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<h1>Cgroups Guide</h1>
<h2>Cgroups Overview</h2>
For a comprehensive description of Linux Control Groups (cgroups) see the
<a href="http://www.kernel.org/doc/Documentation/cgroups/cgroups.txt">
cgroups documentation</A> at kernel.org. Detailed knowledge of cgroups is not
required to use cgroups in SLURM, but a basic understanding of the
following features of cgroups is helpful:
<ul>
<li><b>Cgroup</b> - a container for a set of processes subject to common
controls or monitoring, implemented as a directory and a set of files
(state objects) in the cgroup
virtual filesystem.</li>
<li><b>Subsystem</b> - a module, typically a resource controller, that applies
a set of parameters to the cgroups in a hierarchy.</li>
<li><b>Hierarchy</b> - a set of cgroups organized in a tree structure, with one
or more associated subsystems.</li>
<li><b>State Objects</b> - pseudofiles that represent the state of a cgroup or
apply controls to a cgroup:
<ul>
<li><i>tasks</i> - identifies the processes (PIDs) in the cgroup.
<li><i>release_agent</i> - specifies the location of the script or program to
be called when the cgroup becomes empty.</li>
<li><i>notify_on_release</i> - controls whether the release_agent is called for
the cgroup.</li>
<li>additional state objects specific to each subsystem.</li>
</ul>
</ul>
<br>
<h2>Use of Cgroups in SLURM</h2>
SLURM provides cgroup versions of a number of plugins.
<ul>
<li>proctrack (process tracking)</li>
<li>task (task management)</li>
<li>jobacct_gather (job accounting statistics)</li>
The cgroup plugins can provide a number of benefits over the
other more standard plugins, as described below.
</ul>
<br>
<h2>SLURM Cgroups Configuration Overview</h2>
There are several sets of configuration options for SLURM cgroups:
<ul>
<li><a href="slurm.conf.html">slurm.conf</a> provides options to enable the
cgroup plugins. Each plugin may be enabled or disabled independently
of the others.</li>
<li><a href="cgroup.conf.html">cgroup.conf</a> provides general options that
are common to all cgroup plugins, plus additional options that apply only to
specific plugins.</li>
<li>Additional configuration is required to enable automatic removal of SLURM
cgroups when they are no longer in use.
See <a href="#cleanup">Cleanup of SLURM Cgroups</a> below for details.</li>
</ul>
<a name="available"></a>
<br>
<h2>Currently Available Cgroup Plugins</h2>
<h3>proctrack/cgroup plugin</h3>
The proctrack/cgroup plugin is an alternative to other proctrack
plugins such as proctrack/linux for process tracking and
suspend/resume capability. proctrack/cgroup uses the freezer subsystem
which is more reliable for tracking and control than proctrack/linux.
<p>
To enable this plugin, configure the following option in slurm.conf:
<pre>ProctrackType=proctrack/cgroup</pre>
</p>
There are no specific options for this plugin in cgroup.conf, but the general
options apply. See the <a href="cgroup.conf.html">cgroup.conf</a> man page for
details.
<h3>task/cgroup plugin</h3>
The task/cgroup plugin is an alternative other task plugins such as
task/affinity plugin for task management. task/cgroup provides the
following features:
<ul>
<li>The ability to confine jobs and steps to their allocated cpuset.</li>
<li>The ability to bind tasks to sockets, cores and threads within their step's
allocated cpuset on a node.</li>
<ul>
<li>Supports block and cyclic distribution of allocated cpus to tasks for
binding.</li>
</ul>
<li>The ability to confine jobs and steps to specific memory resources.</li>
<li>The ability to confine jobs to their allocated set of generic resources
(gres devices).</li>
</ul>
The task/cgroup plugin uses the cpuset, memory and devices subsystems.
<p>
To enable this plugin, configure the following option in slurm.conf:
<pre>TaskPlugin=task/cgroup</pre>
</p>
There are many specific options for this plugin in cgroup.conf.  The general
options also apply. See the <a href="cgroup.conf.html">cgroup.conf</a> man page
for details.
<h3>jobacct_gather/cgroup plugin</h3>
<b>At present, jobacct_gather/cgroup should be considered experimental.</b>
<p>
The jobacct_gather/cgroup plugin is an alternative to the jobacct_gather/linux
plugin for the collection of accounting statistics for jobs, steps and tasks.
The cgroup plugin may provide improved performance over jobacct_gather/linux.
jobacct_gather/cgroup uses the cpuacct and memory subsystems. Note: the cpu and
memory statistics collected by this plugin do not represent the same resources
as the cpu and memory statistics collected by the jobacct_gather/linux plugin
(sourced from /proc stat).
<p>
To enable this plugin, configure the following option in slurm.conf:
<pre>JobacctGatherType=jobacct_gather/cgroup</pre>
</p>
There are no specific options for this plugin in cgroup.conf, but the general
options apply. See the <a href="cgroup.conf.html">cgroup.conf</a> man page for
details.
<br><br>
<h2>Organization of SLURM Cgroups</h2>
SLURM cgroups are organized as follows.  A base directory (mount point) is
created at /cgroup, or as configured by the <i>CgroupMountpoint</I> option in
<a href="cgroup.conf.html">cgroup.conf</a>. All cgroup
hierarchies are created below this base directory. A separate hierarchy is
created for each cgroup subsystem in use. The name of the root cgroup in each
hierarchy is the subsystem name. A cgroup named <i>slurm</i> is created below
the root cgroup in each hierarchy. Below each <i>slurm</i> cgroup, cgroups for
SLURM users, jobs, steps and tasks are created dynamically as needed. The names
of these cgroups consist of a prefix identifying the SLURM entity (user, job,
step or task), followed by the relevant numeric id. The following example shows
the path of the task cgroup in the cpuset hierarchy for taskid#2 of stepid#0 of
jobid#123 for userid#100, using the default base directory (/cgroup):
<p><pre>/cgroup/cpuset/slurm/uid_100/job_123/step_0/task_2</pre></p>
Note that this structure applies to a specific compute node. Jobs that use more
than one node will have a cgroup structure on each node.
<a name="cleanup"></a>
<br><br>
<h2>Cleanup of SLURM Cgroups</h2>
Linux provides a mechanism for the automatic removal of a cgroup when its
state changes from non-empty to empty.  A cgroup is empty when no processes are
attached to it and it has no child cgroups.  The SLURM cgroups implementation
allows this mechanism to be used to automatically remove the relevant SLURM
cgroups when tasks, steps and jobs terminate. To enable this automatic removal
feature, follow these steps:
<ul>
<li>If desired, configure the location of the SLURM Cgroup release agent
directory. This is done using the <i>CgroupReleaseAgentDir</i> option in
<a href="cgroup.conf.html">cgroup.conf</a>.
The default location is /etc/slurm/cgroup.</li>
<br>
<pre>
       [sulu] (slurm) etc> cat cgroup.conf | grep CgroupReleaseAgentDir
       CgroupReleaseAgentDir="/etc/slurm/cgroup"
</pre>
<li>Create the common release agent file.  This file should be named
<i>release_common</i>.  An example script for this file is provided in the
SLURM delivery at etc/cgroup.release_common.example. The example script will
automatically remove user, job, step and task cgroups as they become empty. The
file must have execute permission for root.</li><br>
<li>Create release agent files for each cgroup subsystem to be used by SLURM.
This depends on which cgroup plugins are enabled.  For example, the
proctrack/cgroup plugin uses the <i>freezer</i> subsystem.  See
<a href="#available">Currently Available Cgroup Plugins</a> above to find out
which subsystems are used by each plugin. The name of each release agent file
must be of the form <i>release_&lt;subsystem name&gt;</i>. These files should
be created as symbolic links to the common release agent file,
<i>release_common</i>.  The files must have execute permission for root. See
the following example.</li>
<br>
<pre>
       [sulu] (slurm) etc> ls -al /etc/slurm/cgroup
       total 12
       drwxr-xr-x 2 root root 4096 2010-04-23 14:55 .
       drwxr-xr-x 4 root root 4096 2010-07-22 14:48 ..
       -rwxrwxrwx 1 root root  234 2010-04-23 14:52 release_common
       lrwxrwxrwx 1 root root   32 2010-04-23 11:04 release_cpuset -> /etc/slurm/cgroup/release_common
       lrwxrwxrwx 1 root root   32 2010-04-23 11:03 release_freezer -> /etc/slurm/cgroup/release_common

</pre>
</ul>
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<p style="text-align:center;">Last modified 6 June 2012</p>

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