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https://issues.apache.org/jira/browse/MAPREDUCE-1639?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12850562#action_12850562
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Joydeep Sen Sarma commented on MAPREDUCE-1639:
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following up on the previous comment - reading the paper again closely - it's
no longer clear to me that the paper actually implements what i filed or
whether it's closer to how Owen interpreted it. But i think it's true that
sorting only within hash buckets only is fundamentally lower complexity than
sorting everything together.
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i am also wondering how different this is from current data path. today we
anyway bucket map outputs into one bucket per reducer and sort therein. this is
simply saying that we have lots of buckets per reducer (not one) - we are still
sorting inside each of those - but because we are sorting much smaller sets -
the complexity is lowered.
so on the mapper side - we can (sort of) just pretend that the number of
reducers is much much higher. Except when it's time to send off the data to the
reducer (or spill) - we send a bunch of buckets. but this is not transparent to
the merge code - it now has to merge taking the bucketid into consideration. if
we prefixed each map-output key with the bucketid - then it's transparent to
the merge code. so definitely some changes required - but maybe tractable?
> 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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