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From "Gabriel Reid (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CRUNCH-213) Add sharded join functionality
Date Sat, 08 Jun 2013 19:32:20 GMT

    [ https://issues.apache.org/jira/browse/CRUNCH-213?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13678828#comment-13678828
] 

Gabriel Reid commented on CRUNCH-213:
-------------------------------------

Josh, thanks for looking at it.

1) Yep, thanks for catching that on hadoop-2. It is indeed an issue with the generation of
the TaskAttemptContext, and I'll upload a patch in a couple of minutes.

2) The default join strategy caches the values for a single key at a time in memory from the
left table, not the whole table, so replicating the left side won't result in more memory
being used for this caching. The general intention is that the left table is (much) smaller
than the right table, so by replicating the left table and sharding the right table, the size
of the shuffle stays nearly the same, whereas replicating the right table would make the shuffle
much larger (depending on the proportions of the left and right table of course).

3) Correct, there's no default sampling-based strategy yet. Like you thought, I wanted to
keep this functionality possible, but not worry about it specifically yet.
                
> Add sharded join functionality
> ------------------------------
>
>                 Key: CRUNCH-213
>                 URL: https://issues.apache.org/jira/browse/CRUNCH-213
>             Project: Crunch
>          Issue Type: New Feature
>            Reporter: Gabriel Reid
>            Assignee: Gabriel Reid
>         Attachments: CRUNCH-213.patch, CRUNCH-213.patch
>
>
> Performing joins where a large proportion of the values on one or both sides of the join
are mapped to a single key can result in poor performance, as one (or a small number) of reducers
end up handling most of the joining work, leaving the rest of the cluster idle.
> Sharded joining should be added to allow splitting up join keys, thereby distributing
values mapped to a single key over multiple reducer partitions.

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