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From "Gabriel Reid (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CRUNCH-588) Modify HFileUtils to flex on affected regions for hfiles, rather than all regions
Date Tue, 19 Jan 2016 16:51:39 GMT

    [ https://issues.apache.org/jira/browse/CRUNCH-588?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15106996#comment-15106996
] 

Gabriel Reid commented on CRUNCH-588:
-------------------------------------

Sounds interesting -- I'm guessing that this would require an initial scan over all data in
order to determine which regions are going to be receiving data. 

Is this issue happening because you've got a huge HTable in comparison to the amount of data
that you're loading in? Or is the data skew to particular regions just due to how the data
is organized in your HTable?

> Modify HFileUtils to flex on affected regions for hfiles, rather than all regions
> ---------------------------------------------------------------------------------
>
>                 Key: CRUNCH-588
>                 URL: https://issues.apache.org/jira/browse/CRUNCH-588
>             Project: Crunch
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Stephen Durfey
>            Assignee: Josh Wills
>
> HFileUtils when preparing for writing HFiles sets the [number of reducers | https://github.com/apache/crunch/blob/master/crunch-hbase/src/main/java/org/apache/crunch/io/hbase/HFileUtils.java#L422]
equal to the number of regions in the table, and then writes out the start keys for each region
to a sequence file for the TotalOrderPartitioner to consume when partitioning data. This can
result in a very large quantity of reducers that don't do anything due to not having any data
to write to hfiles for the region its partition belonged to. 
> My proposal is to modify HFileUtils, with an optional parameter (or a config, that's
up for debate) to determine which regions data will be loaded into ahead of time, and set
the number of reducers to equal the number of regions, and only write out the start keys for
those affected regions. 
> I have working code to do this on the 0.8.x branch of crunch, as that is what I am currently
on. I can modify it to work on more recent versions, but I wanted to start a discussion around
the viability of this code being contributed back to the community. I am still in process
of capturing metrics around the impact of the change (and trying to get data large enough
to test this out), but at least from a reducer count I have seen substantial drops in my limited
testing so far. For example, I had a job go from 705 reduce tasks during the write down to
36 reduce tasks. 
> I've attached what I have so far as of 0.8.4. I'm going to start working on a version
modified for the latest version of crunch. 



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