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From Vinod Kumar Vavilapalli <vino...@hortonworks.com>
Subject Re: Selecting a task for the tasktracker
Date Thu, 27 Dec 2012 18:11:14 GMT

On top of that, the message indicates that you need to have your scheduler class in the mapred

+Vinod Kumar Vavilapalli
Hortonworks Inc.

On Dec 27, 2012, at 7:38 AM, Hemanth Yamijala wrote:

> Hi,
> Firstly, I am talking about Hadoop 1.0. Please note that in Hadoop 2.x and trunk, the
Mapreduce framework is completely revamped to Yarn (http://hadoop.apache.org/docs/current/hadoop-yarn/hadoop-yarn-site/YARN.html)
and you may need to look at different interfaces for building your own scheduler.
> In 1.0, the primary function of the TaskScheduler is the assignTasks method. Given a
TaskTracker object as input, this method figures out how many free map and reduce slots exist
in that particular tasktracker and selects one or more task that can be scheduled on it. Since
task selection is the primary responsibility and the granularity is at a task level, the class
is called TaskScheduler.
> The method of choosing a job and then a task within the job is customised by the different
schedulers already present in Hadoop. Also, the core logic of selecting a map task with data
locality optimizations is not implemented in the schedulers per se, but they rely on the JobInProgress
object in MapReduce framework for achieving the same.
> To implement your own Scheduler, it may be best to look at the sources of existing schedulers:
JobQueueTaskScheduler, CapacityTaskScheduler or FairScheduler.  In particular, the last two
are in the contrib modules of mapreduce, and hence will be fairly independent to follow. Their
build files will also tell you how to resolve any compile problems like the one you are facing.
> Thanks
> Hemanth  
> On Thu, Dec 27, 2012 at 4:10 PM, Yaron Gonen <yaron.gonen@gmail.com> wrote:
> Hi,
> If I understand correctly, the job scheduler (why is the class called TaskScheduler?)
is responsible for assigning the task whose split is as close as possible to the tasktacker.
> Meaning that the job scheduler is responsible to two things:
> Selecting a job.
> Once a job is selected, assign the closest task to the tasktracker that send the heartbeat.
> Is this correct?
> I want to write my own job scheduler to change the logic above, but it says The type
TaskScheduler is not visible.
> How can I write my own scheduler?
> thanks

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