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https://issues.apache.org/jira/browse/HADOOP-4664?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12678003#action_12678003
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Jothi Padmanabhan commented on HADOOP-4664:
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Matei, while the performance improvement due to parallelization is definitely
something we would like (and that would need HADOOP-4372 as you have pointed
out), the other motivation for this patch is that this might help mitigate the
effect of HADOOP-5286, as Hemanth points out. I will get out a patch for this
soon.
> Parallelize job initialization
> ------------------------------
>
> Key: HADOOP-4664
> URL: https://issues.apache.org/jira/browse/HADOOP-4664
> Project: Hadoop Core
> Issue Type: Improvement
> Components: mapred
> Reporter: Matei Zaharia
> Assignee: Matei Zaharia
> Priority: Blocker
> Fix For: 0.20.0
>
> Attachments: parallel-job-init-v1.patch
>
>
> The job init thread currently initializes one job at a time. However, this is
> a lengthy and partly IO-bound process because all of the job's block
> locations need to be resolved through the namenode and a map of them needs to
> be built. It can take tens of seconds. As a result, the cluster sometimes
> initializes jobs too slowly for full utilization to be achieved, if there are
> many small jobs queued up. It would be better to have a pool of threads that
> initialize multiple jobs in parallel. One thing to be careful of, however, is
> not causing deadlocks or holding locks for too long in these threads.
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