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https://issues.apache.org/jira/browse/MAPREDUCE-1639?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12850714#action_12850714
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Joydeep Sen Sarma commented on MAPREDUCE-1639:
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@Luke - like this line of thinking. I am familiar with Hive - it already does
map side partial aggregates for (most) group-bys and emits them (what you
mentioned). However - for (most) joins and cluster-by - it depends on the
sorting provided by the map-reduce framework. We could alter the these query
plans to execute this algorithm. the downside is pretty obvious: the benefits
would only be available to Hive users (even as the cost of implementation
remains fairly high). it would not be reusable by Pig for example - nor
streaming users, etc.
extrapolating ur idea a bit - we could try to implement this as a library -
something that intercepts the output of the mapper (and combiner) and the input
to the reduce function (that could be inserted into the data path via config
settings). (There's already a configurable output collector interface on the
map output side. not sure about reducer input). We could turn off map-side
sorting in the map-reduce framework itself. the main concern i would have is
that it would seem too hard for these output collectors to implements spills
(/merges) to (/from) disk. Perhaps if that functionality can be extracted out
of the map-reduce core and provided as a library that these output collectors
can use - then it's all feasible.
> Grouping using hashing instead of sorting
> -----------------------------------------
>
> Key: MAPREDUCE-1639
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-1639
> Project: Hadoop Map/Reduce
> Issue Type: New Feature
> Reporter: Joydeep Sen Sarma
>
> most applications of map-reduce care about grouping and not sorting. Sorting
> is a (relatively expensive) way to achieve grouping. In order to achieve just
> grouping - one can:
> - replace the sort on the Mappers with a HashTable - and maintain lists of
> key-values against each hash-bucket.
> - key-value tuples inside each hash bucket are sorted - before spilling or
> sending to Reducer. Anytime this is done - Combiner can be invoked.
> - HashTable is serialized by hash-bucketid. So merges (of either spills or
> Map Outputs) works similar to today (at least there's no change in overall
> compute complexity of merge)
> Of course this hashtable has nothing to do with partitioning. it's just a
> replacement for map-side sort.
> --
> this is (pretty much) straight from the MARS project paper:
> http://www.cse.ust.hk/catalac/papers/mars_pact08.pdf. They report a 45%
> speedup in inverted index calculation using hashing instead of sorting
> (reference implementation is NOT against Hadoop though).
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