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From "Karthik Kambatla (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-1144) JT should not hold lock while writing user history logs to DFS
Date Thu, 11 Oct 2012 20:29:03 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-1144?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13474484#comment-13474484
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Karthik Kambatla commented on MAPREDUCE-1144:
---------------------------------------------

In this particular case of moving history related files, the simplest approach seems to be
to modify the behavior in job-history management. For instance, {{#moveToDone()}} can be executed
asynchronously using a thread-pool; in fact, the method notifies JT of the completion when
it is done moving files.

Should more such situations arise, we can always use a thread-pool in JT for calls to external
entities - DFS in this case, unless it needs to be blocking for correctness. We can cross
that bridge when we get there. 
                
> JT should not hold lock while writing user history logs to DFS
> --------------------------------------------------------------
>
>                 Key: MAPREDUCE-1144
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1144
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: jobtracker
>    Affects Versions: 0.20.1
>            Reporter: Todd Lipcon
>
> I've seen behavior a few times now where the DFS is being slow for one reason or another,
and the JT essentially locks up waiting on it while one thread tries for a long time to write
history files out. The stack trace blocking everything is:
> Thread 210 (IPC Server handler 10 on 7277):
>   State: WAITING
>   Blocked count: 171424
>   Waited count: 1209604
>   Waiting on java.util.LinkedList@407dd154
>   Stack:
>     java.lang.Object.wait(Native Method)
>     java.lang.Object.wait(Object.java:485)
>     org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.flushInternal(DFSClient.java:3122)
>     org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.closeInternal(DFSClient.java:3202)
>     org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.close(DFSClient.java:3151)
>     org.apache.hadoop.fs.FSDataOutputStream$PositionCache.close(FSDataOutputStream.java:67)
>     org.apache.hadoop.fs.FSDataOutputStream.close(FSDataOutputStream.java:106)
>     sun.nio.cs.StreamEncoder.implClose(StreamEncoder.java:301)
>     sun.nio.cs.StreamEncoder.close(StreamEncoder.java:130)
>     java.io.OutputStreamWriter.close(OutputStreamWriter.java:216)
>     java.io.BufferedWriter.close(BufferedWriter.java:248)
>     java.io.PrintWriter.close(PrintWriter.java:295)
>     org.apache.hadoop.mapred.JobHistory$JobInfo.logFinished(JobHistory.java:1349)
>     org.apache.hadoop.mapred.JobInProgress.jobComplete(JobInProgress.java:2167)
>     org.apache.hadoop.mapred.JobInProgress.completedTask(JobInProgress.java:2111)
>     org.apache.hadoop.mapred.JobInProgress.updateTaskStatus(JobInProgress.java:873)
>     org.apache.hadoop.mapred.JobTracker.updateTaskStatuses(JobTracker.java:3598)
>     org.apache.hadoop.mapred.JobTracker.processHeartbeat(JobTracker.java:2792)
>     org.apache.hadoop.mapred.JobTracker.heartbeat(JobTracker.java:2581)
>     sun.reflect.GeneratedMethodAccessor14.invoke(Unknown Source)
> We should try not to do external IO while holding the JT lock, and instead write the
data to an in-memory buffer, drop the lock, and then write.

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