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From "Brice Arnould (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3412) Refactor the scheduler out of the JobTracker
Date Fri, 23 May 2008 16:59:55 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3412?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12599429#action_12599429

Brice Arnould commented on HADOOP-3412:

bq. 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.
You're right. My aim was to move inside the JobScheduler the two structures we might want
to reorder in order to select tasks in a more efficient way. I told myself that it would be
a waste to keep two copies of the same collections (the unordered in the JobTracker, the ordered
in the JobScheduler). But on the other hand, it can make the code more verbose on simple algorithms.
So I propose to write an AbstractJobScheduler with default implementation for {remove,update}TaskTracker
(and maybe also jobs, if it seems reasonable). This way, the schedulers who just need to change
addJob, removeJob and assignTask will be able to reuse a common basis.

bq. 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.
The latest version of the patch (v4) already use reflection to load a scheduler according
to the configuration file. I'm really unsure about the name I choose for the option however
I didn't used Utils.Reflection because it seems to be restricted by design to objects with
a single argument of type Configuration. Moreover, I like better constructor that return objects
"ready for use" :-P. But if you feel that it doesn't respect the Hadoop code style, I can
change that, it's not a problem.

bq. 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.
This description really needs some clarification. getLocksOnJobs() give you the insurance
that every operation on internally stored jobs, namely addJob, removeJob and assignTask, will
wait for you to release the lock before to alter the list. It's intended use is that a user
of this class intending to alter a Job do it in that way :
synchronize (scheduler.getLocksOnJobs()) {
  scheduler.remove (job);
  [Modify job]
I told myself that in this way (add() then remove()), the asymptotic complexity with most
containers would be lesser than it would be if I provided a notifyJobModification() which
ressorted everything (my original intention).

Thanks for your comments, I'm going to post soon a new version of this patch that will take
them in account. Please tell me if you see other errors in that one.

PS: Please excuse me for my English. If I said something misplaced or impolite, it is not
by intention. 

> 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
> 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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