---
layout: default
navsection: userguide
-title: "Parallel tasks"
-navorder: 115
+navmenu: Tutorials
+title: "Parallel Crunch tasks"
+navorder: 15
---
-h1. Tutorial: Parallel tasks
+h1. Tutorial: Parallel Crunch tasks
In the tutorial "writing a crunch script,":tutorial-firstscript.html our script used a "for" loop to compute the md5 hashes for each file in sequence. This approach, while simple, is not able to take advantage of the compute cluster with multiple nodes and cores to speed up computation by running tasks in parallel. This tutorial will demonstrate how to create parallel Crunch tasks.
</code></pre>
</notextile>
-You should now be able to run your new script using Crunch, with "script" referring to our new "parallel-hash.py" script. We will use a different input from our previous examples. We will use @887cd41e9c613463eab2f0d885c6dd96+83@ which consists of three files, "alice.txt", "bob.txt" and "carol.txt" (the example collection used previously in "fetching data from Arvados using Keep":intro-keep.html).
+You should now be able to run your new script using Crunch, with "script" referring to our new "parallel-hash.py" script. We will use a different input from our previous examples. We will use @887cd41e9c613463eab2f0d885c6dd96+83@ which consists of three files, "alice.txt", "bob.txt" and "carol.txt" (the example collection used previously in "fetching data from Arvados using Keep":tutorial-keep.html).
<notextile>
<pre><code>$ <span class="userinput">cat >the_job <<EOF
$ <span class="userinput">arv -h job get --uuid qr1hi-xxxxx-xxxxxxxxxxxxxxx</span>
{
...
- "output":"e2ccd204bca37c77c0ba59fc470cd0f7+162+K@qr1hi",
+ "output":"e2ccd204bca37c77c0ba59fc470cd0f7+162",
...
}
</code></pre>
Because the job ran in parallel, each instance of parallel-hash creates a separate @md5sum.txt@ as output. Arvados automatically collates theses files into a single collection, which is the output of the job:
<notextile>
-<pre><code>$ <span class="userinput">arv keep get e2ccd204bca37c77c0ba59fc470cd0f7+162+K@qr1hi</span>
+<pre><code>$ <span class="userinput">arv keep get e2ccd204bca37c77c0ba59fc470cd0f7+162</span>
md5sum.txt
md5sum.txt
md5sum.txt
-$ <span class="userinput">arv keep get e2ccd204bca37c77c0ba59fc470cd0f7+162+K@qr1hi/md5sum.txt</span>
+$ <span class="userinput">arv keep get e2ccd204bca37c77c0ba59fc470cd0f7+162/md5sum.txt</span>
0f1d6bcf55c34bed7f92a805d2d89bbf alice.txt
504938460ef369cd275e4ef58994cffe bob.txt
8f3b36aff310e06f3c5b9e95678ff77a carol.txt