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From "Olga Natkovich (JIRA)" <j...@apache.org>
Subject [jira] Created: (PIG-274) changing combiner behavior past hadoop 18
Date Tue, 17 Jun 2008 23:02:44 GMT
changing combiner behavior past hadoop 18

                 Key: PIG-274
                 URL: https://issues.apache.org/jira/browse/PIG-274
             Project: Pig
          Issue Type: Bug
            Reporter: Olga Natkovich

In hadoop 18, the way commbiners are handled is changing. The hadoop team agreed to keep things
backward compatible for now but will depricate the current behavior in the future (likely
in hadoop 19) so pig needs to adjust to the new behavior. This should be done in the post
2.0 code base.

Old behavior: combiner is called once and only once per map task
New behavior: combiner can be run 0 or more times on both map and reduce sides. 0 times happens
if only a single <K, V> fits into sort buffer. Multiple time can happen in case of a
hierarchical merge.

The main issue that causes problem for pig is that we would not know in advance whether the
combiner will run 0,1 or more times. This causes several issues:

(1) Lets assume that we compute count. If we enable combiner, reducer expects to get numbers
not values as its input. Hadoop team suggested that we could annotate each tuple with a byte
that tells if it want through combiner. This could be expensive computatinally as well as
will use extra memory. One things to notice is that some algebraics (like SUM, MIN, MAX) don't
care whether the data was precombined as they always to the same thing. Perhaps we can make
algebaic functions declare if they care or not. Then we only anotate the ones that need it.
(2) Since combiner can be called 1 or more times, getInitial and getIntermediate have to do
the same thing. So again, we need to change the interface to reflcat that.
(3) current combiner code assumes that it only works with 1 input. When it runs on the reduce
side, it can be dealing with tuples from multiple inputs. 

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