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

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

On the Importance of Registers for Computability

All consensus hierarchies in the literature assume that we have, in addition to copies of a given object, an unbounded number of registers. But why do we really need these registers? This paper considers what would happen if one attempts to solve consensus using various objects but without any registers. We show that under a reasonable assumption, objects like queues and stacks cannot emulate the missing registers. We also show that, perhaps surprisingly, initialization, shown to have no computational consequences when registers are readily available, is crucial in determining the synchronization power of objects when no registers are allowed. Finally, we show that without registers, the number of available objects affects the level of consensus that can be solved. Our work thus raises the question of whether consensus hierarchies which assume an unbounded number of registers truly capture synchronization power, and begins a line of research aimed at better understanding the interaction between read-write memory and the powerful synchronization operations available on modern architectures.Comment: 12 pages, 0 figure

Relating L-Resilience and Wait-Freedom via Hitting Sets

The condition of t-resilience stipulates that an n-process program is only obliged to make progress when at least n-t processes are correct. Put another way, the live sets, the collection of process sets such that progress is required if all the processes in one of these sets are correct, are all sets with at least n-t processes. We show that the ability of arbitrary collection of live sets L to solve distributed tasks is tightly related to the minimum hitting set of L, a minimum cardinality subset of processes that has a non-empty intersection with every live set. Thus, finding the computing power of L is NP-complete. For the special case of colorless tasks that allow participating processes to adopt input or output values of each other, we use a simple simulation to show that a task can be solved L-resiliently if and only if it can be solved (h-1)-resiliently, where h is the size of the minimum hitting set of L. For general tasks, we characterize L-resilient solvability of tasks with respect to a limited notion of weak solvability: in every execution where all processes in some set in L are correct, outputs must be produced for every process in some (possibly different) participating set in L. Given a task T, we construct another task T_L such that T is solvable weakly L-resiliently if and only if T_L is solvable weakly wait-free

Strong Equivalence Relations for Iterated Models

The Iterated Immediate Snapshot model (IIS), due to its elegant geometrical representation, has become standard for applying topological reasoning to distributed computing. Its modular structure makes it easier to analyze than the more realistic (non-iterated) read-write Atomic-Snapshot memory model (AS). It is known that AS and IIS are equivalent with respect to \emph{wait-free task} computability: a distributed task is solvable in AS if and only if it solvable in IIS. We observe, however, that this equivalence is not sufficient in order to explore solvability of tasks in \emph{sub-models} of AS (i.e. proper subsets of its runs) or computability of \emph{long-lived} objects, and a stronger equivalence relation is needed. In this paper, we consider \emph{adversarial} sub-models of AS and IIS specified by the sets of processes that can be \emph{correct} in a model run. We show that AS and IIS are equivalent in a strong way: a (possibly long-lived) object is implementable in AS under a given adversary if and only if it is implementable in IIS under the same adversary. %This holds whether the object is one-shot or long-lived. Therefore, the computability of any object in shared memory under an adversarial AS scheduler can be equivalently investigated in IIS

Appellate Review of Patent Claim Construction: Should the Federal Circuit Be Its Own Lexicographer in Matters Related to the Seventh Amendment

The Federal Circuit stated in an en banc decision in Cybor Corp. v. FAS Technologies, Inc. that the construction of patent claims is a purely legal issue, and is therefore subject to de novo review on appeal. The Cybor decision reaffirmed the position of the majority of the Federal Circuit which had been announced in its en banc Markman decision, and proclaimed that the de novo standard of review is supported by the Supreme Court\u27s Markman decision, a Seventh Amendment opinion. However, Cybor included strong opposition to a de novo standard of review from some of the judges of the Federal Circuit. Moreover, in subsequent cases, the consistent citation of Cybor in support of the application of a de novo standard of review of claim construction has continued to generate scathing dissents from numerous judges of the Federal Circuit and strong criticism from other members of the bench and bar. Even Congress has taken up the issue, proposing in pending reform legislation to grant trial judges the authority to certify interlocatory appeals on claim construction issues in order to try to stem the tide of high reversal rates following full trials. Nevertheless, the Supreme Court has denied certiorari in at least three recent cases in which the de novo standard of review from Cybor was under attack. While there was a sharp division of opinion among the judges of the Federal Circuit in Cybor, all of the judges relied on the Supreme Court\u27s Markman decision, a Seventh Amendment case, to support differing views. It is clear that there is disagreement among the judges of the Federal Circuit regarding the basis of the holding in the Supreme Court\u27s Markman decision. In Cybor, the majority of the Federal Circuit judges chose to view all the subsidiary questions involved in the construction of patent claims as matters of law, indirectly holding that there are no issues of fact involved in claim construction. Is this view supported by the Supreme Court in its Markman analysis? Has the Federal Circuit engaged in creating unwarranted special rules for patent cases in the area of the Seventh Amendment? Should the Federal Circuit be free to define all the subsidiary questions involved in the construction of patent claims as matters of law, thereby indirectly acting as its own lexicographer with respect to the term fact in the Seventh Amendment? Part I of this Article examines whether the Cybor rule of de novo appellate review of patent claim construction is consistent with Supreme Court precedent, focusing primarily on Seventh Amendment decisions. Part II discusses whether or not it is appropriate for the Federal Circuit to set the boundary between issues of fact and issues of law in patent cases. Finally, Part III addresses the extent to which various proposed standards of appellate review of claim construction are principled, with particular emphasis on the Seventh Amendment

A study of some phases of initiative and leadership of sixth grade children

Thesis (M.A.)--Boston University, 1947. This item was digitized by the Internet Archive

A Single-Enqueuer Wait-Free Queue Implementation

Abstract. We study wait-free linearizable Queue implementations in asynchronous shared-memory systems from other consensus number 2 objects, such as Fetch&Add and Swap. The best previously known im-plementation allows at most two processes to perform Dequeue opera-tions. We provide a new implementation, when only one process performs Enqueue operations and any number of processes perform Dequeue op-erations. A nice feature of this implementation is the fact that both Enqueue and Dequeue operations take constant time.

Prediction of Nitrogen Inputs for Sugar Beet.An Evaluation of Soil Tests and Soil

End of Project ReportCurrently, advice on nitrogen (N) use for tillage crops employs an index system based on crop management. However, there has not been a systematic evaluation of several of its components or of the relevance of soil tests, alone or in conjunction. The objective of the current study, therefore, was to evaluate relevant soil management data, various biological and chemical soil tests, and measurements of Nmin (NH4 + NO3) in the root profile, for prediction of fertiliser N requirements. The data used in the study were derived from a data bank of results of field and laboratory experiments for sugar beet. The following topics were addressed: selection of regression models based on soil management criteria; sustainability of contribution of soil N reserves; limitations of soil tests for N; long-term trends in Nmin and biomass C; relationship of mineral-N flush with biomass C; relevance of Nmin with growing-season rainfall; implications of winter rainfall for residual effects and their justification within the current soil N index.European Union Structural Funding (EAGGF

On the nature of progress

15th International Conference, OPODIS 2011, Toulouse, France, December 13-16, 2011. ProceedingsWe identify a simple relationship that unifies seemingly unrelated progress conditions ranging from the deadlock-free and starvation-free properties common to lock-based systems, to non-blocking conditions such as obstruction-freedom, lock-freedom, and wait-freedom. Properties can be classified along two dimensions based on the demands they make on the operating system scheduler. A gap in the classification reveals a new non-blocking progress condition, weaker than obstruction-freedom, which we call clash-freedom. The classification provides an intuitively-appealing explanation why programmers continue to devise data structures that mix both blocking and non-blocking progress conditions. It also explains why the wait-free property is a natural basis for the consensus hierarchy: a theory of shared-memory computation requires an independent progress condition, not one that makes demands of the operating system scheduler