MapReduce inefficient reducer

Question

What would cause only a single reducer in a MapReduce job apart from all the keys output by the map function being the same?

Answer 1

Possible causes:

1. Your cluster still has the default setting of having only 1 reducer (= default value).
2. Your code explicitly sets the value to be 1 reducer.
3. You are running in local mode (i.e. no cluster at all).

Quote from mapred-default.xml

<property>
  <name>mapred.reduce.tasks</name>
  <value>1</value>
  <description>The default number of reduce tasks per job. Typically set to 99%
  of the cluster's reduce capacity, so that if a node fails the reduces can 
  still be executed in a single wave.
  Ignored when mapred.job.tracker is "local".
  </description>
</property>