[jira] Updated: (HADOOP-3398) ReduceTask::closestPowerOf2 is inefficient

["Chris Douglas (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HADOOP-3398?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Chris Douglas updated HADOOP-3398:
----------------------------------

    Attachment: 3398-0.patch

This returns the same results as the original, save for values <= 0, which neither support.

> 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
>            Priority: Trivial
>             Fix For: 0.18.0
>
>         Attachments: 3398-0.patch
>
>
> ReduceTask computes the "closest power of 2" using loops
> {code}
> private static int getClosestPowerOf2(int value) {
>   final int hob = Integer.highestOneBit(value);
>   return Integer.numberOfTrailingZeros(hob) +
>     Math.min((hob >>> 1) & value, 1);
>   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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