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From Mingxi Wu <Mingxi...@turn.com>
Subject RE: Nested map reduce job
Date Sat, 05 May 2012 21:27:45 GMT
You may not need nested map-reduce job. 

All you need to do is to use keys to partition the permutation. And duplicate the data from
map. 

output.collect(1, value);
output.collect(2, value);
.
.
.
output.collect(n, value);

Then, set your reducer number to n. When you emit data in the mapper, the key is set to the
reducer ID. 
In each reducer, you enumerate the permutation with prefix of reducer id.

You also need to ensure data are hash to the same reduce by implementing 

public class Record implements Writable,WritableComparable<Record>{

    public void readExternal(ObjectInput in) throws IOException { //xxx}
 
    public void writeExternal(ObjectOutput out) throws IOException {
		write(out); 
	}

	public int hashCode() { }
	
}

Hope this helps.

Mingxi

-----Original Message-----
From: venkataswamy [mailto:venkat.taku@gmail.com] 
Sent: Saturday, May 05, 2012 2:17 PM
To: core-user@hadoop.apache.org
Subject: Nested map reduce job


Hi,
   I encountered a strange issue in developing a system. I have data where reducer recieves
about 3 millions values. The reducer emits all the permutations of the values.

Reducer{
   List<values>
   FindPermutations(List<values>)
   foreach( permutation )
       emit( key, permutation )
}


It is feasible to hold values in memory to calculate permutations if the number of values
are low i.e. say less than 10,000. Otherwise, this is not scalable even in computational point
of view.

I tried to write the values into a file and move it to HDFS. Start a new mapreduce job for
permutation from the reducer, this distributes the load of the reducer among available machines.
let me call it as nested mapreduce job. The task waits until the nested job completes and
uses the obtained result to emit the permutations. The parent job's task stills idle, so the
nested job's tasks can run on the same tasktracker, but the tasktracker is not doing it. Is
there a way to signal tasktracker that the current task is paused or sitting idle, but not
to terminate.

All the available tasktrackers are running parent mapreduce job's tasks and the nested mapreduce
job never getting resources to start and falling into deadlock scenario.

I can suspend parent task after starting a nested job for permutations, but it does continue
from the same instruction when it resumes. In simple words, the parent task is not pausing
but suspending.

Anybody got into this situation. If you have any thoughts on it please post it here.


All your help is appreciated.


Thanks,
Venkat




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