Lock-Free and Practical Deques using Single-Word Compare-And-Swap

We present an efficient and practical lock-free implementation of a concurrent deque that is disjoint-parallel accessible and uses atomic primitives which are available in modern computer systems. Previously known lock-free algorithms of deques are either based on non-available atomic synchronization primitives, only implement a subset of the functionality, or are not designed for disjoint accesses. Our algorithm is based on a doubly linked list, and only requires single-word compare-and-swap atomic primitives, even for dynamic memory sizes. We have performed an empirical study using full implementations of the most efficient algorithms of lock-free deques known. For systems with low concurrency, the algorithm by Michael shows the best performance. However, as our algorithm is designed for disjoint accesses, it performs significantly better on systems with high concurrency and non-uniform memory architecture

Time, terror and the technological imagination : Frankenstein's fictional legacy in the scientific age : a thesis presented in partial fulfilment of the requirements for the degree of Master of Arts in English at Massey University, Palmerston North, New Zealand

There is a long-standing belief that there is an opposing discourse between science and the humanities in relation to the future of humankind. Attitudes towards the environment have changed radically in the last 200 years from a natural view to one where we dominate and re-order our environment to suit ourselves and to further the material self-interests of human beings, regardless of cultural and ecological consequences. In order for human beings to properly understand what is happening and why, we must begin to restore the balance between our relationship with Nature and our new technological worldview. The Introduction firstly addresses issues relating to the changing relationship between human beings and their environment over the last two centuries, and how literature and film have accurately predicted our collective future. It is my objective to illustrate how Mary Shelley's Frankenstein has remained one of the most potent pieces of literature foreshadowing the future of humankind, and the timeless quality of the theme of the controller out of control. The main text focuses on Mary Shelley's Frankenstein, and how the novel embodied humankind's growing anxieties and fears about our technological ambivalence, and I give an overview of how Frankenstein has paved the way for further literary and cinematic predictions of our future in artificial and synthesised environments dominated by the new frontier of genetic engineering, artificial intelligence, virtual reality and beyond, and how these technologies will impact on our cultural worldview and the future evolution of humankind

“Colonial Problems, Transnational American Studies”

Excerpt from After American Studies: Rethinking Legacies of Transnational Exceptionalis

The Relative Power of Composite Loop Agreement Tasks

Loop agreement is a family of wait-free tasks that includes set agreement and simplex agreement, and was used to prove the undecidability of wait-free solvability of distributed tasks by read/write memory. Herlihy and Rajsbaum defined the algebraic signature of a loop agreement task, which consists of a group and a distinguished element. They used the algebraic signature to characterize the relative power of loop agreement tasks. In particular, they showed that one task implements another exactly when there is a homomorphism between their respective signatures sending one distinguished element to the other. In this paper, we extend the previous result by defining the composition of multiple loop agreement tasks to create a new one with the same combined power. We generalize the original algebraic characterization of relative power to compositions of tasks. In this way, we can think of loop agreement tasks in terms of their basic building blocks. We also investigate a category-theoretic perspective of loop agreement by defining a category of loops, showing that the algebraic signature is a functor, and proving that our definition of task composition is the "correct" one, in a categorical sense.Comment: 18 page

Tight Bounds for Connectivity and Set Agreement in Byzantine Synchronous Systems

In this paper, we show that the protocol complex of a Byzantine synchronous system can remain $(k - 1)$-connected for up to $\lceil t/k \rceil$ rounds, where $t$ is the maximum number of Byzantine processes, and $t \ge k \ge 1$. This topological property implies that $\lceil t/k \rceil + 1$ rounds are necessary to solve $k$-set agreement in Byzantine synchronous systems, compared to $\lfloor t/k \rfloor + 1$ rounds in synchronous crash-failure systems. We also show that our connectivity bound is tight as we indicate solutions to Byzantine $k$-set agreement in exactly $\lceil t/k \rceil + 1$ synchronous rounds, at least when $n$ is suitably large compared to $t$. In conclusion, we see how Byzantine failures can potentially require one extra round to solve $k$-set agreement, and, for $n$ suitably large compared to $t$, at most that

Well-Structured Futures and Cache Locality

In fork-join parallelism, a sequential program is split into a directed acyclic graph of tasks linked by directed dependency edges, and the tasks are executed, possibly in parallel, in an order consistent with their dependencies. A popular and effective way to extend fork-join parallelism is to allow threads to create futures. A thread creates a future to hold the results of a computation, which may or may not be executed in parallel. That result is returned when some thread touches that future, blocking if necessary until the result is ready. Recent research has shown that while futures can, of course, enhance parallelism in a structured way, they can have a deleterious effect on cache locality. In the worst case, futures can incur $\Omega(P T_\infty + t T_\infty)$ deviations, which implies $\Omega(C P T_\infty + C t T_\infty)$ additional cache misses, where $C$ is the number of cache lines, $P$ is the number of processors, $t$ is the number of touches, and $T_\infty$ is the \emph{computation span}. Since cache locality has a large impact on software performance on modern multicores, this result is troubling. In this paper, however, we show that if futures are used in a simple, disciplined way, then the situation is much better: if each future is touched only once, either by the thread that created it, or by a thread to which the future has been passed from the thread that created it, then parallel executions with work stealing can incur at most $O(C P T^2_\infty)$ additional cache misses, a substantial improvement. This structured use of futures is characteristic of many (but not all) parallel applications

An Empirical Study of Speculative Concurrency in Ethereum Smart Contracts

We use historical data to estimate the potential benefit of speculative techniques for executing Ethereum smart contracts in parallel. We replay transaction traces of sampled blocks from the Ethereum blockchain over time, using a simple speculative execution engine. In this engine, miners attempt to execute all transactions in a block in parallel, rolling back those that cause data conflicts. Aborted transactions are then executed sequentially. Validators execute the same schedule as miners. We find that our speculative technique yields estimated speed-ups starting at about 8-fold in 2016, declining to about 2-fold at the end of 2017, where speed-up is measured using either gas costs or instruction counts. We also observe that a small set of contracts are responsible for many data conflicts resulting from speculative concurrent execution

On the Mailbox Problem

The Mailbox Problem was described and solved by Aguilera, Gafni, and Lamport in their 2010 DC paper with an algorithm that uses two flag registers that carry 14 values each. An interesting problem that they ask is whether there is a mailbox algorithm with smaller flag values. We give a positive answer by describing a mailbox algorithm with 6 and 4 values in the two flag registers