[jira] Commented: (MAPREDUCE-902) Map output merge still uses unnecessary seeks

["Todd Lipcon (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/MAPREDUCE-902?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12766833#action_12766833 ] 

Todd Lipcon commented on MAPREDUCE-902:
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I've seen similar performance issues when there are a high number of reducers.

Christian: which linux IO scheduler are you using? Can you try switching to anticipatory and seeing if the problem improves?

> Map output merge still uses unnecessary seeks
> ---------------------------------------------
>
>                 Key: MAPREDUCE-902
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-902
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: task
>    Affects Versions: 0.20.1
>            Reporter: Christian Kunz
>
> HADOOP-3638 improved the merge of the map output by caching the index files.
> But why not also caching the data files?
> In our use-case scenario, still using hadoop-0.18.3, but HADOOP-3638 would only help partially, an individual map tasks finishes in less than 30 minutes, but needs 4 hours to merge 70 spills for 20,000 partitions (with lzo compression), reading about 10kB from each spill file (which is re-opened for every partition). As this is just a merge sort, there is no reason to not keep the input files open and eliminate seek altogether with sequential access.

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