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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Resolved: (HADOOP-7) MapReduce has a series of problems concerning task-allocation to worker nodes
Date Mon, 06 Feb 2006 18:39:59 GMT
     [ http://issues.apache.org/jira/browse/HADOOP-7?page=all ]
Doug Cutting resolved HADOOP-7:

    Resolution: Fixed

Mike committed this last week.

> MapReduce has a series of problems concerning task-allocation to worker nodes
> -----------------------------------------------------------------------------
>          Key: HADOOP-7
>          URL: http://issues.apache.org/jira/browse/HADOOP-7
>      Project: Hadoop
>         Type: Bug
>  Environment: All
>     Reporter: Mike Cafarella
>  Attachments: jobtracker.patch
> The MapReduce JobTracker is not great at allocating tasks to TaskTracker worker nodes.
> Here are the problems:
> 1) There is no speculative execution of tasks
> 2) Reduce tasks must wait until all map tasks are completed before doing any work
> 3) TaskTrackers don't distinguish between Map and Reduce jobs.  Also, the number of
> tasks at a single node is limited to some constant.  That means you can get weird deadlock
> problems upon machine failure.  The reduces take up all the available execution slots,
but they
> don't do productive work, because they're waiting for a map task to complete.  Of course,
> map task won't even be started until the reduce tasks finish, so you can see the problem...
> 4) The JobTracker is so complicated that it's hard to fix any of these.
> The right solution is a rewrite of the JobTracker to be a lot more flexible in task handling.
> It has to be a lot simpler.  One way to make it simpler is to add an abstraction I'll
> "TaskInProgress".  Jobs are broken into chunks called TasksInProgress.  All the TaskInProgress
> objects must be complete, somehow, before the Job is complete.
> A single TaskInProgress can be executed by one or more Tasks.  TaskTrackers are assigned
> If a Task fails, we report it back to the JobTracker, where the TaskInProgress lives.
 The TIP can then
> decide whether to launch additional  Tasks or not.
> Speculative execution is handled within the TIP.  It simply launches multiple Tasks in
parallel.  The
> TaskTrackers have no idea that these Tasks are actually doing the same chunk of work.
 The TIP
> is complete when any one of its Tasks are complete.

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