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From "Owen O'Malley (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-76) Implement speculative re-execution of reduces
Date Wed, 15 Nov 2006 23:25:40 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-76?page=comments#action_12450232 ] 
Owen O'Malley commented on HADOOP-76:

The propsed PhasedFileSystem needs to support mkdirs in order to work with MapFileOutputFormat.

It will break less user OutputFormats if we also support the read operations:
by passing the request to the underlying FileSystem.

It is confusing to use the TaskInProgress.hasSucceededTask for reduces and TaskInProgress.completes
for maps. I think it would be better to use the completes for both.

Thanks for putting the generic types into activeTasks, but it should look like:

Map<String, String> activeTasks = new HashMap();

The declared type should use the interface and the constructor doesn't need the generic types.

The logic in TaskInProgress.isRunnable is pretty convoluted although I think if you use completes
for reduces, it doesn't need to change.

TaskInProgress.hasRanOnMachine should be hasRunOnMachine.

findNewTask should add a new condition instead of continue:

               } else if (specTarget == -1 &&
                          task.hasSpeculativeTask(avgProgress)) {
+                if(task.hasRanOnMachine(taskTracker)){
+                  continue ;
+                }
                 specTarget = i;

should be:

               } else if (specTarget == -1 &&
                              task.hasSpeculativeTask(avgProgress) &&
                              !task.hasRanOnMachine(taskTracker)) {
                 specTarget = i;

The patch always creates a PhasedFileSystem even when it won't be used because there is no
speculative execution.

@@ -298,7 +305,14 @@

     } finally {
-      out.close(reporter);
+      if( runSpeculative ){
+        out.close(reporter);
+        pfs.commit();
+        pfs.close();
+       }else{
+        out.close(reporter);
+        fs.close();
+      }

"out.close(reporter);" should be lifted out of the branch. And it is usually better to not
close the file system because they are cached and may be used in another context. So I'd drop
the else clause all together.

> Implement speculative re-execution of reduces
> ---------------------------------------------
>                 Key: HADOOP-76
>                 URL: http://issues.apache.org/jira/browse/HADOOP-76
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: mapred
>    Affects Versions: 0.1.0
>            Reporter: Doug Cutting
>         Assigned To: Sanjay Dahiya
>            Priority: Minor
>         Attachments: Hadoop-76.patch, Hadoop-76_1.patch, Hadoop-76_2.patch, Hadoop-76_3.patch,
Hadoop-76_4.patch, Hadoop-76_5.patch, spec_reducev.patch
> As a first step, reduce task outputs should go to temporary files which are renamed when
the task completes.

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