[GitHub] spark issue #16867: [WIP][SPARK-16929] Improve performance when check specul...

[jinxing64 <gi...@git.apache.org>]

Github user jinxing64 commented on the issue:

    https://github.com/apache/spark/pull/16867
  
    @squito
    Thanks a lot for your comments : )
    >When check speculatable tasks in TaskSetManager, current code scan all task infos and sort durations of successful tasks in O(NlogN) time complexity.
    
    `checkSpeculatableTasks` is scheduled every 100ms by `scheduleAtFixedRate`(not `scheduleWithFixedDelay `), thus the interval can be less than 100ms. In my cluster(yarn-cluster mode), if size of the task set is over 300000 and the driver is running on some machine with poor cpu performance, the `Arrays.sort` can take over than 100ms easily. Since `checkSpeculatableTasks` will synchronize `TaskSchedulerImpl`, I suspect that's why my driver hang.
    
    I get median duration by `TreeSet.slice`, which comes from `IterableLike` and cannot jump to the mid position unluckily. The time complexity is O(n) in this pr.
    I can get the mid position by reflection, but I don't want to do that, I think that is harmful for code clarity.


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