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Hadoop >> mail # dev >> Combiner Execution


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Mike Schliep 2013-10-22, 14:50
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Amr Shahin 2013-10-22, 15:27
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Gopal Vijayaraghavan 2013-10-22, 16:02
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RE: Combiner Execution
+1. A node level or rack level or any level intermediate combiner is
fairly straightforward to add in Tez. Please carry over your question to
the Apache Tez dev mailing list [EMAIL PROTECTED] if you are
interested in following that path.

Bikas

-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
Gopal Vijayaraghavan
Sent: Tuesday, October 22, 2013 9:03 AM
To: [EMAIL PROTECTED]
Subject: Re: Combiner Execution

Hi,

I'll answer your questions in reverse.

> According to http://developer.yahoo.com/hadoop/tutorial/module4.html the
output is already combined over all Mappers in a node. But we can not find
how this is happening. Can someone point us to where this combiner is
executed?

You'll find the Combiner runner somewhere buried inside MapTask.java, hunt
for the combinerRunner in there.

The Combiner only combines the output of a single map-task (after
sorting). This kicks in only if there are spills in that 1 map-task >
minSpillsForCombine.

It does not do any cross-task actions and the MR framework (as it is
today) doesn't leave enough room for scheduling a cross-task activity (i.e
MR is strictly bi-partite).

> For a class project my group and I are looking to experiment with
combining the output from Mappers on the same node or in the same rack. We
found the idea at http://wiki.apache.org/hadoop/HadoopResearchProjects.

Your general idea is sort of chalked out in Apache Tez (per-host/per-rack
multi-level combiner trees, which is designed to be more flexible with its
plumbing) -
https://issues.apache.org/jira/browse/TEZ-145

Cheers,
Gopal

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Tsuyoshi OZAWA 2013-10-22, 18:44
NEW: Monitor These Apps!
elasticsearch, apache solr, apache hbase, hadoop, redis, casssandra, amazon cloudwatch, mysql, memcached, apache kafka, apache zookeeper, apache storm, ubuntu, centOS, red hat, debian, puppet labs, java, senseiDB