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From "paul sutter (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-277) Race condition in Configuration.getLocalPath()
Date Tue, 06 Jun 2006 17:32:31 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-277?page=comments#action_12414996 ] 

paul sutter commented on HADOOP-277:
------------------------------------


Sameer -> I think you're right. Ultimately, the synchronize is not good enough, and it
seems that you've got the right solution.

Owen ->  here's the case Naveen described: lets say five directory levels are created by
the call to mkdirs(), and there are two threads entering mkrdirs() at about the same time.
Will one of the two threads exit before the fifth directory is created? I dont have the source
code, but it seems to me that it could. If that would happen, that thread will step ahead
to the next directory because when it does the exists() check, the directory will not exist.
Which leads to Sameer's point that even the synchronize is not good enough.





> Race condition in Configuration.getLocalPath()
> ----------------------------------------------
>
>          Key: HADOOP-277
>          URL: http://issues.apache.org/jira/browse/HADOOP-277
>      Project: Hadoop
>         Type: Bug

>  Environment: linux, 64 bit, dual core, 4x400GB disk, 4GB RAM
>     Reporter: paul sutter
>  Attachments: hadoop-277.patch, hadoop-task_1_r_9.log, mkdirs.patch
>
> (attached: a patch to fix the problem, and a logfile showing the problem occuring twice)
> There is a race condition in Configuration.java:
>        Path file = new Path(dirs[index], path);
>        Path dir = file.getParent();
>        if (fs.exists(dir) || fs.mkdirs(dir)) {
>          return file;
> If two threads simultaneously process this code with the same target directory, fs.exists()
will return false, but from fs.mkdirs() only one of the two threads will return true. From
the Java documentation:
>  "returns: true if and only if the directory was created, along with all necessary parent
directories; false otherwise"
> That is, if the first thread successfully creates the directory, the second will not,
and therefore return false, even though the directory exists.
> This was really happening. We use four temporary directories, and we had reducers failing
all over the place with  bizarre impossible errors. I modified the ReduceTaskRunner to output
the filename that it creates to find the problem, and the log output is below.
> Here you can see copies initiated for two files that hash to the same temp directory,
simultaneously. map_4.out is created in the correct directory (/data2...), but map_15.out
is created in the next directory (/data3...) becuase of this race condition. Minutes later,
when the appender tries to locate the file, that race condition does not occur (the directory
already exists), and the appender looks for the file map_15.out in the correct directory,
where it does not exist.
> 060605 142414 task_0001_r_000009_1 Copying task_0001_m_000004_0 output from rmr05.
> 060605 142414 task_0001_r_000009_1 Copying task_0001_m_000015_0 output from rmr04.
> ...
> 060605 142416 task_0001_r_000009_1 done copying task_0001_m_000004_0 output from rmr05
into /data2/tmp/mapred/local/task_0001_r_000009_1/map_4.out
> ...
> 060605 142418 task_0001_r_000009_1 done copying task_0001_m_000015_0 output from rmr04
into /data3/tmp/mapred/local/task_0001_r_000009_1/map_15.out
> ...
> 060605 142531 task_0001_r_000009_1 0.31808624% reduce > append > /data2/tmp/mapred/local/task_0001_r_000009_1/map_4.out
> ...
> 060605 142725 task_0001_r_000009_1 java.io.FileNotFoundException: /data2/tmp/mapred/local/task_0001_r_000009_1/map_15.out

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