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Arun C Murthy commented on HADOOP-3840:
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bq. There's also a warning about no new tests, but I'm not sure how to include those because
the patch is mostly refactoring.
Matei - it would really nice to attach a junit test since this is a new feature - maybe along
the lines of HADOOP-2214?
> Support pluggable speculative execution
> ---------------------------------------
>
> Key: HADOOP-3840
> URL: https://issues.apache.org/jira/browse/HADOOP-3840
> Project: Hadoop Core
> Issue Type: Improvement
> Components: mapred
> Reporter: Matei Zaharia
> Assignee: Matei Zaharia
> Priority: Minor
> Attachments: HADOOP-3840-v1.txt, HADOOP-3840-v2.patch
>
>
> HADOOP-3412 introduced an way to plug in a job scheduler for MapReduce. However, the
job schedulers all use JobInProgress.obtainNewMapTask or obtainNewReduceTask to select tasks
to run from each job, which uses a threshold-based speculative execution algorithm that has
several shortcomings (see JIRAs about the scheduler not speculating tasks that freeze after
having 80% progress for example). As a first step towards supporting better speculative execution
policies while not breaking backwards compatibility, it makes sense to make the speculative
execution policy pluggable. Luckily this is easy - we just need an interface around obtainNewMapTask
and obtainNewReduceTask. This JIRA suggests adding a TaskSelector abstract class which, given
a TaskTracker and a JobInProgress, chooses a task to run on the tracker. A default implementation
that uses the current methods in JobInProgress is provided. Both TaskSchedulers in trunk are
changed to use TaskSelector.
> In addition, there are methods to count how many speculative tasks a job needs, since
TaskInProgress.hasSpeculative() may not work if we change the algorithm for selecting speculative
tasks. This count is needed for some schedulers, such as a fair scheduler.
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