The notion of an anonymous shared memory, introduced by Taubenfeld in PODC 2017, considers that processes use different names for the same memory location. As an example, a location name A used by a process p and a location name B ≠ A used by another process q can correspond to the very same memory location X, and similarly for the names B used by p and A used by q which may (or may not) correspond to the same memory location Y ≠ X. In this context, the PODC paper presented a 2-process symmetric deadlock-free mutual exclusion (mutex) algorithm and a necessary condition on the size m of the anonymous memory for the existence of such an n-process algorithm. This condition states that m must be belongs to M(n) {1} where M(n)= {m: ∀ ℓ: (1) < ℓ ≤ n: gcd(ℓ,m)=1). Symmetric means here that,process identities define a specific data type which allows a process to check only if two identities are equal or not.

We investigate the time and space complexity of detecting and preventing ABAs in shared memory algorithms for systems with n processes and bounded base objects. To that end, we define ABA-detecting registers, which are similar to normal read/write registers, except that they allow a process q to detect with a read operation, whether some process wrote the register since q's last read. ABA-detecting registers can be implemented trivially from a single unbounded register, but we show that they have a high complexity if base objects are bounded: An obstruction-free implementation of an ABA-detecting single bit register cannot be implemented from fewer than n-1 bounded registers. Moreover, bounded CAS objects (or more generally, conditional read-modify-write primitives) offer little help to implement ABA-detecting single bit registers: We prove a linear time-space tradeoff for such implementations. We show that the same time-space tradeoff holds for implementations of single bit LL/SC primitives from bounded writable CAS objects. This proves that the implementations of LL/SC/VL by Anderson and Moir (1995) as well as Jayanti and Petrovic (2003) are optimal. We complement our lower bounds with tight upper bounds: We give an implementation of ABA-detecting registers from n+1 bounded registers, which has step complexity O(1). We also show that (bounded) LL/SC/VL can be implemented from a single bounded CAS object and with O(n) step complexity. Both upper bounds are asymptotically optimal with respect to their time-space product.

We devise a technique for augmenting shared objects in the standard n-process shared memory model with a linearizable Write{} operation, using bounded space and optimal worst-case step complexity. We provide a transformation of any shared object SW supporting only sequential Write{} operations into an object $W$ that supports concurrent Write{} operations. This transformation requires O(n2) SW objects and O(n2) O(log n)-bit registers, and each method (including Write{}) has, up to a constant additive term, the same time complexity as the corresponding method on object $SW$. Our implementation is deterministic, wait-free, and uses only shared registers (supporting atomic read and write operations). To the best of our knowledge, similarly efficient general constructions are not known even if stronger primitives such as CAS or LL/SC are available.

We present a new technique for reclaiming memory in concurrent shared memory algorithms with n asynchronous processes. Our methodology can be applied in the same settings as hazard pointers [10], but provides better worst-case guarantees: For the same tasks for which hazard pointers have expected constant amortized complexity, our technique guarantees constant time in the worst-case.

The development of peer to peer overlay networks applications has attracted an immense interest from the research community in recent years. Several challenging issues have to be resolved in order to provide accessible, efficient and scalable inter-peer communication. Achieving resilience so as to reduce the disconnection probability, is among the most demanding issues to provide a robust and omnipresent service to peer to peer applications. This paper attempts to address this issue, by proposing a graph-theoretic model using the well-known star interconnection network with sub-logarithmic degree characteristics, which not only facilitate scalability problem, but also achieves maximum connectivity compared to the other existing graph-based methods. The simulation results confirm that the proposed solution attains a higher degree of resiliency compared to other existing topologies.