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https://issues.apache.org/jira/browse/HADOOP-3412?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12604906#action_12604906
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Tom White commented on HADOOP-3412:
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This looks good. A few comments:
1. You can change AbstractJobScheduler and FifoJobScheduler's constructors to
be no-arg constructors. The JobTracker isn't being used, and if you make
AbstractJobScheduler implement Configurable, the the Configuration will be
automatically called if you use ReflectionUtils#newInstance (this is the
standard Hadoop pattern).
2. This needs some unit tests. It should be possible to write a unit test for
FifoJobScheduler since it is relatively isolated.
3. In AbstractJobScheduler#assignTask you move to the next step if the
following condition is true:
{code}
(step == 0 || step == 2) && mapTasksNumber > maximumMapLoad
|| (step == 1 || step == 3) && reduceTasksNumber > maximumReduceLoad
{code}
Should these checks be >= (greater than or equal to) so the maximum load isn't
exceeded?
4. Rather than having AbstractJobScheduler manage maxTasksPerJob, it might be
better to have a subclass of FifoJobScheduler that limits tasks per job
(TaskLimitedFifoJobScheduler). Or if this is possible through composition
rather than inheritance then even better. Then FifoJobScheduler would then just
preserve the current behaviour.
> 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.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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