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https://issues.apache.org/jira/browse/HADOOP-4474?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12641634#action_12641634
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dhruba borthakur commented on HADOOP-4474:
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Is there are scalability to be gained if the heartbeat processing occurs under
a read-lock? If it can be so arranged (especially for heartbeats that do not
ask for a new task) then multiple threads in the JT can be responding to
heartbeats at the same time.
> JT should not iterate through all jobs in every heartbeat to find a cleanup
> or setup task
> -----------------------------------------------------------------------------------------
>
> Key: HADOOP-4474
> URL: https://issues.apache.org/jira/browse/HADOOP-4474
> Project: Hadoop Core
> Issue Type: Improvement
> Components: mapred
> Reporter: Vivek Ratan
>
> On every heartbeat, the JT first looks to see if it can run a setup or
> cleanup task, before calling a Scheduler to get a Map or Reduce task. The JT
> maintains a hashmap of JobInProgress objects (which can be waiting, running,
> or completed). It iterates through this hashmap on each heartbeat to find a
> setup or cleanup task. This linear search can be be very expensive,
> especially with large clusters where the number of jobs is high. There are
> lots of obvious ways to cut down on this linear search.
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