Upper and Lower Bounds on the Space Complexity of Detectable Object

TL;DR

This paper investigates the space complexity of recoverable, detectable concurrent objects in non-volatile memory systems, providing new bounds and implementations that highlight the necessity of auxiliary external state for detectability.

Contribution

It introduces the first wait-free bounded-space detectable object implementations and proves lower bounds on bit complexity and the need for external auxiliary state in detectable algorithms.

Findings

First wait-free bounded-space detectable objects provided

Lower bound of Ω(N) bits for detectable CAS implementations

Detectability requires auxiliary external state in recoverable algorithms

Abstract

The emergence of systems with non-volatile main memory (NVM) increases the interest in the design of \emph{recoverable concurrent objects} that are robust to crash-failures, since their operations are able to recover from such failures by using state retained in NVM. Of particular interest are recoverable algorithms that, in addition to ensuring object consistency, also provide \emph{detectability}, a correctness condition requiring that the recovery code can infer if the failed operation was linearized or not and, in the former case, obtain its response. In this work, we investigate the space complexity of detectable algorithms and the external support they require. We make the following three contributions. First, we present the first wait-free bounded-space detectable read/write and CAS object implementations. Second, we prove that the bit complexity of every $N$-process obstruction-free detectable CAS implementation, assuming values from a domain of size at least $N$, is $\Omega(N)$. Finally, we prove that the following holds for obstruction-free detectable implementations of a large class of objects: their recoverable operations must be provided with \emph{auxiliary state} -- state that is not required by the non-recoverable counterpart implementation -- whose value must be provided from outside the operation, either by the system or by the caller of the operation. In contrast, this external support is, in general, not required if the recoverable algorithm is not detectable.