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Date: Fri, 16 May 2008 13:43:55 -0700 (PDT)
From: "Tsz Wo (Nicholas), SZE (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-3398) ReduceTask::closestPowerOf2 is
 inefficient
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    [ https://issues.apache.org/jira/browse/HADOOP-3398?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12597606#action_12597606 ] 

Tsz Wo (Nicholas), SZE commented on HADOOP-3398:
------------------------------------------------

Mathematically, we should throw an exception (Math.log() returns NaN for -ve numbers and -infinity for 0) when value <= 0 since logarithm is undefined.

> ReduceTask::closestPowerOf2 is inefficient
> ------------------------------------------
>
>                 Key: HADOOP-3398
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3398
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: mapred
>            Reporter: Chris Douglas
>            Assignee: Chris Douglas
>            Priority: Trivial
>             Fix For: 0.18.0
>
>         Attachments: 3398-0.patch, 3398-1.patch, 3398-2.patch
>
>
> ReduceTask computes the "closest power of 2" using loops
> {code}
> private static int getClosestPowerOf2(int value) {
>   int power = 0;
>   int approx = 1;
>   while (approx < value) {
>     ++power;
>     approx = (approx << 1);
>   }
>   if ((value - (approx >> 1)) < (approx - value)) {
>     --power;
>   }
>   return power;
> }
> {code}
> This could be improved.

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