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From Robert Evans <ev...@yahoo-inc.com>
Subject Re: Combiner timing out
Date Fri, 04 Nov 2011 16:11:58 GMT
There was a change that went into 0.20.205 https://issues.apache.org/jira/browse/MAPREDUCE-2187
where after so many inputs to the combiner progress is automatically reported.  I looked through
the code for 0.20.205 and from what I can see the CombineOutputCollector should be getting
an instance of TaskReporter.  What version of Hadoop are you running?  Are you using the old
APIs in the mapred package or the newer APIs in the mapreduce java package?

--Bobby Evans

On 11/4/11 1:20 AM, "Christopher Egner" <cegner@apple.com> wrote:

Hi all,

Let me preface this with my understanding of how tasks work.

If a task takes a long time (default 10min) and demonstrates no progress, the task tracker
will decide the process is hung, kill it, and start a new attempt.  Normally, one uses a Reporter
instance's progress method to provide progress updates and avoid this. For a streaming mapper,
the Reporter class is org.apache.hadoop.mapred.Task$TaskReporter and this works well.  Streaming
is even set up to take progress, status, and counter updates from stderr, which is really
cool.

However, for combiner tasks, the class is org.apache.hadoop.mapred.Reporter$1.  The first
subclass in this particular java file is the Reporter.NULL class, which ignores all updates.
 So even if a combiner task is updating its reporter in accordance with docs (see postscript),
its updates are ignored and it dies at 10 minutes.  Or one sets mapred.task.timeout very high,
allowing truly hung tasks to go unrecognised for much longer.

At least this is what I've been able to put together from reading code and searching the web
for docs (except hadoop jira which has been down for a while - my bad luck).

So am I understanding this correctly?  Are there plans to change this?  Or reasons that combiners
can't have normal reporters associated to them?

Thanks for any help,
Chris

http://hadoop.apache.org/common/docs/r0.20.2/mapred_tutorial.html#Reporter
http://www.cloudera.com/blog/2009/05/10-mapreduce-tips/ (cf tip 7)
http://hadoop.apache.org/common/docs/r0.18.3/streaming.html#How+do+I+update+counters+in+streaming+applications%3F
http://hadoop.apache.org/common/docs/r0.20.0/mapred-default.html  (cf mapred.task.timeout)


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