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From "eric baldeschwieler (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-362) tasks can get lost when reporting task completion to the JobTracker has an error
Date Tue, 25 Jul 2006 02:48:15 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-362?page=comments#action_12423221 ] 
            
eric baldeschwieler commented on HADOOP-362:
--------------------------------------------

+1 to the second idea (unique hashcode / client thread, forcing serialization the server via
mapping it to the same thread ID)


> tasks can get lost when reporting task completion to the JobTracker has an error
> --------------------------------------------------------------------------------
>
>                 Key: HADOOP-362
>                 URL: http://issues.apache.org/jira/browse/HADOOP-362
>             Project: Hadoop
>          Issue Type: Bug
>          Components: mapred
>            Reporter: Devaraj Das
>         Assigned To: Owen O'Malley
>         Attachments: lost-status-updates.patch
>
>
> Basically, the JobTracker used to lose some updates about successful map tasks and it
would assume that the tasks are still running (the old progress report is what it used to
display in the web page). Now this would cause the reduces to also wait for the map output
and they would never receive the output. This would cause the job to appear as if it was hung.
>  
> The following piece of code sends the status of tasks to the JobTracker:
>  
>             synchronized (this) {
>                 for (Iterator it = runningTasks.values().iterator();
>                      it.hasNext(); ) {
>                     TaskInProgress tip = (TaskInProgress) it.next();
>                     TaskStatus status = tip.createStatus();
>                     taskReports.add(status);
>                     if (status.getRunState() != TaskStatus.RUNNING) {
>                         if (tip.getTask().isMapTask()) {
>                             mapTotal--;
>                         } else {
>                             reduceTotal--;
>                         }
>                         it.remove();
>                     }
>                 }
>             }
>  
>             //
>             // Xmit the heartbeat
>             //
>            
>             TaskTrackerStatus status =
>               new TaskTrackerStatus(taskTrackerName, localHostname,
>                                     httpPort, taskReports,
>                                     failures);
>             int resultCode = jobClient.emitHeartbeat(status, justStarted);
>  
>  
> Notice that the completed TIPs are removed from runningTasks data structure. Now, if
the emitHeartBeat threw an exception (if it could not communicate with the JobTracker till
the IPC timeout expires) then this update is lost. And the next time it sends the hearbeat
this completed task's status is missing and hence the JobTracker doesn't know about this completed
task. So, one solution to this is to remove the completed TIPs from runningTasks after emitHeartbeat
returns. Here is how the new code would look like:
>  
>  
>             synchronized (this) {
>                 for (Iterator it = runningTasks.values().iterator();
>                      it.hasNext(); ) {
>                     TaskInProgress tip = (TaskInProgress) it.next();
>                     TaskStatus status = tip.createStatus();
>                     taskReports.add(status);
>                 }
>             }
>  
>             //
>             // Xmit the heartbeat
>             //
>  
>             TaskTrackerStatus status =
>               new TaskTrackerStatus(taskTrackerName, localHostname,
>                                     httpPort, taskReports,
>                                     failures);
>             int resultCode = jobClient.emitHeartbeat(status, justStarted);
>             synchronized (this) {
>                 for (Iterator it = runningTasks.values().iterator();
>                      it.hasNext(); ) {
>                     TaskInProgress tip = (TaskInProgress) it.next();
>                     if (tip.runstate != TaskStatus.RUNNING) {
>                         if (tip.getTask().isMapTask()) {
>                             mapTotal--;
>                         } else {
>                             reduceTotal--;
>                         }
>                         it.remove();
>                     }
>                 }
>             }
>  

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