There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

We will review some of the major results in random graphs and some of the more challenging open problems. We will cover algorithmic and structural questions. We will touch on newer models, including those related to the WWW.

Random graphs

物件導向軟體之架構(Object-Oriented Software Construction)探討

In 1974 an article appeared in Science magazine with the dry-sounding title “Judgment Under Uncertainty: Heuristics and Biases” by a pair of psychologists who were not well known outside their discipline of decision theory. In it Amos Tversky and Daniel Kahneman introduced the world to Prospect Theory, which mapped out how humans actually behave when faced with decisions about gains and losses, in contrast to how economists assumed that people behave. Prospect Theory turned Economics on its head by demonstrating through a series of ingenious experiments that people are much more concerned with losses than they are with gains, and that framing a choice from one perspective or the other will result in decisions that are exactly the opposite of each other, even if the outcomes are monetarily the same. Prospect Theory led cognitive psychology in a new direction that began to uncover other human biases in thinking that are probably not learned but are part of our brain’s wiring.

Thinking fast and slow.

This paper describes a new replication algorithm that is able to tolerate Byzantine faults. We believe that Byzantinefault-tolerant algorithms will be increasingly important in the future because malicious attacks and software errors are increasingly common and can cause faulty nodes to exhibit arbitrary behavior. Whereas previous algorithms assumed a synchronous system or were too slow to be used in practice, the algorithm described in this paper is practical: it works in asynchronous environments like the Internet and incorporates several important optimizations that improve the response time of previous algorithms by more than an order of magnitude. We implemented a Byzantine-fault-tolerant NFS service using our algorithm and measured its performance. The results show that our service is only 3% slower than a standard unreplicated NFS.

/pdf/practical-byzantine-fault-tolerance-58gb0de0hd.pdf

Practical Byzantine fault tolerance

It has been considered bon ton to blame locks for their fragility, especially since researchers identified obstruction-freedom: a progress condition that precludes locking while being weak enough to raise the hope for good performance. This paper attenuates this hope by establishing lower bounds on the complexity of obstruction-free implementations in contention-free executions: those where obstruction-freedom was precisely claimed to be effective. Through our lower bounds, we argue for an inherent cost of concurrent computing without locks.We first prove that obstruction-free implementations of a large class of objects, using only overwriting or trivial primitives in con-tention-free executions, have Ω(n) space complexity and Ω(log2n) (obstruction-free) step complexity. These bounds apply to implementations of many popular objects, including variants of fetch&add, counter, compare&swap, and LL/SC. When arbitrary primitives can be applied in contention-free executions, we show that, in any implementation of binary consensus, or any perturbable object, the number of distinct base objects accessed and memory stalls incurred by some process in a contention free execution is Ω(√n). All these results hold regardless of the behavior of processes after they become aware of contention. We also prove that, in any obstruction-free implementation of a perturbable object in which processes are not allowed to fail their operations, the number of memory stalls incurred by some process that is unaware of contention is Ω(n).

Synchronizing without locks is inherently expensive

Transactional memory (TM) is a promising paradigm for concurrent programming. This paper is an overview of our recent theoretical work on defining a theory of TM. We first recall some TM correctness properties and then overview results on the inherent power and limitations of TMs.

/pdf/transactional-memory-glimmer-of-a-theory-3nhgq7orlh.pdf

Transactional Memory: Glimmer of a Theory

A universal construction is an algorithm that transforms any object with a sequential specification into a wait-free linearizable implementation of that object. This paper presents a novel universal construction algorithm for a message-passing system with process crash failures. Our algorithm relies on two fine-grained underlying abstractions: a weak form of leader election, and a one-shot form of register.Our algorithm is indulgent, efficient and generic. Being indulgent intuitively means that the algorithm preserves consistency even if the underlying system is asynchronous for arbitrary periods of time. Compared to other indulgent universal constructions, our algorithm uses fewer messages and gives rise to less work in steady-state. Our algorithm is generic in two senses: (1) although it is devised for a crash-stop model, it can be easily ported to various crash-recovery models, and (2) although it is optimized for steady-state periods, it can easily be extended to trade-off between steady-state performance and fail-over time.

/pdf/an-efficient-universal-construction-for-message-passing-2zc1g5a4yh.pdf

An Efficient Universal Construction for Message-Passing Systems

We show that it is impossible to design a transactional memory system which ensures parallelism, i.e. transactions do not need to synchronize unless they access the same application objects, while ensuring very little consistency, i.e. a consistency condition, called weak adaptive consistency, introduced here and which is weaker than snapshot isolation, processor consistency, and any other consistency condition stronger than them (such as opacity, serializability, causal serializability, etc.), and very little liveness, i.e. that transactions eventually commit if they run solo.

/pdf/the-pcl-theorem-transactions-cannot-be-parallel-consistent-12xnlwytj4.pdf

The PCL theorem: transactions cannot be parallel, consistent and live

We consider the polling problem in a social network where participants care about their reputation: they do not want their vote to be disclosed nor their misbehaving, if any, to be publicly exposed. Assuming this reputation concern, we show that a simple secret sharing scheme, combined with verification procedures, can efficiently enable polling without the need for any central authority or heavyweight cryptography.

More specifically, we present DPol, a simple and scalable distributed polling protocol where misbehaving nodes are exposed with a non-zero probability and the probability of dishonest participants violating privacy is balanced with their impact on the accuracy of the polling result. The trade-off is captured by a generic parameter of the protocol, an integer k we call the privacy parameter , so that in a system of N nodes with $B dishonest participants, the probability of disclosing a participant's vote is bounded by (B /N ) k + 1, whereas the impact on the polling result is bounded by (6k + 2) B .

We report on the deployment of DPolover 400 PlanetLab nodes. The polling result suffers a relative error of less than 10% in the face of message losses, crashes and asynchrony inherent in PlanetLab. In the presence of dishonest nodes, our experiments show that the impact on the polling result is (4k + 1) B on average, consistently lower that the theoretical bound of (6k + 2) B .

https://hal.inria.fr/inria-00429678/document

Rachid Guerraoui

Papers

Synchronizing without locks is inherently expensive

Transactional Memory: Glimmer of a Theory

An Efficient Universal Construction for Message-Passing Systems

The PCL theorem: transactions cannot be parallel, consistent and live

Decentralized Polling with Respectable Participants