arXiv subjects

Jian Lu

Publications and source records attributed to Jian Lu.

101 records · Page 6Linked to original sources

arXiv2015

Almost Strong Consistency: "Good Enough" in Distributed Storage Systems

A consistency/latency tradeoff arises as soon as a distributed storage system replicates data. For low latency, modern storage systems often settle for weak consistency conditions, which provide little, or even worse, no guarantee for data consistency. In this paper we propose the notion of almost strong consistency as a better balance option for the consistency/latency tradeoff. It provides both deterministically bounded staleness of data versions for each read and probabilistic quantification on the rate of "reading stale values", while achieving low latency. In the context of distributed storage systems, we investigate almost strong consistency in terms of 2-atomicity. Our 2AM (2-Atomicity Maintenance) algorithm completes both reads and writes in one communication round-trip, and guarantees that each read obtains the value of within the latest 2 versions. To quantify the rate of "reading stale values", we decompose the so-called "old-new inversion" phenomenon into concurrency patterns and read-write patterns, and propose a stochastic queueing model and a "timed balls-into-bins model" to analyze them, respectively. The theoretical analysis not only demonstrates that "old-new inversions" rarely occur as expected, but also reveals that the read-write pattern dominates in guaranteeing such rare data inconsistencies. These are further confirmed by the experimental results, showing that 2-atomicity is "good enough" in distributed storage systems by achieving low latency, bounded staleness, and rare data inconsistencies.

cs.DC↗

arXiv2015

On small bases for which $1$ has countably many expansions

Let $q\in(1,2)$. A $q$-expansion of a number $x$ in $[0,\frac{1}{q-1}]$ is a sequence $(δ_i)_{i=1}^\infty\in\{0,1\}^{\mathbb{N}}$ satisfying $$ x=\sum_{i=1}^\infty\frac{δ_i}{q^i}.$$ Let $\mathcal{B}_{\aleph_0}$ denote the set of $q$ for which there exists $x$ with a countable number of $q$-expansions, and let $\mathcal{B}_{1, \aleph_0}$ denote the set of $q$ for which $1$ has a countable number of $q$-expansions. In \cite{Sidorov6} it was shown that $\min\mathcal{B}_{\aleph_0}=\min\mathcal{B}_{1,\aleph_0}=\frac{1+\sqrt{5}}{2},$ and in \cite{Baker} it was shown that $\mathcal{B}_{\aleph_0}\cap(\frac{1+\sqrt{5}}{2}, q_1]=\{ q_1\}$, where $q_1(\approx1.64541)$ is the positive root of $x^6-x^4-x^3-2x^2-x-1=0$. In this paper we show that the second smallest point of $\mathcal{B}_{1,\aleph_0}$ is $q_3(\approx1.68042)$, the positive root of $x^5-x^4-x^3-x+1=0$. Enroute to proving this result we show that $\mathcal{B}_{\aleph_0}\cap(q_1, q_3]=\{ q_2, q_3\}$, where $q_2(\approx1.65462)$ is the positive root of $x^6-2x^4-x^3-1=0$.

math.NT↗

arXiv2013

Low-temperature fabrication of brown TiO2 with enhanced photocatalytic activities under visible light

Titanium dioxide is a photocatalytic substance of great practical importance. However, with its bandgap in the ultraviolet (UV) regime, native forms (undoped) of TiO2 generally exhibits poor photocatalytic activities under visible light. Here we report a facile one-step low-temperature method to treat native TiO2 with NaH in a solution-based protocol. The NaH treatment effectively induces the Ti(III) species and oxygen vacancies into the TiO2 host lattice, and enables the bandgap of TiO2 to be conveniently adjusted from the UV region to the red end of the visible spectrum. The modified TiO2 exhibited significantly enhanced photocatalytic capability under visible light, and lead to faster photo-degradation of organic chemical material. Compared with other ways to reduce the bandgap of TiO2, the approach reported here provides unique advantages for safe, large-scale and economic production of narrow-bandgap TiO2 materials.

physics.chem-ph↗

arXiv2013

Verifying PRAM Consistency over Read/Write Traces of Data Replicas

Data replication technologies enable efficient and highly-available data access, thus gaining more and more interests in both the academia and the industry. However, data replication introduces the problem of data consistency. Modern commercial data replication systems often provide weak consistency for high availability under certain failure scenarios. An important weak consistency is Pipelined-RAM (PRAM) consistency. It allows different processes to hold different views of data. To determine whether a data replication system indeed provides PRAM consistency, we study the problem of Verifying PRAM Consistency over read/write traces (or VPC, for short). We first identify four variants of VPC according to a) whether there are Multiple shared variables (or one Single variable), and b) whether write operations can assign Duplicate values (or only Unique values) for each shared variable; the four variants are labeled VPC-SU, VPC-MU, VPC-SD, and VPC-MD. Second, we present a simple VPC-MU algorithm, called RW-CLOSURE. It constructs an operation graph $\mathcal{G}$ by iteratively adding edges according to three rules. Its time complexity is $O(n^5)$, where n is the number of operations in the trace. Third, we present an improved VPC-MU algorithm, called READ-CENTRIC, with time complexity $O(n^4)$. Basically it attempts to construct the operation graph $\mathcal{G}$ in an incremental and efficient way. Its correctness is based on that of RW-CLOSURE. Finally, we prove that VPC-SD (so is VPC-MD) is $\sf{NP}$-complete by reducing the strongly $\sf{NP}$-complete problem 3-PARTITION to it.

cs.DC↗

arXiv2013

Want a Good Answer? Ask a Good Question First!

Community Question Answering (CQA) websites have become valuable repositories which host a massive volume of human knowledge. To maximize the utility of such knowledge, it is essential to evaluate the quality of an existing question or answer, especially soon after it is posted on the CQA website. In this paper, we study the problem of inferring the quality of questions and answers through a case study of a software CQA (Stack Overflow). Our key finding is that the quality of an answer is strongly positively correlated with that of its question. Armed with this observation, we propose a family of algorithms to jointly predict the quality of questions and answers, for both quantifying numerical quality scores and differentiating the high-quality questions/answers from those of low quality. We conduct extensive experimental evaluations to demonstrate the effectiveness and efficiency of our methods.

cs.DB↗

arXiv2013

Enabling Context-awareness by Predicate Detection in Asynchronous Pervasive Computing Environments

Pervasive applications are involving more and more autonomous computing and communicating devices, augmented with the abilities of sensing and controlling the logical / physical environment. To enable context-awareness for such applications, we are challenged by the intrinsic asynchrony among the context collecting devices. To this end, we introduce the predicate detection theory and propose the Predicate-Detection-based Context-Awareness (PD-CA) framework, in which: a) logical time is used to explicitly cope with the asynchrony; b) specification of predicates enables the applications to express contextual properties of their concerns; c) online and incremental predicate detection algorithms effectively enable context-awareness at runtime. Under the guidance of the PD-CA framework, we present the design and implementation of the MIPA middleware, which shields the applications from the burden of processing the asynchronous contexts. We also demonstrate how PD-CA simplifies the development of context-aware applications. Experimental evaluations show the performance of MIPA in supporting context-aware applications despite of the asynchrony.

cs.DC↗

arXiv2012

MaTrust: An Effective Multi-Aspect Trust Inference Model

Trust is a fundamental concept in many real-world applications such as e-commerce and peer-to-peer networks. In these applications, users can generate local opinions about the counterparts based on direct experiences, and these opinions can then be aggregated to build trust among unknown users. The mechanism to build new trust relationships based on existing ones is referred to as trust inference. State-of-the-art trust inference approaches employ the transitivity property of trust by propagating trust along connected users. In this paper, we propose a novel trust inference model (MaTrust) by exploring an equally important property of trust, i.e., the multi-aspect property. MaTrust directly characterizes multiple latent factors for each trustor and trustee from the locally-generated trust relationships. Furthermore, it can naturally incorporate prior knowledge as specified factors. These factors in turn serve as the basis to infer the unseen trustworthiness scores. Experimental evaluations on real data sets show that the proposed MaTrust significantly outperforms several benchmark trust inference models in both effectiveness and efficiency.

cs.DB↗

arXiv2011

Design of a Sliding Window over Asynchronous Event Streams

The proliferation of sensing and monitoring applications motivates adoption of the event stream model of computation. Though sliding windows are widely used to facilitate effective event stream processing, it is greatly challenged when the event sources are distributed and asynchronous. To address this challenge, we first show that the snapshots of the asynchronous event streams within the sliding window form a convex distributive lattice (denoted by Lat-Win). Then we propose an algorithm to maintain Lat-Win at runtime. The Lat-Win maintenance algorithm is implemented and evaluated on the open-source context-aware middleware we developed. The evaluation results first show the necessity of adopting sliding windows over asynchronous event streams. Then they show the performance of detecting specified predicates within Lat-Win, even when faced with dynamic changes in the computing environment.

cs.DC↗

arXiv2011

Spin Selective Purcell Effect in a Quantum Dot Microcavity System

We demonstrate the selective coupling of a single quantum dot exciton spin state with the cavity mode in a quantum dot-micropillar cavity system. By tuning an external magnetic field, the Zeeman splitted exciton spin states coupled differently with the cavity due to field manipulated energy detuning. We found a 26 times increase in the emission intensity of spin-up exciton state with respect to spin-down exciton state at resonance due to Purcell effect, which gives rise to the selective enhancement of light emission with the circular polarization degree up to 93%. A four-level rate equation model is developed and quantitatively agrees well with our experimental data. Our results pave the way for the realization of future quantum light sources and the quantum information processing applications.

physics.optics↗

arXiv2009

Checking Behavioral Consistency Constraints for Pervasive Context in Asynchronous Environments

Context consistency checking, the checking of specified constraint on properties of contexts, is essential to context-aware applications. In order to delineate and adapt to dynamic changes in the pervasive computing environment, context-aware applications often need to specify and check behavioral consistency constraints over the contexts. This problem is challenging mainly due to the distributed and asynchronous nature of pervasive computing environments. Specifically, the critical issue in checking behavioral constraints is the temporal ordering of contextual activities. The contextual activities usually involve multiple context collecting devices, which are fully-decentralized and interact in an asynchronous manner. However, existing context consistency checking schemes do not work in asynchronous environments, since they implicitly assume the availability of a global clock or relay on synchronized interactions. To this end, we propose the Ordering Global Activity (OGA) algorithm, which detects the ordering of the global activities based on predicate detection in asynchronous environments. The essence of our approach is the message causality and its on-the-fly coding as logic vector clocks in asynchronous environments. We implement the Middleware Infrastructure for Predicate detection in Asynchronous environments (MIPA), over which the OGA algorithm is implemented and evaluated. The evaluation results show the impact of asynchrony on the checking of behavioral consistency constraints, which justifies the primary motivation of our work. They also show that OGA can achieve accurate checking of behavioral consistency constraints in dynamic pervasive computing environments.

cs.SE↗

arXiv2008

High Quality Factor Silicon Cantilever Driven by PZT Actuator for Resonant Based Mass Detection

A high quality factor (Q-factor) piezoelectric lead zirconat titanate (PZT) actuated single crystal silicon cantilever was proposed in this paper for resonant based ultra-sensitive mass detection. Energy dissipation from intrinsic mechanical loss of the PZT film was successfully compressed by separating the PZT actuator from resonant structure. Excellent Q-factor, which is several times larger than conventional PZT cantilever, was achieved under both atmospheric pressure and reduced pressures. For a 30 micrometer-wide 100 micrometer-long cantilever, Q-factor was measured as high as 1113 and 7279 under the pressure of 101.2 KPa and 35 Pa, respectively. Moreover, it was found that high-mode vibration can be realized by the cantilever for the pursuit of great Q-factor, while support loss became significant because of the increased vibration amplitude at the actuation point. An optimized structure using node-point actuation was suggested then to suppress corresponding energy dissipation.

cs.OH↗