On the robustness of Herlihy's hierarchy

Abstract

A wait-free hierarchy maps object types to levels in the union of {1,2,3,...} and {∞}, and has the following property: if a type T is at level N, then, for all types T′, there is a wait-free implementation of an object of type T′, for N processes, using only registers and objects of type T. The infinite hierarchy defined by Herlihy is an example of a wait-free hierarchy. A wait-free hierarchy is robust if it has the following property: if a type T is at level N, and S is a finite set of types belonging to levels N-1 or lower, then there is no wait-free implementation of an object of type T, for N processes, using any number and any combination of objects belonging to the types in S. Robustness implies that there are no clever ways of combining weak shared objects to obtain stronger ones. Contrary to what many researchers believe, we prove that Herlihy's hierarchy is not robust. We then define some natural variants of Herlihy's hierarchy, which are also infinite wait-free hierarchies. With the exception of one, which is still open, these are not robust either. We conclude with the open question of whether a non-trivial robust wait-free hierarchies exists.