[jira] [Created] (SPARK-18967) Locality preferences should be used when scheduling even when delay scheduling is turned off

["Imran Rashid (JIRA)" <ji...@apache.org>]

Imran Rashid created SPARK-18967:
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             Summary: Locality preferences should be used when scheduling even when delay scheduling is turned off
                 Key: SPARK-18967
                 URL: https://issues.apache.org/jira/browse/SPARK-18967
             Project: Spark
          Issue Type: Bug
          Components: Scheduler
    Affects Versions: 2.1.0
            Reporter: Imran Rashid
            Assignee: Imran Rashid


If you turn delay scheduling off by setting {{spark.locality.wait=0}}, you effectively turn off the use the of locality preferences when there is a bulk scheduling event.  {{TaskSchedulerImpl}} will use resources based on whatever random order it decides to shuffle them, rather than taking advantage of the most local options.

This happens because {{TaskSchedulerImpl}} offers resources to a {{TaskSetManager}} one at a time, each time subject to a maxLocality constraint.  However, that constraint doesn't move through all possible locality levels -- it uses [{{tsm.myLocalityLevels}} |https://github.com/apache/spark/blob/1a64388973711b4e567f25fa33d752066a018b49/core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala#L360].  And {{tsm.myLocalityLevels}} [skips locality levels completely if the wait == 0 | https://github.com/apache/spark/blob/1a64388973711b4e567f25fa33d752066a018b49/core/src/main/scala/org/apache/spark/scheduler/TaskSetManager.scala#L953].  So with delay scheduling off, {{TaskSchedulerImpl}} immediately jumps to giving tsms the offers with {{maxLocality = ANY}}.

*WORKAROUND*: instead of setting {{spark.locality.wait=0}}, use {{spark.locality.wait=1ms}}.  The one downside of this is if you have tasks that actually take less than 1ms.  You could even run into SPARK-18886.  But that is a relatively unlikely scenario for real workloads.



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