Distributed Shared Memory abstraction (DSM) is traditionally realized through a distributed memory consistency system(MCS) on top of a message passing system. In this paper we analyze the impossibility of efficient partial replication implementation of causally consistent DSM. Efficiency is discussed in terms of control information that processes have to propagate to maintain consistency. We introduce the notions of share graph and hoop to model variable distribution and the concept of dependency chain to characterize processes that have to manage information about a variable even though they do not read or write that variable. Then, we consider PRAM, a consistency criterion weaker enough to allow efficient partial replication implementations and strong enough to solve interesting problems. Finally, we illustrate the power of PRAM with the Bellman- Ford shortest path algorithm.