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From "Matei Zaharia (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3412) Refactor the scheduler out of the JobTracker
Date Mon, 30 Jun 2008 20:50:47 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3412?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12609364#action_12609364
] 

Matei Zaharia commented on HADOOP-3412:
---------------------------------------

While the idea of splitting up the TaskScheduler and JobQueue is  
great, isn't having a getJobQueue() in TaskScheduler needlessly  
complicating the interface? Now, when somebody wants to write a new  
scheduling algorithm, they may have to write two classes, a  
TaskScheduler and a JobQueue, and figure out how they'll interact. I'd  
suggest having the add(JobInProgress) and remove(JobInProgress)  
methods directly in the TaskScheduler, and removing getJobQueue(), so  
that the JobTracker only calls methods of the TaskScheduler object.  
The DefaultTaskScheduler can include a JobQueue that it passes the add/ 
remove methods to, but people writing new schedulers should have the  
option to do something different.

Here are two concrete cases where this would be useful:
- Some TaskSchedulers, even if they use a JobQueue, might want to run  
code in response to the add() and remove() methods (updating data  
structures, deciding to preempt existing jobs because a new one, etc).  
Right now this is not possible unless the JobQueue tells them it was  
updated.
- Some schedulers might not logically have a job queue. For example,  
we might imagine a scheduler that sorts tasks, not jobs, based on  
locality and on whether the task is speculative.

Matei





> 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
>             Fix For: 0.19.0
>
>         Attachments: JobScheduler.patch, JobScheduler_v2.patch, JobScheduler_v3.patch,
JobScheduler_v3b.patch, JobScheduler_v4.patch, JobScheduler_v5.patch, JobScheduler_v6.1.patch,
JobScheduler_v6.2.patch, JobScheduler_v6.3.patch, JobScheduler_v6.4.patch, JobScheduler_v6.patch,
JobScheduler_v7.1.patch, JobScheduler_v7.patch, RackAwareJobScheduler.java, SimpleResourceAwareJobScheduler.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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