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: Improvement
Components: task
Affects Versions: 0.21.0, 0.20.2, 0.20.1
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.
