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https://issues.apache.org/jira/browse/MAPREDUCE-2094?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12918851#action_12918851
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Niels Basjes commented on MAPREDUCE-2094:
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I just noticed that the Yahoo Hadoop tutorial "[Module 5: Advanced MapReduce
Features |http://developer.yahoo.com/hadoop/tutorial/module5.html]"; shows a
code example for defining your own
[FileInputFormat|http://developer.yahoo.com/hadoop/tutorial/module5.html#fileformat].
The shown example code implements a derivative using FileInputFormat and
LineRecordReader without overruling isSplittable ... I expect this tutorial
code to lead people into this bug.
Since this bug will only become apparent when using large "non splittable"
(gzipped) input files it is also important to notice that almost no one will
have a (unit) test that will trip on this bug.
> 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
> Affects Versions: 0.20.1, 0.20.2, 0.21.0
> Reporter: Niels Basjes
>
> 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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