.tgz</a>
</li>
<li><a href="download/2011.1/SHA256SUMS">SHA256 check sums</a>
</li>
</ul>
<h4>Major changes (since LITMUS<sup>RT</sup> 2010.2):</h4>
<ul class="notopmargin">
<li>
Rebased LITMUS<sup>RT</sup> from Linux 2.6.34 to Linux 2.6.36.
</li>
<li>
Added support for the ARM architecture (tested on a PB11MPCore baseboard with a four-core ARM11 MPCore CPU).
</li>
<li>
Feather-Trace devices are now allocated dynamically and are properly registered with <span class="src">sysfs</span>. This avoids bugs due to major device number collisions and removes the need for manual device node creation (on a system with standard <span class="src">udev</span> rules).
</li>
<li>
Improved debug tracing output and made trace buffer size configurable.
</li>
<li>
Various bug fixes concerning C-EDF cluster size changes. The cluster size
can now be configured with the file <span class="src">/proc/litmus/plugins/C-EDF/cluster</span>.
</li>
<li>
Various <span class="src">KConfig</span> cleanups and improvements.
</li>
<li>
Dropped <span class="src">scons</span> as the build system for <span class="src">liblitmus</span>
and reverted to makefiles.
</li>
<li>
Added cscope and TAG file generation to <span class="src">liblitmus</span>.
</li>
<li>
<span class="src">st_trace</span> can now be controlled with signals (part of <span class="src">ft_tools</span>).
</li>
</ul>
</div>
<p>
Please note that the current implementation is a <em>prototype</em> with
certain limitations. It is not secure in a multiuser context,
<em>i.e.</em>, real-time system calls do not require superuser
privileges.
</p>
<p class="nobottommargin">
Older releases: <a href="litmus2010.html">LITMUS<sup>RT</sup> 2010 series</a>, <a href="litmus2008.html">LITMUS<sup>RT</sup> 2008 series</a>, <a href="litmus2007.html">LITMUS<sup>RT</sup> 2007 series</a>.
</p>
</div>
<h2 id="install">Installation</h2>
<div class="box">
<p class="notopmargin">
The current release of LITMUS<sup>RT</sup> 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<sup>RT</sup>
real-time API. Note that the current implementation works on the
Intel (both x86-32 and x86-64) and on the ARM architecture (ARMV6).
</p>
<h3>Patching the Kernel</h3>
<p class="notopmargin">
The extension to the Linux kernel is released as a patch against Linux
2.6.36. To install the LITMUS<sup>RT</sup> kernel, first <a
href="http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.36.tar.bz2">download the Linux
kernel 2.6.36</a> and untar it in a directory of your choice (hereafter
referred to as <span class="src">$DIR</span>). Second, apply the
LITMUS<sup>RT</sup> patch (see <a href="#download">Section Download</a>)
and configure, compile, and install the kernel as usual. The patch is <span
class="src">-p1</span> applicable.
To summarize, the LITMUS<sup>RT</sup> kernel can be obtained, patched, and
compiled with the following commands:
</p>
<pre class="shell">cd $DIR
# get Linux 2.6.36
wget http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.36.tar.bz2
tar xjf linux-2.6.36.tar.bz2
wget http://www.cs.unc.edu/~anderson/litmus-rt/download/2011.1/litmus-rt-2011.1.patch
mv linux-2.6.36 litmus-rt
# apply the LITMUS RT patch
cd litmus-rt
patch -p1 < ../litmus-rt-2011.1.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
# - disable "Write protect kernel read-only data structures" (in kernel debug)
make menuconfig
# compile the kernel
make bzImage
make modules
# proceed to install kernel, build initrd, etc.
...
</pre>
<p>
When configuring the kernel, note that there is a menu (at the very end of the list)
with LITMUS<sup>RT</sup>-specific configuration options. For reference, we provide sample <a href="download/2011.1/x86_64-config">64-bit configuration</a> that is known to work under KVM.
</p>
<h3>Libraries</h3>
<p class="notopmargin">
The user-space library for real-time tasks, <span class="src">liblitmus</span>,
depends on the LITMUS<sup>RT</sup> kernel kernel and provides its own build system. Run <span class="src">make help</span> for instructions. Note that
you will have to create a <span class="src">.config</span> file in the library directory to point the build system to the kernel sources.
For reference, we provide a sample <a href="download/2011.1/liblitmus-config">liblitmus configuration file</a>.
</p>
<p class="notopmargin">
To summarize, the <span class="src">liblitmus</span> can be obtained and
compiled with the following commands:
</p>
<pre class="shell">
cd $DIR
wget http://www.cs.unc.edu/~anderson/litmus-rt/download/2011.1/liblitmus-2011.1.tgz
tar xzf liblitmus-2011.1.tgz
cd liblitmus
wget http://www.cs.unc.edu/~anderson/litmus-rt/download/2011.1/liblitmus-config
mv liblitmus-config .config
make
</pre>
<h3>Tools</h3>
<p class="notopmargin">
The Feather-Trace tools (<span class="src">ft_tools</span>) depend on the <span class="src">liblitmus</span> build system. Before the <span class="src">ft_tools</span> project can be built, you must have successfully compiled <span class="src">liblitmus</span> first.
For reference, we provide a sample <a href="download/2011.1/ft_tools-config">ft_tools configuration file</a>.
</p>
<p class="notopmargin">
To summarize, the <span class="src">ft_tools</span> can be obtained and
compiled with the following commands:
</p>
<pre class="shell">
cd $DIR
wget http://www.cs.unc.edu/~anderson/litmus-rt/download/2011.1/ft_tools-2011.1.tgz
tar xzf ft_tools-2011.1.tgz
cd ft_tools
wget http://www.cs.unc.edu/~anderson/litmus-rt/download/2011.1/ft_tools-config
mv ft_tools-config .config
make
</pre>
<p class="nobottommargin">
Please refer to the <a href="#doc">documentation</a> on how to use the LITMUS<sup>RT</sup>
real-time API as provided by <span class="src">liblitmus</span>.
</p>
</div>
<h2 id="doc">Documentation</h2>
<div class="box">
<p class="notopmargin">
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
<a href="http://www.cs.unc.edu/~anderson/papers/rtlws07.pdf">“LITMUS<sup>RT</sup>:
A Status Report”</a>.
</p>
<h3>Real-Time Scheduling Policies</h3>
<p class="qa">
The kernel contains the following real-time scheduling policy implementations:
</p>
<ul>
<li>
PFAIR, an implementation of the PD<sup>2</sup> algorithm,
</li>
<li>
PSN-EDF, a partitioned EDF (P-EDF) implementation with support for the real-time synchronization protocol
FMLP,
</li>
<li>
GSN-EDF, a global EDF (G-EDF) implementation with support for the real-time synchronization protocol
FMLP,
</li>
<li>
C-EDF (Clustered EDF), a hybrid of G-EDF and P-EDF, and
</li>
<li>
Linux, a placeholder policy that disables all real-time functionality added by the LITMUS<sup>RT</sup> patch.
</li>
</ul>
<p>
Only one policy can be active at any time. Initially (<em>i.e.,</em> during and after boot), the "Linux" policy is active.
You can use the tool <span class="src">showsched</span> (part of <span class="src">liblitmus</span>) to display
the name of the currently active policy.
</p>
<h3>Changing the Active Policy</h3>
<p class="qa">
You can use the tool <span class="src">setsched</span> (part of <span class="src">liblitmus</span>)
to select a new plugin at run time.
</p>
<div class="screenshot">
<img src="gfx/setsched.png" alt="Screen shot of setsched"/>
</div>
<p>
Only root can change the active policy, and only when there are no real-time tasks present.
</p>
<p>
If you do not have the <span class="src">dialog</span> utility installed, then you can still used <span class="src">setsched</span> by passing the desired scheduling policy as a commandline parameter, <em>e.g.</em> type <span class="src"> setsched PFAIR </span> to activate the PFAIR plugin.
</p>
<p>The list of loaded plugins is exported at <span class="src">/proc/litmus/plugins/loaded</span>; the active plugin is reported at <span class="src">/proc/litmus/active_plugin</span>.
</p>
<h3>Selecting the C-EDF Cluster Size</h3>
<p class="qa">
The C-EDF plugin can create clusters based on the cache topology. When the plugin is activated (see above), it configures clusters based on the value ("L1", "L2", "L3", or "ALL") last written to <span class="src">/proc/litmus/plugins/C-EDF/cluster</span>. Note that the C-EDF must be reloaded (for example by switching to the Linux plugin and back to C-EDF) to enact a change to the desired cluster size; changing the cluster size while C-EDF is active is not supported.
</p>
<h3>Writing Real-Time Tasks</h3>
<p class="qa">
The user space library that provides the LITMUS<sup>RT</sup> API,
<span class="src">liblitmus</span>, contains two example real-time tasks
(<span class="src">base_task.c</span> and
<span class="src">base_mt_task.c</span>)
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 at these example
programs.
</p>
<h3>Tracing Overheads and Scheduling Decisions</h3>
<p class="qa">LITMUS<sup>RT</sup> provides numerous tracing facilities that are discussed in-depth in the tutorial <a href="doc/tracing.html">Tracing with LITMUS<sup>RT</sup></a>.
</p>
<p class="nobottommargin">
Please contact the <a href="#collaborators">current maintainer</a> if you have any
questions.
</p>
</div>
<h2 id="development">Development</h2>
<div class="box">
<p class="nomargin">
Patches and suggestions are very welcome!
Both the LITMUS<sup>RT</sup> kernel and liblitmus are available
as a public <a href="http://git-scm.com/">git</a> repository at
<a href="public-repository/index.html">http://www.cs.unc.edu/~anderson/litmus-rt/public-repository</a>.
</p>
</div>
<h2 id="credits">Credits</h2>
<div class="box">
<div style="float: right;">
<a href="http://validator.w3.org/check?uri=referer"><img
src="http://www.w3.org/Icons/valid-xhtml10"
alt="Valid XHTML 1.0 Strict" height="31" width="88"/></a>
</div>
<p class="nomargin">
Linux is a registered trademark of Linus Torvalds. <br /> The
LITMUS<sup>RT</sup> logo was designed by Jasper McChesney of <a href="http://www.breakforsense.net/">Break for Sense Design</a>. <br />
Web design by Björn Brandenburg.
</p>
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