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From "Brice Arnould (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-3412) Refactor the scheduler out of the JobTracker
Date Mon, 30 Jun 2008 13:21:45 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-3412?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Brice Arnould updated HADOOP-3412:
----------------------------------

    Attachment: JobScheduler_v7.1.patch

*Tom White*
bq. Unless we're absolutely sure that we've got the interface right, I think JobQueue and
TaskScheduler should be abstract classes. See https://issues.apache.org/jira/browse/HADOOP-1230?focusedCommentId=12573958#action_12573958
You mean that interfaces are easier to make evolve because they can provide default implementations
to the methods we will add ?
I didn't thought of that. I made the change.

bq. We don't need iterator() and getSortedJobs() - iterator() is sufficient.
getSortedJobs() allows to bias the choice of the job by the characteristics of the TaskTracker,
something that appeared to be useful when I played with the API. This new proposition however
provides a default implementation for it.

bq. What did you think of the idea of having JobLimitedTaskScheduler?
Not a problem to add it, but I see two way of doing so without duplicating most of assignTask()
:
 * By composition, adding a JobFilter subclass with two methods : isAcceptable (job, step)
and getNumberOfSteps(). The first would tell if a job is right for the step we're in and the
second the number of steps we need.
 * By inheritance, providing isAcceptable and getNumberOfSteps as methods of the DefaultTaskScheduler.

Both are easy to implement but that new level of abstraction seems contrary to the KISS principle,
except if we really need it for other filters. For now, when limits are disabled, the TaskScheduler
just do one more test line 104, and one other line 124 when limits are enabled. That might
not justify the creation of another class and a filter concept (again : excepted if we need
them for something else).

I fixed the warnings. Thanks for your advices. It's instructive ^^

> 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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