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.t...@gmail.com]
Sent: Saturday, May 05, 2012 2:17 PM
To: core-u...@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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