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From "Niels Basjes (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (MAPREDUCE-2094) org.apache.hadoop.mapreduce.lib.input.FileInputFormat: isSplitable implements unsafe default behaviour that is different from the documented behaviour.
Date Sun, 27 Jul 2014 20:20:39 GMT

     [ https://issues.apache.org/jira/browse/MAPREDUCE-2094?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Niels Basjes updated MAPREDUCE-2094:
------------------------------------

    Attachment: MAPREDUCE-2094-20140727-svn.patch

Created new patch file via the svn route.
In this the binary file is now an ASCII file. This is possible because the content of the
file is irrelevant, only the filename and the size (> 3 bytes) are what matters.

> org.apache.hadoop.mapreduce.lib.input.FileInputFormat: isSplitable implements unsafe
default behaviour that is different from the documented behaviour.
> -------------------------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: MAPREDUCE-2094
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-2094
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: task
>            Reporter: Niels Basjes
>            Assignee: Niels Basjes
>         Attachments: MAPREDUCE-2094-2011-05-19.patch, MAPREDUCE-2094-20140727-svn.patch,
MAPREDUCE-2094-20140727.patch, MAPREDUCE-2094-FileInputFormat-docs-v2.patch
>
>
> When implementing a custom derivative of FileInputFormat we ran into the effect that
a large Gzipped input file would be processed several times. 
> A near 1GiB file would be processed around 36 times in its entirety. Thus producing garbage
results and taking up a lot more CPU time than needed.
> It took a while to figure out and what we found is that the default implementation of
the isSplittable method in [org.apache.hadoop.mapreduce.lib.input.FileInputFormat | http://svn.apache.org/viewvc/hadoop/mapreduce/trunk/src/java/org/apache/hadoop/mapreduce/lib/input/FileInputFormat.java?view=markup
] is simply "return true;". 
> This is a very unsafe default and is in contradiction with the JavaDoc of the method
which states: "Is the given filename splitable? Usually, true, but if the file is stream compressed,
it will not be. " . The actual implementation effectively does "Is the given filename splitable?
Always true, even if the file is stream compressed using an unsplittable compression codec.
"
> For our situation (where we always have Gzipped input) we took the easy way out and simply
implemented an isSplittable in our class that does "return false; "
> Now there are essentially 3 ways I can think of for fixing this (in order of what I would
find preferable):
> # Implement something that looks at the used compression of the file (i.e. do migrate
the implementation from TextInputFormat to FileInputFormat). This would make the method do
what the JavaDoc describes.
> # "Force" developers to think about it and make this method abstract.
> # Use a "safe" default (i.e. return false)



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