[jira] Commented: (HADOOP-4664) Parallelize job initialization

["Vivek Ratan (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-4664?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12648074#action_12648074 ] 

Vivek Ratan commented on HADOOP-4664:
-------------------------------------

Just as an FYI, we're doing something similar in the Capacity Scheduler (HADOOP-4513). We're initializing jobs asynchronously, and have a thread per queue, so jobs in different queues get initialized in parallel. 

> 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
>         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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