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https://issues.apache.org/jira/browse/MAPREDUCE-2094?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Niels Basjes updated MAPREDUCE-2094:
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Description:
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)
was:
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)
> 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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