[jira] [Updated] (MAPREDUCE-2094) org.apache.hadoop.mapreduce.lib.input.FileInputFormat: isSplitable implements unsafe default behaviour that is different from the documented behaviour.

["Gian Merlino (JIRA)" <ji...@apache.org>]

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

Gian Merlino updated MAPREDUCE-2094:
------------------------------------

    Attachment: MAPREDUCE-2094-FileInputFormat-docs.patch

We just hit this bug too. I'd really prefer a safer default (like "return false" or something like Niels's patch) but if that is not doable, better docs would have helped. The current docs imply that the default implementation handles splittable vs non-splittable files, even though it doesn't.

I've attached some wording that I think is more clear.

> 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-FileInputFormat-docs.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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