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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-2094) org.apache.hadoop.mapreduce.lib.input.FileInputFormat: isSplitable implements unsafe default behaviour that is different from the documented behaviour.
Date Thu, 19 May 2011 19:37:47 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-2094?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13036399#comment-13036399
] 

Todd Lipcon commented on MAPREDUCE-2094:
----------------------------------------

I think this is still incompatible.

I think the majority of file-based input formats *should* be splittable - it's the whole reason
that MR scales to big files. It's the minority of cases where files shouldn't be splittable.

IMO the correct fix for this issue is to do a better job of documenting FileInputFormat.isSplitable()
and the class-level javadoc on FileInputFormat. This way, those who implement it themselves
will take note that they need to override it.

> 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
>
>
> 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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