If the job is small, the application master may choose to run them in the same JVM as itself, since it judges the overhead of allocating new containers and running tasks in them as outweighing the gain to be had in running them in parallel, compared to running them sequentially on one node. (This is different to Mapreduce 1, where small jobs are never run on a single task tracker.)
Such a job is said to be Uberized, or run as an Uber task.
