By default, Hadoop launches a new JVM for each map or reduce job and runs the map or reduce task in parallel. When the map or reduce job is a lightweight job running only for a few seconds, the JVM startup process could be a significant overhead. Hadoop has an option to reuse the JVM by sharing the JVM to run mapper/reducer serially instead of in parallel. JVM reuse applies to map or reduce tasks in the same job. Tasks from different jobs will always run in a separate JVM. To enable reuse, we can set the maximum number of tasks for a single job for JVM reuse using the following property. Its default value is 1. If set to -1, there is no limit:
> SET mapreduce.job.jvm.numtasks=5;
