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Re: Selecting a task for the tasktracker

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 (
and you may need to look at different interfaces for building your own

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.

On Thu, Dec 27, 2012 at 4:10 PM, Yaron Gonen <[EMAIL PROTECTED]> 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:
>    1. Selecting a job.
>    2. 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