---
layout: default
navsection: installguide
title: Install the Crunch dispatcher

...

The dispatcher normally runs on the same host/VM as the API server.

h2. Test the Arvados job queue

Crunch dispatches work from the job queue on the Arvados API server.  Before you start installing the Crunch dispatcher, now's a good time to check that the API server and Git server can coordinate to create job records.  Run these commands *on your shell server* to create a collection, and a job to calculate the MD5 checksum of every file in it:

<notextile>
<pre><code>~$ <span class="userinput">echo 'Hello, Crunch!' | arv-put --portable-data-hash -</span>
&hellip;
d40c7f35d80da669afb9db1896e760ad+49
~$ <span class="userinput">read -rd $'\000' newjob &lt;&lt;EOF; arv job create --job "$newjob"
{"script_parameters":{"input":"d40c7f35d80da669afb9db1896e760ad+49"},
 "script_version":"0988acb472849dc0",
 "script":"hash",
 "repository":"arvados"}
EOF</span>
</code></pre>
</notextile>

If you get the error

<pre>
ArgumentError: Specified script_version does not resolve to a commit
</pre>

it often means that the API server can't read the specified repository&mdash;either because it doesn't exist, or because the user running the API server doesn't have permission to read the repository files.  Check the API server's log (@/var/www/arvados-api/current/log/production.log@) for details, and double-check the instructions in the "Git server installation guide":install-arv-git-httpd.html.

If everything goes well, the API server should create a job record, and your @arv@ command will output the JSON for that record.  It should have state @Queued@ and script_version @0988acb472849dc08d576ee40493e70bde2132ca@.  If the job JSON includes those fields, you can proceed to install the Crunch dispatcher and a compute node.  This job will remain queued until you install those services.

h2. Perl SDK dependencies

Install the Perl SDK on the controller.

* See "Perl SDK":{{site.baseurl}}/sdk/perl/index.html page for details.

h2. Python SDK dependencies

Install the Python SDK and CLI tools on controller and all compute nodes.

* See "Python SDK":{{site.baseurl}}/sdk/python/sdk-python.html page for details.

h2(#slurm). Set up SLURM

On the API server, install SLURM and munge, and generate a munge key.

On Debian-based systems:

<notextile>
<pre><code>~$ <span class="userinput">sudo /usr/bin/apt-get install slurm-llnl munge</span>
~$ <span class="userinput">sudo /usr/sbin/create-munge-key</span>
</code></pre>
</notextile>

On Red Hat-based systems:

<notextile>
<pre><code>~$ <span class="userinput">sudo yum install slurm munge slurm-munge</span>
</code></pre>
</notextile>

Now we need to give SLURM a configuration file.  On Debian-based systems, this is installed at @/etc/slurm-llnl/slurm.conf@.  On Red Hat-based systems, this is installed at @/etc/slurm/slurm.conf@.  Here's an example @slurm.conf@:

<notextile>
<pre>
ControlMachine=uuid_prefix.your.domain
SlurmctldPort=6817
SlurmdPort=6818
AuthType=auth/munge
StateSaveLocation=/tmp
SlurmdSpoolDir=/tmp/slurmd
SwitchType=switch/none
MpiDefault=none
SlurmctldPidFile=/var/run/slurmctld.pid
SlurmdPidFile=/var/run/slurmd.pid
ProctrackType=proctrack/pgid
CacheGroups=0
ReturnToService=2
TaskPlugin=task/affinity
#
# TIMERS
SlurmctldTimeout=300
SlurmdTimeout=300
InactiveLimit=0
MinJobAge=300
KillWait=30
Waittime=0
#
# SCHEDULING
SchedulerType=sched/backfill
SchedulerPort=7321
SelectType=select/linear
FastSchedule=0
#
# LOGGING
SlurmctldDebug=3
#SlurmctldLogFile=
SlurmdDebug=3
#SlurmdLogFile=
JobCompType=jobcomp/none
#JobCompLoc=
JobAcctGatherType=jobacct_gather/none
#
# COMPUTE NODES
NodeName=DEFAULT
PartitionName=DEFAULT MaxTime=INFINITE State=UP

NodeName=compute[0-255]
PartitionName=compute Nodes=compute[0-255] Default=YES Shared=YES
</pre>
</notextile>

h3. SLURM configuration essentials

Whenever you change this file, you will need to update the copy _on every compute node_ as well as the controller node, and then run @sudo scontrol reconfigure@.

*@ControlMachine@* should be a DNS name that resolves to the SLURM controller (dispatch/API server). This must resolve correctly on all SLURM worker nodes as well as the controller itself. In general SLURM is very sensitive about all of the nodes being able to communicate with the controller _and one another_, all using the same DNS names.

*@NodeName=compute[0-255]@* establishes that the hostnames of the worker nodes will be compute0, compute1, etc. through compute255.
* There are several ways to compress sequences of names, like @compute[0-9,80,100-110]@. See the "hostlist" discussion in the @slurm.conf(5)@ and @scontrol(1)@ man pages for more information.
* It is not necessary for all of the nodes listed here to be alive in order for SLURM to work, although you should make sure the DNS entries exist. It is easiest to define lots of hostnames up front, assigning them to real nodes and updating your DNS records as the nodes appear. This minimizes the frequency of @slurm.conf@ updates and use of @scontrol reconfigure@.

Each hostname in @slurm.conf@ must also resolve correctly on all SLURM worker nodes as well as the controller itself. Furthermore, the hostnames used in the configuration file must match the hostnames reported by @hostname@ or @hostname -s@ on the nodes themselves. This applies to the ControlMachine as well as the worker nodes.

For example:
* In @slurm.conf@ on control and worker nodes: @ControlMachine=uuid_prefix.your.domain@
* In @slurm.conf@ on control and worker nodes: @NodeName=compute[0-255]@
* In @/etc/resolv.conf@ on control and worker nodes: @search uuid_prefix.your.domain@
* On the control node: @hostname@ reports @uuid_prefix.your.domain@
* On worker node 123: @hostname@ reports @compute123.uuid_prefix.your.domain@

h3. Automatic hostname assignment

If your worker node bootstrapping script (see "Installing a compute node":install-compute-node.html) does not send the worker's current hostname, the API server will choose an unused hostname from the set given in @application.yml@, which defaults to @compute[0-255]@.

If it is not feasible to give your compute nodes hostnames like compute0, compute1, etc., you can accommodate other naming schemes with a bit of extra configuration.

If you want Arvados to assign names to your nodes with a different consecutive numeric series like @{worker1-0000, worker1-0001, worker1-0002}@, add an entry to @application.yml@; see @/var/www/arvados-api/current/config/application.default.yml@ for details. Example:
* In @application.yml@: <code>assign_node_hostname: worker1-%<slot_number>04d</code>
* In @slurm.conf@: <code>NodeName=worker1-[0000-0255]</code>

If your worker hostnames are already assigned by other means, and the full set of names is known in advance, have your worker node bootstrapping script (see "Installing a compute node":install-compute-node.html) send its current hostname, rather than expect Arvados to assign one.
* In @application.yml@: <code>assign_node_hostname: false</code>
* In @slurm.conf@: <code>NodeName=alice,bob,clay,darlene</code>

If your worker hostnames are already assigned by other means, but the full set of names is _not_ known in advance, you can use the @slurm.conf@ and @application.yml@ settings in the previous example, but you must also update @slurm.conf@ (both on the controller and on all worker nodes) and run @sudo scontrol reconfigure@ whenever a new node comes online.

h2. Enable SLURM job dispatch

In your API server's @application.yml@ configuration file, add the line @crunch_job_wrapper: :slurm_immediate@ under the appropriate section.  (The second colon is not a typo.  It denotes a Ruby symbol.)

h2. Crunch user account

Run @sudo adduser crunch@.  The crunch user should have the same UID, GID, and home directory on all compute nodes and on the dispatcher (API server).

h2. Run the Crunch dispatcher service

To dispatch Arvados jobs:

* The API server script @crunch-dispatch.rb@ must be running.
* @crunch-job@ needs the installation path of the Perl SDK in its @PERLLIB@.
* @crunch-job@ needs the @ARVADOS_API_HOST@ (and, if necessary, @ARVADOS_API_HOST_INSECURE@) environment variable set.

Install runit to monitor the Crunch dispatch daemon.  {% include 'install_runit' %}

Install the script below as the run script for the Crunch dispatch service, modifying it as directed by the comments.

<notextile>
<pre><code>#!/bin/sh
set -e

rvmexec=""
## Uncomment this line if you use RVM:
#rvmexec="/usr/local/rvm/bin/rvm-exec default"

export ARVADOS_API_HOST=<span class="userinput">uuid_prefix.your.domain</span>
export CRUNCH_DISPATCH_LOCKFILE=/var/lock/crunch-dispatch
export HOME=$(pwd)
export RAILS_ENV=production

## Uncomment this line if your cluster uses self-signed SSL certificates:
#export ARVADOS_API_HOST_INSECURE=yes

# This is the path to docker on your compute nodes. You might need to
# change it to "docker", "/opt/bin/docker", etc.
export CRUNCH_JOB_DOCKER_BIN=<span class="userinput">docker.io</span>

fuser -TERM -k $CRUNCH_DISPATCH_LOCKFILE || true
cd /var/www/arvados-api/current
exec $rvmexec bundle exec ./script/crunch-dispatch.rb 2>&1
</code></pre>
</notextile>