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From "Ari Rabkin (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3412) Refactor the scheduler out of the JobTracker
Date Fri, 23 May 2008 16:15:56 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3412?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12599416#action_12599416
] 

Ari Rabkin commented on HADOOP-3412:
------------------------------------

I agree that the scheduler is currently excessively difficult to modify and reason about.
 Something like this could be very useful in scheduler development.  I have a couple thought
about the details your patch.

Why is the scheduler responsible for managing the task tracker statuses?  Shouldn't that stay
in JobTracker?  Though you'd still need updateTaskTrackerStatus() in the JobScheduler so that
the scheduler can do something smart if there's been a change in status.  

It might be wise to add an initialize() to the JobScheduler interface so that JobSchedulers
can be written using only the default constructor.  This would make it easier to push the
choice of scheduler into a config file; you'd just list a class name and the system could
use reflection to load the scheduler and start it.  

For which structures do you intend getLockOnJobs() to return a lock?  Saying that the scheduler
"won't move anything" is a little open-ended.  I don't see why the scheduler would ever need
to write to externally-visible structures.



> Refactor the scheduler out of the JobTracker
> --------------------------------------------
>
>                 Key: HADOOP-3412
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3412
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: mapred
>            Reporter: Brice Arnould
>            Assignee: Brice Arnould
>            Priority: Minor
>         Attachments: JobScheduler.patch, JobScheduler_v2.patch, JobScheduler_v3.patch,
JobScheduler_v3b.patch, JobScheduler_v4.patch, RackAwareJobScheduler.java
>
>
> First I would like warn you that my proposition is assumed to be very naive. I just hope
that reading it won't make you lose time.
> h4. The aim
> It seems to me that improving Hadoop scheduling could be very profitable. But, it is
hard to implement and compare schedulers, because the scheduling logic is mixed within the
rest of the JobTracker.
> This bug is the first step of an attempt to improve the Hadoop scheduler. It re-implements
the current scheduling algorithm in a separate class called JobScheduler. This new class is
instantiated in the JobTracker.
> h4. Bug fixed as a side effects
> This patch probably cannot be submited as it is.
> A first difficulty is that it does not have exactly the same behaviour than the current
JobTracker. More precisely, it doesn't re-implement things like code that seems to be never
called or concurency problems.
> I wrote TOCONFIRM where my proposition differ from the current implementation, so you
can find them easily.
> I know that fixing bugs silently is bad. So, independently of what you decide about this
patch, I will open issues for bugs that you confirm.
> h4. Other side effects
> Another side effect of this patch is to add documentation about each step of the scheduling.
I hope that it will help future improvement by lowering the level required to contribute to
the scheduler.
> It also reduces the complexity and the granularity of the JobTracker (making it more
parallel).
> h4. The future
> If you feel that this is a step the right direction, I will try to propose a JobSchedulerInterface
that many JobSchedulers could implement and to propose alternatives to the current « FifoJobScheduler
».  If some of you have ideas about that please tell ^^ I will also open issues for things
marked as FIXME in the patch.

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