The Space Complexity of Scannable Objects with Bounded Components

A fundamental task in the asynchronous shared memory model is obtaining a consistent view of a collection of shared objects while they are being modified concurrently by other processes. A scannable object addresses this problem. A scannable object is a sequence of readable objects called components, each of which can be accessed independently. It also supports the Scan operation, which simultaneously reads all of the components of the object. In this paper, we consider the space complexity of an n-process, k-component scannable object implementation from objects with bounded domain sizes. If the value of each component can change only a finite number of times, then there is a simple lock-free implementation from k objects. However, more objects are needed if each component is fully reusable, i.e. for every pair of values v, v\u27, there is a sequence of operations that changes the value of the component from v to v\u27. We considered the special case of scannable binary objects, where each component has domain {0, 1}, in PODC 2021. Here, we present upper and lower bounds on the space complexity of any n-process implementation of a scannable object O with k fully reusable components from an arbitrary set of objects with bounded domain sizes. We construct a lock-free implementation from k objects of the same types as the components of O along with ?n/b? objects with domain size 2^b. By weakening the progress condition to obstruction-freedom, we construct an implementation from k objects of the same types as the components of O along with ?n/(b-1)? objects with domain size b. When the domain size of each component and each object used to implement O is equal to b and n ? b^k - bk + k, we prove that 1/2? (k + (n-1)/b - log_b n) objects are required. This asymptotically matches our obstruction-free upper bound. When n > b^k - bk + k, we prove that 1/2? (b^{k-1} - {(b-1)k + 1}/b) objects are required. We also present a lower bound on the number of objects needed when the domain sizes of the components and the objects used by the implementation are arbitrary and finite

Survey of local algorithms

A local algorithm is a distributed algorithm that runs in constant time, independently of the size of the network. Being highly scalable and fault-tolerant, such algorithms are ideal in the operation of large-scale distributed systems. Furthermore, even though the model of local algorithms is very limited, in recent years we have seen many positive results for non-trivial problems. This work surveys the state-of-the-art in the field, covering impossibility results, deterministic local algorithms, randomised local algorithms, and local algorithms for geometric graphs.Peer reviewe

Electronic Voting

This open access book LNCS 13353 constitutes the proceedings of the 7th International Conference on Electronic Voting, E-Vote-ID 2022, held in Bregenz, Austria, in October 2022. The 10 full papers presented were carefully reviewed and selected from 39 submissions. The conference collected the most relevant debates on the development of Electronic Voting, from aspects relating to security and usability through to practical experiences and applications of voting systems, also including legal, social, or political aspects, amongst others

Optimisation problems in wireless sensor networks : Local algorithms and local graphs

This thesis studies optimisation problems related to modern large-scale distributed systems, such as wireless sensor networks and wireless ad-hoc networks. The concrete tasks that we use as motivating examples are the following: (i) maximising the lifetime of a battery-powered wireless sensor network, (ii) maximising the capacity of a wireless communication network, and (iii) minimising the number of sensors in a surveillance application. A sensor node consumes energy both when it is transmitting or forwarding data, and when it is performing measurements. Hence task (i), lifetime maximisation, can be approached from two different perspectives. First, we can seek for optimal data flows that make the most out of the energy resources available in the network; such optimisation problems are examples of so-called max-min linear programs. Second, we can conserve energy by putting redundant sensors into sleep mode; we arrive at the sleep scheduling problem, in which the objective is to find an optimal schedule that determines when each sensor node is asleep and when it is awake. In a wireless network simultaneous radio transmissions may interfere with each other. Task (ii), capacity maximisation, therefore gives rise to another scheduling problem, the activity scheduling problem, in which the objective is to find a minimum-length conflict-free schedule that satisfies the data transmission requirements of all wireless communication links. Task (iii), minimising the number of sensors, is related to the classical graph problem of finding a minimum dominating set. However, if we are not only interested in detecting an intruder but also locating the intruder, it is not sufficient to solve the dominating set problem; formulations such as minimum-size identifying codes and locating–dominating codes are more appropriate. This thesis presents approximation algorithms for each of these optimisation problems, i.e., for max-min linear programs, sleep scheduling, activity scheduling, identifying codes, and locating–dominating codes. Two complementary approaches are taken. The main focus is on local algorithms, which are constant-time distributed algorithms. The contributions include local approximation algorithms for max-min linear programs, sleep scheduling, and activity scheduling. In the case of max-min linear programs, tight upper and lower bounds are proved for the best possible approximation ratio that can be achieved by any local algorithm. The second approach is the study of centralised polynomial-time algorithms in local graphs – these are geometric graphs whose structure exhibits spatial locality. Among other contributions, it is shown that while identifying codes and locating–dominating codes are hard to approximate in general graphs, they admit a polynomial-time approximation scheme in local graphs

Concurrency Awareness in a P2P Wiki System

Awareness, collaborative system, wiki, distributed wiki, P2PInternational audienceCurrently, Wikis are the most popular form of collaborative editors. Recently, some researches have proposed fully decentralized wikis relying on peer-to-peer networks. This new architecture subtly changes the behavior of wiki in case of concurrent changes. While traditional wikis ensure that all pages have been reviewed by a user, some pages in a P2P wiki are produced by the system through a automatic merge. To overcome this problem, this paper introduces concurrency awareness, a mechanism that makes users aware about the status of wiki pages. The particular context of a P2P wiki system makes traditional awareness mechanisms inadequate and requires the desgin of a new one

Electronic Voting

This open access book LNCS 13353 constitutes the proceedings of the 7th International Conference on Electronic Voting, E-Vote-ID 2022, held in Bregenz, Austria, in October 2022. The 10 full papers presented were carefully reviewed and selected from 39 submissions. The conference collected the most relevant debates on the development of Electronic Voting, from aspects relating to security and usability through to practical experiences and applications of voting systems, also including legal, social, or political aspects, amongst others

Concurrency Awareness in a P2P Wiki System

Currently, Wikis are the most popular form of collaborative editors. Recently, some researches have been done to shift from traditional centralized architecture to fully decentralized wikis relying on peer-to-peer networks. This architecture improves scalability and fault-tolerance, but subtly changes the behavior of wiki in case of concurrent changes. While traditional wikis ensure that all wiki pages have been reviewed by, at least, a human, some pages in P2P wiki systems can be the result of an automatic merge done by the system. This forces P2P wiki systems to integrate a concurrency awareness system to notify users about the status of wiki pages. The particular context of a P2P wiki system makes traditional awareness mechanisms inadequate. In this paper, we present a new concurrency awareness mechanism designed for P2P wiki systems

Building Scalable Architectures Using Emerging Memory Technologies

A confluence of trends is reshaping computing today. On one end, the massive amounts of data being generated by the proliferation of sensing and internet services are creating a demand for better computer architectures and systems. The other stream of the confluence is the nanotechnology advances that are unearthing new memory device technologies with the potential to replace (or be combined with) conventional memories. Given these trends, this thesis examines emerging memory device technologies that provide a unique opportunity to build computer architectures with efficient and scalable data storage and processing capabilities. The associated memory architectures of these new systems promise to offer distinctive features such as intrinsic non-volatility, highly dense memory structures, extremely low-power consumption and even embedded processing capabilities. Among others, some examples of emerging memory technologies with such features are PCM, 3D Xpoint, STT-RAM and ReRAM. A central question with the new memory architectures built with emerging memory technologies is whether or not the resultant systems are scalable. Towards answering this question, this thesis identifies that conventional memory architecture specific scaling methods may not directly apply in case of emerging memory technologies. These methods were developed mostly for SRAM and DRAM, and today, they do not provide the desired outcomes for emerging memory technologies. As a result, there exist fundamental unsolved problems concerning scalability in building memory architectures. Unfortunately, this means that even though emerging memory technologies provide distinctive features, they may be largely left untapped. Given the scalability concerns, this thesis then advocates a scalability-first approach for building computer architectures using emerging memory technologies while being aware of the limitations and opportunities associated with them. As demonstrations of the scalability-first approach, the thesis discusses several scalability problems encountered in systems using emerging memory technologies. It also brings out potential solutions for each of these problems in the form of novel techniques and tools. For instance, the thesis discusses the problem and a solution for scaling write order enforcement mechanisms for data persistence on large non-volatile main memory systems, followed by the problem and a potential solution for scaling write bandwidth and thereby reducing memory interference on systems with dense non-volatile memory caches. Also discussed are methods for scaling system architectures with in-memory processing capability subject to its operational complexity and other limits. The proposed scalability-first approach points to prospects and ways for better adoption of emerging memory technologies within existing systems. The approach and the solutions also lead to likely transition paths to even more scalable and markedly different systems of the future