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From "Bikas Saha (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-4607) Race condition in ReduceTask completion can result in Task being incorrectly failed
Date Thu, 06 Sep 2012 16:55:08 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-4607?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13449806#comment-13449806
] 

Bikas Saha commented on MAPREDUCE-4607:
---------------------------------------

I see what you mean. Thats what I had figured earlier from your comments. If we are going
to make these changes then I would say lets go the full way and make each test explicitly
create the mock task instead of doing it in the setup() where there is no context. My original
change was not good for readability but was the minimal local change. Your change is good
for readability but we are stopping midway by making future changers have to think on why
the reduce test has been special cased. So IMO lets make each test create mock tasks as appropriate
in context. Its going to be same wrt perf but a lot better to read. In a sense the original
tests were only using mappers and the current setup method made sense. Introducing reducer
tests invalidates that approach. What do you think?
                
> Race condition in ReduceTask completion can result in Task being incorrectly failed
> -----------------------------------------------------------------------------------
>
>                 Key: MAPREDUCE-4607
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-4607
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>    Affects Versions: 2.1.0-alpha
>            Reporter: Bikas Saha
>            Assignee: Bikas Saha
>         Attachments: MAPREDUCE-4607.1.patch, MAPREDUCE-4607.2.patch, MAPREDUCE-4607.3.patch,
MAPREDUCE-4607.patch
>
>
> Problem reported by chackaravarthy in MAPREDUCE-4252
> This problem has been handled when speculative task launched for map task and other attempt
got failed (not killed)
> Can the similar kind of scenario can happen in case of reduce task?
> Consider the following scenario for reduce task in case of speculation (one attempt got
killed):
> 1. A task attempt is started.
> 2. A speculative task attempt for the same task is started.
> 3. The first task attempt completes and causes the task to transition to SUCCEEDED.
> 4. Then speculative task attempt will be killed because of the completion of first attempt.
> As a result, internal error will be thrown from this attempt (TaskImpl.MapRetroactiveKilledTransition)
and hence task attempt failure leads to job failure.
> TaskImpl.MapRetroactiveKilledTransition
> if (!TaskType.MAP.equals(task.getType())) {
>         LOG.error("Unexpected event for REDUCE task " + event.getType());
>         task.internalError(event.getType());
>       }
> So, do we need to have following code in MapRetroactiveKilledTransition also just like
in MapRetroactiveFailureTransition.
> if (event instanceof TaskTAttemptEvent) {
>         TaskTAttemptEvent castEvent = (TaskTAttemptEvent) event;
>         if (task.getState() == TaskState.SUCCEEDED &&
>             !castEvent.getTaskAttemptID().equals(task.successfulAttempt)) {
>           // don't allow a different task attempt to override a previous
>           // succeeded state
>           return TaskState.SUCCEEDED;
>         }
>       }
> please check whether this is a valid case and give your suggestion.

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