팀 매니저님께서 결과를 보시며 의문을 표하시기에, 조사해봄. 일단 Hadoop Definite Guide 책에서 Combiner Function에 대해 찾아보면, Many MapReduce jobs are limited by the bandwidth available on the cluster, so it pays to minimize the data transferred between map and reduce tasks. Hadoop allows the user to specify a combiner function to be run on the map output—the combiner function’s output forms the input to the reduce function. from ..