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<h1>Topology</h1>

<p>SLURM version 2.0 can be configured to support topology-aware resource
allocation to optimize job performance.
There are two primary modes of operation, one to optimize performance on
systems with a three-dimensional torus interconnect and another for
a hierarchical interconnect.</p>

<p>SLURM's native mode of resource selection is to consider the nodes
as a one-dimensional array. 
Jobs are allocated resources on a best-fit basis.
For larger jobs, this minimizes the number of sets of consecutive nodes
allocated to the job.</p>

<h2>Three-dimension Topology</h2>

<p>Some larger computers rely upon a three-dimensional torus interconnect.
The IBM BlueGene computers is one example of this which has highly
constrained resource allocation scheme, essentially requiring that 
jobs be allocated a set of nodes logically having a rectangular shape.
SLURM has a plugin specifically written for BlueGene to select appropriate
nodes for jobs, change network switch routing, boot nodes, etc as described
in the <a href="bluegene.html">BlueGene User and Administrator Guide</a>.</p>

<p>The Sun Constellation and Cray XT systems also have three-dimensional 
torus interconnects, but do not require that jobs execute in adjacent nodes.
On those systems, SLURM only needs to allocate resources to a job which 
are nearby on the network. 
SLURM accomplishes this using a 
<a href="http://en.wikipedia.org/wiki/Hilbert_curve">Hilbert curve</a> 
to map the nodes from a three-dimensional space into a one-dimensional
space.
SLURM's native best-fit algorithm is thus able to achieve a high degree
of locality for jobs.
For more information, see SLURM's documentation for
<a href="sun_const.html">Sun Constellation</a> and
<a href="cray.html">Cray XT</a> systems.</p>

<h2>Hierarchical Networks</h2>

<p>SLURM can also be configured to allocate resources to jobs on a 
hierarchical network to minimize network contention.
The basic algorithm is to identify the lowest level switch in the
hierarchy that can satisfy a job's request and then allocate resources
on its underlying leaf switches using a best-fit algorithm.
Use of this logic requires a configuration setting of 
<i>TopologyPlugin=topology/tree</i>.</p>

<p>At some point in the future SLURM code may be provided to 
gather network topology information directly.
Now the network topology information must be included 
in a <i>topology.conf</i> configuration file as shown in the 
examples below.
The first example describes a three level switch in which 
each switch has two children. 
Note that the <i>SwitchName</i> values are arbitrary and only
used to bookkeeping purposes, but a name must be specified on
each line.
The leaf switch descriptions contain a <i>SwitchName</i> field
plus a <i>Nodes</i> field to identify the nodes connected to the
switch.
Higher-level switch descriptions contain a <i>SwitchName</i> field
plus a <i>Switches</i> field to identify the child switches.
SLURM's hostlist expression parser is used, so the node and switch
names need not be consecutive (e.g. "Nodes=tux[0-3,12,18-20]"
and "Swithces=s[0-2,4-8,12]" will parse fine).
</p>

<p>An optional LinkSpeed option can be used to indicate the 
relative performance of the link. 
The units used are arbitrary and this information is currently not used.
It may be used in the future to optimize resource allocations.</p>

<p>The first example shows what a topology would look like for an
eight node cluster in which all switches have only two children as
shown in the diagram (not a very realistic configuration, but 
useful for an example).</p>

<pre>
# topology.conf
# Switch Configuration
SwitchName=s0 Nodes=tux[0-1]
SwitchName=s1 Nodes=tux[2-3]
SwitchName=s2 Nodes=tux[4-5]
SwitchName=s3 Nodes=tux[6-7]
SwitchName=s4 Switches=s[0-1]
SwitchName=s5 Switches=s[2-3]
SwitchName=s6 Switches=s[4-5]
</pre>
<img src=topo_ex1.gif width=600>

<p>The next example is for a network with two levels and 
each switch has four connections.</p>
<pre>
# topology.conf
# Switch Configuration
SwitchName=s0 Nodes=tux[0-3]   LinkSpeed=900
SwitchName=s1 Nodes=tux[4-7]   LinkSpeed=900
SwitchName=s2 Nodes=tux[8-11]  LinkSpeed=900
SwitchName=s3 Nodes=tux[12-15] LinkSpeed=1800
SwitchName=s4 Switches=s[0-3]  LinkSpeed=1800
SwitchName=s5 Switches=s[0-3]  LinkSpeed=1800
SwitchName=s6 Switches=s[0-3]  LinkSpeed=1800
SwitchName=s7 Switches=s[0-3]  LinkSpeed=1800
</pre>
<img src=topo_ex2.gif width=600>

<p style="text-align:center;">Last modified 24 March 2009</p>

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