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From "Runping Qi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3442) QuickSort may get into unbounded recursion
Date Tue, 10 Jun 2008 10:30:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3442?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12603826#action_12603826
] 

Runping Qi commented on HADOOP-3442:
------------------------------------

bq. The 2*log(n) heuristic was intended to bail out of a worst case, not to protect against
the StackOverflowError. The recursion on the system stack is already limited to log(n), but
quicksort will always have O(n^2) cases that this needs to handle. I'm happy to discuss an
appropriate value for k, but k*log(n) seems appropriate to this purpose.

I guess it is safe to choose a larger k. For example, with k = 8, the stack depth is bounded
by 250 in practice, but a lot fewer cases of input data patterns will need to invoke the heapsort.
Even with all these fixes, I still believe  it is good to add randomization to pivotal selection
(or randomize the input data upfront).

> QuickSort may get into unbounded recursion
> ------------------------------------------
>
>                 Key: HADOOP-3442
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3442
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: mapred
>    Affects Versions: 0.17.0
>            Reporter: Runping Qi
>            Assignee: Chris Douglas
>            Priority: Blocker
>             Fix For: 0.17.1, 0.18.0
>
>         Attachments: 3442-0.patch, 3442-0v17.patch, 3442-1.patch, 3442-2.patch, 3442-3.patch,
CheckSortBuffer.java, HADOOP-3442.patch, overflow.zip, spillbuffers.patch
>
>


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