hadoop-mapreduce-issues mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
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 Tue, 28 Sep 2010 09:45:34 GMT

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

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

    Issue Type: Bug  (was: Improvement)

> 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 contradcition 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. " 
> For our situation (where we always have Gzipped input) we took the easy way out and simply
implemented an isSplittable inour 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 (and therfor the entire FileInputFormat
class) abstract.
> # Use a "safe" default (i.e. return false)

-- 
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.


Mime
View raw message