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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2560) Processing multiple input splits per mapper task
Date Fri, 31 Oct 2008 22:13:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2560?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12644443#action_12644443
] 

dhruba borthakur commented on HADOOP-2560:
------------------------------------------

Hi Eric, You are right, this could have unwelcome performance characteristics in certain cases.
We will have to measure this first. 
I am actually more inclined to solve my problem using HADOOP-4565.

> Processing multiple input splits per mapper task
> ------------------------------------------------
>
>                 Key: HADOOP-2560
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2560
>             Project: Hadoop Core
>          Issue Type: Bug
>            Reporter: Runping Qi
>            Assignee: dhruba borthakur
>         Attachments: multipleSplitsPerMapper.patch
>
>
> Currently, an input split contains a consecutive chunk of input file, which by default,
corresponding to a DFS block.
> This may lead to a large number of mapper tasks if the input data is large. This leads
to the following problems:
> 1. Shuffling cost: since the framework has to move M * R map output segments to the nodes
running reducers, 
> larger M means larger shuffling cost.
> 2. High JVM initialization overhead
> 3. Disk fragmentation: larger number of map output files means lower read throughput
for accessing them.
> Ideally, you want to keep the number of mappers to no more than 16 times the number of
 nodes in the cluster.
> To achive that, we can increase the input split size. However, if a split span over more
than one dfs block,
> you lose the data locality scheduling benefits.
> One way to address this problem is to combine multiple input blocks with the same rack
into one split.
> If in average we combine B blocks into one split, then we will reduce the number of mappers
by a factor of B.
> Since all the blocks for one mapper share a rack, thus we can benefit from rack-aware
scheduling.
> Thoughts?

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