hadoop-common-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "paul sutter (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-277) Race condition in Configuration.getLocalPath()
Date Tue, 06 Jun 2006 01:12:29 GMT
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

(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

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
060605 142418 task_0001_r_000009_1 done copying task_0001_m_000015_0 output from rmr04 into
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

This message is automatically generated by JIRA.
If you think it was sent incorrectly contact one of the administrators:
For more information on JIRA, see:

View raw message