Configuring and Tuning MapReduce's Shuffle
Once you have outgrown your small Hadoop cluster it’s worth tuning some of the shuffle configurables to ensure that your performance keeps up with the physical growth of your cluster. The figure below shows key configurables in the shuffle stage in Hadoop versions 1.x and earlier, and identifies those that should be tuned.
You can read more about these configurables and their default values by looking at mapred-default.xml. My book Hadoop in Practice (Manning Publications) in chapter 6 discusses how some of the configuration values in the figure should be tweaked when you start working with mid to large-size Hadoop clusters.