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From "Malcolm Matalka" <mmata...@millennialmedia.com>
Subject RE: Potential race condition (Hadoop 18.3)
Date Mon, 02 Mar 2009 21:15:36 GMT
I have a situation which may be related.  I am running hadoop 0.18.1.  I
am on a cluster with 5 machines and testing on very small input of 10
lines.  Mapper produces either 1 or 0 output per line of input yet
somehow I get 18 lines of output from the reducer.  For example I have
one input where the key is:

However, I added a print to the reducer to print keys right before
calling output.collect and I have 3 instances of this key being printed.

I have turned speculative execution off and still get this.

Does this sound related?  A known bug?  Something I'm missing?  Fixed in

- Malcolm

-----Original Message-----
From: Koji Noguchi [mailto:knoguchi@yahoo-inc.com] 
Sent: Monday, March 02, 2009 15:59
To: core-user@hadoop.apache.org
Subject: RE: Potential race condition (Hadoop 18.3)


If you're using getOutputPath, try replacing it with getWorkOutputPath.



-----Original Message-----
From: Ryan Shih [mailto:ryan.shih@gmail.com] 
Sent: Monday, March 02, 2009 11:01 AM
To: core-user@hadoop.apache.org
Subject: Potential race condition (Hadoop 18.3)

Hi - I'm not sure yet, but I think I might be hitting a race condition
Hadoop 18.3. What seems to happen is that in the reduce phase, some of
tasks perform speculative execution but when the initial task completes
successfully, it sends a kill to the new task started. After all is said
done, perhaps one in every five or ten which kill their second task ends
with zero or truncated output.  When I code it to turn off speculative
execution, the problem goes away. Are there known race conditions that I
should be aware of around this area?

Thanks in advance,

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