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

["Hadoop QA (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/MAPREDUCE-2094?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14525562#comment-14525562 ] 

Hadoop QA commented on MAPREDUCE-2094:
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|| Vote || Subsystem || Runtime || Comment ||
| {color:blue}0{color} | pre-patch |  14m 31s | Pre-patch trunk compilation is healthy. |
| {color:green}+1{color} | @author |   0m  0s | The patch does not contain any @author tags. |
| {color:green}+1{color} | tests included |   0m  0s | The patch appears to include 3 new or modified test files. |
| {color:green}+1{color} | javac |   7m 27s | There were no new javac warning messages. |
| {color:green}+1{color} | javadoc |   9m 37s | There were no new javadoc warning messages. |
| {color:green}+1{color} | release audit |   0m 22s | The applied patch does not increase the total number of release audit warnings. |
| {color:green}+1{color} | checkstyle |   1m  3s | There were no new checkstyle issues. |
| {color:red}-1{color} | whitespace |   0m  1s | The patch has 9  line(s) that end in whitespace. Use git apply --whitespace=fix. |
| {color:green}+1{color} | install |   1m 35s | mvn install still works. |
| {color:green}+1{color} | eclipse:eclipse |   0m 32s | The patch built with eclipse:eclipse. |
| {color:green}+1{color} | findbugs |   1m 15s | The patch does not introduce any new Findbugs (version 2.0.3) warnings. |
| {color:green}+1{color} | mapreduce tests |   1m 34s | Tests passed in hadoop-mapreduce-client-core. |
| | |  38m  0s | |
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|| Subsystem || Report/Notes ||
| Patch URL | http://issues.apache.org/jira/secure/attachment/12658039/MAPREDUCE-2094-20140727-svn.patch |
| Optional Tests | javadoc javac unit findbugs checkstyle |
| git revision | trunk / 6ae2a0d |
| whitespace | https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/5609/artifact/patchprocess/whitespace.txt |
| hadoop-mapreduce-client-core test log | https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/5609/artifact/patchprocess/testrun_hadoop-mapreduce-client-core.txt |
| Test Results | https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/5609/testReport/ |
| Java | 1.7.0_55 |
| uname | Linux asf906.gq1.ygridcore.net 3.13.0-36-lowlatency #63-Ubuntu SMP PREEMPT Wed Sep 3 21:56:12 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux |
| Console output | https://builds.apache.org/job/PreCommit-MAPREDUCE-Build/5609/console |


This message was automatically generated.

> 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-2094-20140727-svn.patch, MAPREDUCE-2094-20140727.patch, MAPREDUCE-2094-FileInputFormat-docs-v2.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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