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From "Ben Roling (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CRUNCH-515) Decrease probability of collision on Crunch temp directories
Date Thu, 30 Apr 2015 22:10:07 GMT

    [ https://issues.apache.org/jira/browse/CRUNCH-515?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14522369#comment-14522369
] 

Ben Roling commented on CRUNCH-515:
-----------------------------------

{quote}
I don't see any obvious way for me to trace back from the stray directories to the jobs that
created them to be able to do analytics to identify which jobs are leaving behind the most
stray directories.
{quote}

A teammate mentioned the relatively obvious fact that the tmp paths are encoded into the job
names of the jobs that use them based on the way that Crunch includes plan information in
the job name.  As such, I can join the stray directory list against the job names to try to
analyze which types of jobs are leaving the most stray directories.  This doesn't work perfectly
for reasons such as limits on job name length but it gets me some good information.  I just
started looking at a set of such data from stray temp directories left yesterday in our dev
environment and all of the jobs I could match to were successful jobs.  I can also get a good
idea of which development teams are responsible for these jobs to follow up on what they might
be doing (or not doing) to cause the strays much more frequently than other jobs.

> Decrease probability of collision on Crunch temp directories
> ------------------------------------------------------------
>
>                 Key: CRUNCH-515
>                 URL: https://issues.apache.org/jira/browse/CRUNCH-515
>             Project: Crunch
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 0.8.4, 0.11.0
>            Reporter: Ben Roling
>            Assignee: Josh Wills
>         Attachments: CRUNCH-515-1.patch
>
>
> I've heard reports of failures of Crunch pipelines at our organization due to collision
on temp directories.
> Take the following stack trace from an old internal email thread I dug up as an example:
> {noformat}
> 2015-04-02 04:45:49,208 INFO org.apache.crunch.hadoop.mapreduce.lib.jobcontrol.CrunchControlledJob:
org.apache.hadoop.mapred.FileAlreadyExistsException: Output directory /tmp/crunch-686245394/p2/output
already exists
>     at org.apache.hadoop.mapreduce.lib.output.FileOutputFormat.checkOutputSpecs(FileOutputFormat.java:132)
>     at org.apache.hadoop.mapred.JobClient$2.run(JobClient.java:1013)
>     at org.apache.hadoop.mapred.JobClient$2.run(JobClient.java:974)
>     at java.security.AccessController.doPrivileged(Native Method)
>     at javax.security.auth.Subject.doAs(Subject.java:394)
>     at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1438)
>     at org.apache.hadoop.mapred.JobClient.submitJobInternal(JobClient.java:974)
>     at org.apache.hadoop.mapreduce.Job.submit(Job.java:582)
>     at org.apache.crunch.hadoop.mapreduce.lib.jobcontrol.CrunchControlledJob.submit(CrunchControlledJob.java:340)
>     at org.apache.crunch.hadoop.mapreduce.lib.jobcontrol.CrunchJobControl.startReadyJobs(CrunchJobControl.java:277)
>     at org.apache.crunch.hadoop.mapreduce.lib.jobcontrol.CrunchJobControl.pollJobStatusAndStartNewOnes(CrunchJobControl.java:316)
>     at org.apache.crunch.impl.mr.exec.MRExecutor.monitorLoop(MRExecutor.java:113)
>     at org.apache.crunch.impl.mr.exec.MRExecutor.access$000(MRExecutor.java:55)
>     at org.apache.crunch.impl.mr.exec.MRExecutor$1.run(MRExecutor.java:84)
>     at java.lang.Thread.run(Thread.java:682)
> {noformat}
> What we found in this case is the pre-existing directory was rather old.  It hung around
because we're doing a poor job of cleaning old garbage out of our HDFS /tmp directory.  We
intend to set up a job to delete stuff older than a couple of weeks or so out of /tmp but
I think the chances of a collision will still be high enough that failures like this might
still happen on occasion.
> The temp directory Crunch chooses is a random 31-bit value:
> https://github.com/apache/crunch/blob/apache-crunch-0.11.0/crunch-core/src/main/java/org/apache/crunch/impl/dist/DistributedPipeline.java#L326
> I say 31 bit value because it comes from a 32-bit random integer but only includes positive
values, thereby excluding 1 bit.
> The following blog post shows some probabilities for 32-bit hash collisions, which are
essentially the same problem:
> http://preshing.com/20110504/hash-collision-probabilities/
> Since we're dealing with 31 bits instead of 32 the probabilities will be higher than
expressed there for 32 bits.  Even with 32 bits the probability of collision is 1 in 100 with
just 9292 values.
> I have not done any thorough investigation to understand why, but in our production environment
we have a lot of Crunch jobs and we are leaving 200-300 stray Crunch temp directories per
day.  Depending on how aggressive we get with a scheduled job to clean old stuff out of temp
we could still have a realistic chance of hitting a collision.
> My proposal is to change the random integer component of the temp path to a UUID or something
similar to make it drastically more unlikely that a collision will ever occur regardless of
whether or not "/tmp" is ever cleaned up.



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