Wait-free snapshots in real-time systems: algorithms and performance

Abstract

Snap-shot mechanisms are used to read a globally consistent set of variable values. Methods based on locking are penalized by blocking, which typically leads to difficulties in guaranteeing deadlines of high priority tasks. A method which combines the predictability of locking-based methods with the low interference (no blocking) of lock-free methods is desirable. In this paper we present one such method, based and the concept of wait-freeness. A wait-free method is a lock-free method which is guaranteed to correctly complete in a bounded number of steps. The price to pay for this predictability in the timing domain is the need for more than one copy of the shared objects. We evaluate our method analytically by formulating and comparing schedulability equations for snapshots in systems using lock-based lock-free and our wait-free method. We also outline ideas to study the effects of using the different snapshot methods in distributed (CAN-based) systems.