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From "Matei Zaharia (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5170) Set max map/reduce tasks on a per-job basis, either per-node or cluster-wide
Date Thu, 07 May 2009 19:10:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5170?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12707048#action_12707048
] 

Matei Zaharia commented on HADOOP-5170:
---------------------------------------

Hemanth, if you submit another job that is also CPU-bound, it may interfere with the first.
However, if you submit one that is IO-bound, it will be fine. This task limit feature isn't
meant to solve the general resource allocation problem, only to give you a way to limit resource
consumption if you know that you have one job with very resource-intensive tasks and many
jobs with less resource-intensive tasks. Because it's such a simple feature, I think it's
a good one to add before building any kind of automatic resource-aware scheduling. It will
solve many users' problems in the short term, as evidenced by the number of votes and watchers.

> Set max map/reduce tasks on a per-job basis, either per-node or cluster-wide
> ----------------------------------------------------------------------------
>
>                 Key: HADOOP-5170
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5170
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: mapred
>            Reporter: Jonathan Gray
>         Attachments: tasklimits.patch
>
>
> There are a number of use cases for being able to do this.  The focus of this jira should
be on finding what would be the simplest to implement that would satisfy the most use cases.
> This could be implemented as either a per-node maximum or a cluster-wide maximum.  It
seems that for most uses, the former is preferable however either would fulfill the requirements
of this jira.
> Some of the reasons for allowing this feature (mine and from others on list):
> - I have some very large CPU-bound jobs.  I am forced to keep the max map/node limit
at 2 or 3 (on a 4 core node) so that I do not starve the Datanode and Regionserver.  I have
other jobs that are network latency bound and would like to be able to run high numbers of
them concurrently on each node.  Though I can thread some jobs, there are some use cases that
are difficult to thread (scanning from hbase) and there's significant complexity added to
the job rather than letting hadoop handle the concurrency.
> - Poor assignment of tasks to nodes creates some situations where you have multiple reducers
on a single node but other nodes that received none.  A limit of 1 reducer per node for that
job would prevent that from happening. (only works with per-node limit)
> - Poor mans MR job virtualization.  Since we can limit a jobs resources, this gives much
more control in allocating and dividing up resources of a large cluster.  (makes most sense
w/ cluster-wide limit)

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