There are two potential reasons why we might be concerned about the size of MPI. The first is that potential users might equate size with complexity and decide that MPI is too complicated to bother learning. The second is that vendors might decide that MPI is too difficult to implement. The design of MPI addresses the first of these concerns by adopting a layered approach. For example, novices can avoid having to worry about groups and communicators by performing all communication in the pre-defined communicator MPI_COMM_WORLD. In fact, most existing message-passing applications can be ported to MPI simply by converting the communication routines on a one-for-one basis (although the resulting MPI application may not be optimally efficient). To allay the concerns of potential implementors the MPI Forum at one stage considered defining a core subset of MPI known as the MPI subset that would be substantially smaller than MPI and include just the point-to-point communication routines and a few of the more commonly-used collective communication routines. However, early work by Lusk, Gropp, Skjellum, Doss, Franke and others on early implementations of MPI showed that it could be fully implemented without a prohibitively large effort . Thus, the rationale for the MPI subset was lost, and this idea was dropped.