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https://issues.apache.org/jira/browse/HADOOP-2960?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Allen Wittenauer resolved HADOOP-2960.
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Resolution: Won't Fix
Closing at won't fix, given the -1.
> A mapper should use some heuristics to decide whether to run the combiner
> during spills
> ---------------------------------------------------------------------------------------
>
> Key: HADOOP-2960
> URL: https://issues.apache.org/jira/browse/HADOOP-2960
> Project: Hadoop Common
> Issue Type: Bug
> Reporter: Runping Qi
>
> Right now, the combiner, if set, will be called for each spill, no mapper
> whether the combiner can actually reduce the values.
> The mapper should use some heuristics to decide whether to run the combiner
> during spills.
> One of such heuristics is to check the the ratio of the nymber of keys to
> the number of unique keys in the spill.
> The combiner will be called only if that ration exceeds certain threshold
> (say 2).
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