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From "Vivek Ratan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4472) Should we move out the creation of setup/cleanup tasks from JobInProgress.initTasks()?
Date Wed, 29 Oct 2008 07:00:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4472?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12643425#action_12643425
] 

Vivek Ratan commented on HADOOP-4472:
-------------------------------------

bq. If we think that doing HADOOP-4513 addresses the issue to do with initializing too many
jobs simultaneously in the capacity scheduler, then I propose that we don't fix this issue
at all for now. Instead we should look at HADOOP-4421. Thoughts?

Agreed. Just to sum up. A job can start running (i.e., we can schedule the first of its map
or reduce tasks) when it's M/R tasks are initialized and when its setup task is complete.
Each of these (running of the setup task, calling initTasks()) can potentially take a while.
The benefit of moving the creation of the setup task outside of initTasks() is the ability
to run the setup task independently of when initTasks() is called. IN particular, we'd like
to run the setup tasks as early as possible. Or at the very least, avoid running the setup
task after initTasks(), as these two are independent. This approach is beneficial if either
the setup task or initTasks(), or both, take a long time.

However, achieving this separation is not easy. If we want to preserve current semantics and
not change any interfaces, then we need to update a job's state to RUNNING either when the
setup task is done or when initTasks() completes. Since the job's state change requires notifications
to listeners, and initTasks() does not have access to listeners, this notification is going
to be hard to do without changing some interfaces. See HADOOP-4521 for a related issue. 

Given that the long-term solution is not clear, and could be quite different, we should drop
this proposal and look at a larger solution in the context of HADOOP-4421. 

> Should we move out the creation of setup/cleanup tasks from JobInProgress.initTasks()?

> ---------------------------------------------------------------------------------------
>
>                 Key: HADOOP-4472
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4472
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: mapred
>            Reporter: Vivek Ratan
>            Assignee: Amareshwari Sriramadasu
>
> JobInProgress.initTasks() creates TIPs for map and reduce tasks, and also the newly-introduced
setup and cleanup tasks. initTasks() is called by the schedulers, as for reasons of memory
optimizations, schedulers may choose to initialize M/R tasks at various moments (the Capacity
Scheduler, for example, calls initTasks() just when it considers a job for running). One can
say that Schedulers 'own' the initialization of M/R tasks in a job. Furthermore the JT 'owns'
the setup and cleanup tasks (it schedules them, and Schedulers are unaware of these tasks).
This causes a problematic dependency between the JT and a Scheduler. For example, the Capacity
Scheduler calls initTasks() and immediately calls JobInProgress.obtainNewMapTask for a map
task. This is a problem today, because we cannot run any map or reduce tasks before the setup
task is run, which the Capacity Scheduler is not aware of. 
> Either all Schedulers are explicitly aware of setup/cleanup tasks and their dependencies
with M/R tasks (in which case, Schedulers 'own' the creation and scheduling of all these tasks
correctly), or the JT 'owns' the setup/cleanup tasks and Schedulers are completely unaware
of them (in which case, the creation of setup/cleanup tasks must be moved out of initTasks
into a separate method which is called by the JT). 
> I think the latter is the right way to go (unless we implement HADOOP-4421, in which
case the former option may be viable as well). 

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