A Machine Learning Approach to POS Tagging We have applied the inductive learning of statistical decision trees and relaxation labeling to the Natural Language Processing (NLP) task of morphosyntactic disambiguation (Part Of Speech Tagging). The learning process is supervised and obtains a language model oriented to resolve POS ambiguities, consisting of a set of statistical decision trees expressing distribution of tags and words in some relevant contexts. The acquired decision trees have been directly used in a tagger that is both relatively simple and fast, and which has been tested and evaluated on the Wall Street Journal (WSJ) corpus with competitive accuracy. However, better results can be obtained by translating the trees into rules to feed a flexible relaxation labeling based tagger. In this direction we describe a tagger which is able to use information of any kind (n-grams, automatically acquired constraints, linguistically motivated manually written constraints, etc.), and in particular to incorporate the machine-learned decision trees. Simultaneously, we address the problem of tagging when only limited training material is available, which is crucial in any process of constructing, from scratch, an annotated corpus. We show that high levels of accuracy can be achieved with our system in this situation, and report some results obtained when using it to develop a 5.5 million words Spanish corpus from scratch. Part of Speech (POS) Tagging is a very basic and well known Natural Language Processing (NLP) problem which consists of assigning to each word of a text the proper morphosyntactic tag in its context of appearance. It is very useful for a number of NLP applications: as a preprocessing step to syntactic parsing, in information extraction and retrieval (e.g. document classification in internet searchers), text to speech systems, corpus linguistics, etc.. The base of POS tagging is that many words being ambiguous regarding their POS, in most cases they can be completely disambiguated by taking into account an adequate context. For instance, in the sample sentence presented in Table 1 , the word shot is disambiguated as a past participle because it is preceded by the auxiliary was. Although in this case the word is disambiguated simply by looking at the preceding tag, it must be taken into account that the preceding word could be ambiguous, or that the necessary context could be much more complicated than merely the preceding word. Furthermore, there are even cases in which the ambiguity is non-resolvable using only morphosyntactic features of the context, and require semantic and/or pragmatic knowledge. The comparison of POS taggers is a delicate issue since many factors can affect the final accuracy figures: different sizes for training and test sets, evaluation over different corporaand thus, different tag distributions-, different tag sets, etc. Even when comparing systems under similar conditions, results might be non-conclusive if noisy material is used for the test (see , for details). Nevertheless, an orienting comparison between our system and other current taggersusing the same corpus (WSJ) and training and test sets with similar sizes-can be sketched. To do so, we selected a representative sample of the most accurate statistical and machinelearning taggers that can be found in the literature: Daelemans et al. (1996) report an accuracy of 96.4% (which, taking into account that 5.5% of the test set are unknown words, with an accuracy rate of 90.6%, yields a figure of about 96.7% over known words). Similarly, Brill (1994) obtains a 97.2% over known words, Weischedel et al. (1993) get 96.7% and Ratnaparkhi (1996) reports 96.7%.. Although our figures are higher than the ones reported by the previous systems-we obtained 97.36% on known words over WSJ with similar training sizes-, we believe that quantitative comparison is not conclusive in this case, since conditions are not similar enough to enable us to state that the reported difference in accuracy is significant.. Nevertheless, we do believe that a qualitative comparison can be performed, and that our system presents the following advantages:. • Flexibility: It enables a rich feature representation, which can easily incorporate different sources of available information (morhpological, syntax, semantics, etc.) . It also provides a numerical weight or probability estimation for each tag.. • Decision Trees Acquisition: A possible way of acquiring statistical knowledge is the presented algorithm for DT acquisition, which has been proved to be more accurate and compact than other representations-such that of Daelemans et al. (1996) -for the POS tagging task.. • Linguistic Knowledge: Our system is able to deal with linguistic knowledge when expressed in the form of regular context constraints. Thus, it is able to take advantage of the robustness of statistical systems and the coverage of infrequent cases that linguistic rules can provide.. • Simultaneous disambiguation: It also enables the simultaneous disambiguation of different features (i.e. POS tags, senses, etc.) as described by Padró (1998) .. The main drawback of our hybrid system is the computational cost of applying the context rules. This causes a speed vs. accuracy trade-off which may make the system unsuitable for cases in which high tagging speed and high accuracy are required.. Note that the above comparisons only mention taggers based on automatic model acquisition. We have intentionally avoided the comparison with linguistic taggers because we think that both approaches such be considered in a complementary-rather than exclusive-way. It is a fact that years of work on machine learning for tagging have resulted in taggers that significantly underperform the linguistic taggers based on hand-written rules in the style of Constraint Grammars (Karlsson et al., 1995) . We think that automatic taggers will never be able to fully compete with manual linguistic taggers. This may be explained by the coverage of infrequent cases and exceptions which-due to their low frequency-are much difficult to acquire in a statistical approach. However, it is also accepted that linguistic taggers have a much higher acquisition cost and that they are less portable. The hybrid approach takes advantage of both models, achieving a more adequate balance of acquisition cost and accuracy.

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Random Access Broadcast: Stability and Throughput Analysis A wireless network in which packets are broadcast to a group of receivers through use of a random access protocol is considered in this work. The relation to previous work on networks of interacting queues is discussed and subsequently, the stability and throughput regions of the system are analyzed and presented. A simple network of two source nodes and two destination nodes is considered first. The broadcast service process is analyzed assuming a channel that allows for packet capture and multipacket reception. In this small network, the stability and throughput regions are observed to coincide. The same problem for a network with N sources and M destinations is considered next. The channel model is simplified in that multipacket reception is no longer permitted. Bounds on the stability region are developed using the concept of stability rank and the throughput region of the system is compared to the bounds. Our results show that as the number of destination nodes increases, the stability and throughput regions diminish. Additionally, a previous conjecture that the stability and throughput regions coincide for a network of arbitrarily many sources is supported for a broadcast scenario by the results presented in this work. In this work we have investigated the information-carrying ability of source nodes which compete for access to a shared channel in order to broadcast messages to a common set of destination nodes.. The scenario we consider is relevant to broadcast applications and network discovery and control. We formulated the problem as a network of queues which interact through the shared channel and investigated the stability and throughput regions of the source nodes. Our results strengthen an unproven conjecture that the stability and throughput regions of finite-user random access coincide. We also demonstrate that the throughput and stable throughput regions diminish as the number of source or destination nodes grows. Our intention in this work is to present results relating to the scaling laws for wireless networks, which often show a degradation in throughput as the number of nodes increases. Our results show a similar trend from a multicast stability point of view.

B I O B O B O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O B O O O O O O O B O O O B O O O O O O O O O O O O O O O O O O O O O B O B O O O O O O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O B O O O O O O O O O O O O O B O B O O O B I O O O O O O B O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O B O O O


Security of social information from query analysis in DaaS Databases as a Service (DaaS) are recently in the spotlight as an essential component of the cloud computing framework. We can more easily develop applications to facilitate users' collaboration than without DaaS. Unfortunately, DaaS brings a new risk of data compromise because most data are currently stored and managed by the service provider.Securing users' information, the data are generally encrypted at a trusted client. Although encryption ensures that no one can sneak a look at the data, malicious users still have opportunities to take users' relational information. The relational information, which we name social information as opposed to personal information, has not been adequately examined, but it is becoming important because of improvement of social analysis. As described herein, we rst introduce an attack model of obtaining social information from analyses of query logs in DaaS; then we provide a solution to the problem. Our method converts some dierent queries in the same query, so that malicious attackers cannnot analyze the query logs. We also dene an overhead cost of the translation and propose a method that can optimize the conversion using extensible hashing. Database as a Service (DaaS) is a component of the cloud computing framework and it is attracting someone's atten- As described herein, we introduce a new privacy problem security of social information to produce safer DaaS than that which services already provide. Social information is information about relation between users: it diers from personal information because personal information is information identifying individual users. Therefore, it has been treated as unimportant. Nevertheless, many methods of analyzing social networks are studied so that the relation information can present risk. We believe that discussing social information is important to retain privacy.. Based on these situations, we introduce an attack method of exposing social information from analyzing queries. We propose a method to defend the attack by rewriting the queries. Our proposal has two parameters related to safety and cost; users can choose them according to their demand.. Moreover, we evaluate our proposal using an experiment based on real information: an open directory. Based on the result, a specic pair of parameters yields high security and low overhead. However, our method must incur large costs depending on circumstances: more improvement is necessary.. We envision our future work as improvement of the attack method and query conversion to protect privacy from stronger attacks.

O O B I O O O O B O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O B O O O B I O O O B O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O B O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O


On smoothing three-dimensional Monte Carlo ion implantation simulation results Abstract-An algorithm for smoothing results of three-dimensional (3-D) Monte Carlo ion implantation simulations and translating them from the grid used for the Monte Carlo simulation to an arbitrary unstructured 3-D grid is presented. This algorithm is important for joining various process simulation steps, where data have to be smoothed or transferred from one grid to another. Furthermore, it is important for integrating the ion implantation simulator into a process flow. One reason for using different grids is that for certain Monte Carlo simulation methods, using orthogrids is mandatory because of performance reasons.The algorithm presented sweeps a small rectangular grid over the points of the new tetrahedral grid and uses approximation by generalized Bernstein polynomials. This approach was put on a mathematically sound basis by proving several properties of these polynomials. It does not suffer from the adverse effects of least squares fits of polynomials of fixed degree as known from the response surface method.The most important properties of Bernstein polynomials generalized to cuboid domains are presented, including uniform convergence, an asymptotic formula, and the variation diminishing property. The smoothing algorithm which works very fast is described and, in order to show its applicability, the resulting values of a 3-D real world implantation example are given and compared with those of a least squares fit of a multivariate polynomial of degree two, which yielded unusable results. A FTER a Monte Carlo simulation of ion implantation on an orthogrid, the question arises how to translate the resulting values, i.e., concentrations, to an unstructured grid. In the Monte Carlo simulation, an orthogrid is commonly used in order to achieve workable simulation times, since calculating point locations, i.e., tracing the position of ions, dominates performance. For other subsequent simulations via, e.g., the finite element method, it is mandatory to use different, unstructured grids. Furthermore, the resulting values have to be smoothed in order to provide suitable input for the simulation of subsequent process steps like diffusion.. Thus, an algorithm for smoothing Monte Carlo ion implantation results has to meet the following demands.. • It has to work with unstructured target grids. • It must provide suitable smoothing.. • Since the number of grid points in the target grid is usually large, it must not be computationally expensive. One simple approach is to perform a least squares fit of a multivariate polynomial of a fixed degree, usually two, and to hope that this polynomial is a suitable approximation providing proper smoothing. This is known as the response surface methodology (RSM) [1] approach and has been used to a great extent in computer-aided design applications, but it often does not work satisfactorily. In order to solve this problem, generalizations of Bernstein polynomials were devised and their properties proven. Hence, a fast algorithm based on these polynomials was developed and applied to real world examples, where its advantages can be seen. Before describing this method, we will have a closer look at the RSM approach. The RSM approach will be compared with the proposed algorithm since least squares fits are a popular method.. The RSM approach can be summarized as follows. Let be a continuous function on a multidimensional interval . On the interval , a rectangular grid with points is chosen. The RSM approximation of is the multivariate polynomial of a degree two or higher fixed degree, which is determined by a least squares fit such that is minimal. Computing the coefficients of is a well-known procedure [1] . RSM has been used extensively in computeraided design applications, e.g., in [2] - [13] .. Although it can be argued that the RSM approximation is based on a truncated Taylor series expansion for a multivariate function , it is important to note that this is a local approximation and quite different from a least squares fit for several points. In the Taylor series expansion, convergence occurs when the number of terms and, thus, the degree of the polynomial increases, whereas in the RSM approach, the degree of the approximating polynomial is fixed to an arbitrary low value. Increasing the degree is possible of course, but the choice is still arbitrary and the number of coefficients and, thus, the number of points required for the least squares fit increases abundantly.. Furthermore, the RSM suffers from the fact that a polynomial of fixed degree cannot preserve the global properties of the original function: the set of all polynomials of a certain fixed maximal degree is not dense in , compact. There 0278-0070/03$17.00 © 2003 IEEE do not exist any rigorous statements about approximating or smoothing properties. Moreover, using more and more points for the least squares fit is not a remedy and generally does not improve the RSM polynomial, while the computational effort is increased. A simple example for this fact is the exponential function.. Although the RSM approach can be improved by transforming the variables before fitting the polynomials, it has to be known a priori which transformations are useful and should be considered. If this knowledge is available, it can of course be applied to other approximation approaches as well.. Finally, an advantage of the RSM approach is the simple structure of the approximations: it is easy to deal with polynomials of degree two. However, in the algorithm proposed in the following, no polynomials have to be constructed explicitly and the computational effort for doing least squares fits is eliminated as well.. In the following, the properties of generalized Bernstein polynomials are discussed and the algorithm is described in detail. Finally, the two approaches are compared by looking at the results of a three-dimensional (3-D) ion implantation example with about 80 000 grid points. In summary, the properties of polynomials of fixed degree arising from least square fits were compared to those of general-ized Bernstein polynomials on multidimensional intervals. The properties of the Bernstein polynomials were proven and presented, and it was found that these fulfill the requirements for approximations needed for smoothing Monte Carlo simulation results and translating them from ion implantation orthogrids to unstructured grids.. The polynomials and the algorithm devised provide the following benefits. First, they converge uniformly when the number of base points goes to infinity. Second, an asymptotic formula gives information about their rate of convergence. Third, total variation is decreased and the approximations do not oscillate more often about any straight line than the original function. This assures suitable smoothing. Fourth, the algorithm works very fast and is easy to implement using the specialized formula given, since the calculation of the actual approximating polynomials is avoided.. Finally, the new algorithm and its RSM counterpart were compared in a real world Monte Carlo ion implantation example, and the new algorithm was found to yield superior results, which can immediately be used for further simulations.

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Reasoning on temporal class diagrams: Undecidability results This paper introduces a temporal class diagram language useful to model temporal varying data. The atemporal portion of the language contains the core constructors available in both EER diagrams and UML class diagrams. The temporal part of the language is able to distinguish between temporal and atemporal constructs, and it has the ability to represent dynamic constraints between classes. The language is characterized by a model-theoretic (temporal) semantics. Reasoning services as logical implication and satisfiability are also defined. We show that reasoning on finite models is different from reasoning on unrestricted ones. Then, we prove that reasoning on temporal class diagrams is an undecidable problem on both unrestricted models and on finite ones. Conceptual data modeling describes an application domain in a declarative and reusable way by specifying constraints on the use of the data and possibly drawing new information from it. Recently, a number of conceptual modeling languages has emerged as de facto standards; in particular, we mention entity-relationship (ER) for the relational data model, UML and ODMG for the object-oriented data model, and RDF, DAML+OIL and OWL for the web ontology languages. In this paper we deal with temporally extended conceptual data models developed to abstract the temporal aspects of information. Many temporal models have been developed (in particular to help designing temporal databases) and a summary of results achieved in the area can be found in two good surveys [14, 17] .. Here we propose a temporal class diagram formalism equipped with both a linear and a graphical syntax along with a model-theoretic semantics. The atemporal portion of the language contains the core constructors available in most of the conceptual models mentioned above. Essentially, Classes and Relationships are first class citizens. Classes can be organized in disjoint and/or covering generalized hierarchies. Relations between classes are modeled through n-ary relationships. Full cardinality constraints can be specified on the participation of classes into relationships. The temporal part of the language supports valid time for classes and relationships in the line of TIMEER [13] and ERT [20] , while supporting dynamic constraints for classes as presented in MADS [19] . This paper moves from previous works of the author where the temporal conceptual model ER VT has been formally characterized [1, 4] . Starting from such formalization we devise here a temporal modeling language as a sub-language of ER VT with the main intention to investigate whether reasoning over temporal diagrams is decidable. In addition to the classical EER constructors (the Extended EntityRelationship data model, see [11] ), the language proposed here is able to express the following temporal constraints:. Timestamping. The data model distinguishes between snapshot constructs Y i.e., each of their instances has a global lifespan Y and temporary constructs Y i.e., each of their instances have a limited lifespan. Dynamic Constructs. They apply to classes by capturing the object migration from a source class to a target class. They are also called transition constraints [15] and they describe how an object can change its class membership from one class to another. For example, an object in the Employee class may later migrate to become an object of the Manager class.. The main result illustrated here is that reasoning on temporal conceptual models is undecidable providing the diagrams are able both: (a) To distinguish between temporal and non-temporal constructs; and (b) to represent dynamic constraints between classes, i.e., classes whose instances migrate to other classes. This result is different from a similar one presented in [5] . Indeed, in [5] the authors showed that temporal diagrams expressed in the ER VT modeling language can be embedded into the temporal description logic DLR US Y where U, S extend DLR [8] with the until and since temporal modalities Y and that reasoning in DLR US was undecidable. On the other hand, here we prove that even reasoning just on temporal class diagrams (and thus on ER VT schemas) is undecidable. The undecidability result is proved via a reduction of the Halting Problem. In particular, we proceed by first showing that the halting problem can be encoded as a Knowledge Base (KB) in ALC F Y where F extends the description logic ALC with the future temporal modality Y and then proving that such a KB in ALC F can be captured by a schema expressed in our temporal class diagram. Note that, in [12] the undecidability of ALC F is proved using both: (a) Complex axioms Y i.e., Axioms can be combined using boolean and modal operators; and (b) both global and local axioms Y i.e., Axioms can be either true at all time or true at some time, respectively. Since the temporal class diagram is able to encode just simple global axioms, we modify the proof presented in [12] by showing that checking concept satisfiability w.r.t. an ALC F KB made by just simple global axioms is an undecidable problem. This new result on temporal description logics reduces the gap between decidable and undecidable languages.. We also show that temporal class diagrams do not enjoy the Finite Model Property (FMP) (that is, a class could be satisfiable only in models with an infinite domain). The negative undecidability result also holds when reasoning is restricted to finite models. Still the halting problem can be reduced to satisfiability of temporal class diagrams in finite models.. The paper is organized as follow. The temporal description logic ALC F is briefly introduced in section 2. Section 3 gives a formal presentation of temporal class diagrams along with a running example. The reasoning services over temporal class diagrams are defined in section 4. That reasoning in presence of dynamic constraints is undecidable in both unrestricted and finite models is proved in sections 5 and 6, respectively. Section 7 makes final conclusions and summarizes the complexity results already obtained when dealing with temporal data languages with different expressivity power. In the conclusions an interesting open problem related to the complexity of reasoning is finally mentioned. We formally introduced a data modeling language useful to represent timevarying data. The language is equipped with a linear and graphical syntax and a model-theoretic semantics. A relevant aspect of the proposed formalism is the possibility to formally specify reasoning tasks based on the associated semantics. Reasoning problems as class, relationship and schema satisfiability and logical implication have been described.. We then investigated the complexity of reasoning on temporal models and we found that such problem is undecidable as soon as the language is able to distinguish between temporal and atemporal constructs (in particular, whether the language captures temporal relationships) and has the ability to represent dynamic constraints between classes. While temporal class diagrams do not enjoy the finite model property we prove that even reasoning on finite models is undecidable.. The main reason behind the undecidability result is the possibility to postulate that a binary relation does not vary in time. Indeed, it has been shown in [5] that temporal diagrams expressed in the ER VT modeling language can be embedded into the temporal description logic DLR US . While DLR US is undecidable, the fragment, DLR À US , of DLR US deprived of the ability to talk about temporal persistence of n-ary relations, for n U _ 2 2, is decidable. Indeed, reasoning in DLR À US is an EXPTIMEcomplete problem [5] . This result gives us an useful scenario where reasoning over temporal schemas becomes decidable. In particular, if we forbid timestamping for relationships (i.e., relationships are just unmarked) reasoning on temporal models with both concept timestamping and full evolution constraints can be reduced to reasoning over DLR À US . The problem of reasoning in this setting is complete for EXPTIME since the EXPTIME-complete problem of reasoning with ALC knowledge bases can be captured by such schemas [7] .. It is an open problem whether reasoning is still decidable by regaining timestamping for relationships (and maintaining timestamping for classes) but dropping evolution constraints altogether. We have a strong feeling that this represents a decidable scenario since it is possible to encode temporal schemas without evolution constraints by using a combination between the description logic DLR and the epistemic modal logic S5. Decidability results have been proved for the sub-logic ALC S5 [12] . But, it is an open problem whether this result still holds for the more complex logic DLR S5 .

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


The labor economics of paid crowdsourcing We present a model of workers supplying labor to paid crowdsourcing projects. We also introduce a novel method for estimating a worker's reservation wage-the key parameter in our labor supply model. We tested our model by presenting experimental subjects with real-effort work scenarios that varied in the offered payment and difficulty. As predicted, subjects worked less when the pay was lower. However, they did not work less when the task was more time-consuming. Interestingly, at least some subjects appear to be "target earners," contrary to the assumptions of the rational model. The strongest evidence for target earning is an observed preference for earning total amounts evenly divisible by 5, presumably because these amounts make good targets. Despite its predictive failures, we calibrate our model with data pooled from both experiments. We find that the reservation wages of our sample are approximately log normally distributed, with a median wage of $1.38/hour. We discuss how to use our calibrated model in applications. Crowdsourcing is a form of "peer production" [2] that outsources work traditionally performed by an employee to an "undefined, generally large group of people in the form of an open call" [11] . Despite the successes and the perceived Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. EC '10, June 7-11, 2010 , Cambridge, Massachusetts, USA. Copyright 2010 ACM 978-1-60558-822-3/10/06 ...$10.00.. promise of crowdsourcing, 1 would-be users face a serious practical challenge: they need to attract a crowd. Crowdsourcing projects have used a variety of inducements, including entertainment [21] , information [1, 13] , and the chance to be altruistic and win attention from others [12] . Until recently, it was extremely difficult to offer money to a crowd, but with the advent of online labor markets such as oDesk, Elance and Amazon's Mechanical Turk (AMT), buyers can now easily pay workers with cash [8] .. Compared to cash, non-monetary crowdsourcing incentives are limited in at least two ways. First, some nonmonetary incentives depend on the nature of the task or the identity of the proposer, which limits their usefulness. For example, tasks such as classifying web pages, transcribing scanned documents and validating search results are ideal for crowdsourcing, yet they are unlikely to attract volunteers because they are tedious and the private benefits come only to the proposer. Second, even when appropriate non-monetary incentives exist, they are hard to adjust. For example, assuming that we could compute a "fun" labor supply elasticity, , it is not clear how we could make a task 10(1/ )% more fun in order to boost provision by 10%. With cash incentives, computing supply elasticities is straightforward and making price adjustments is easy.. The possibility of cash payments raises a design question: how does a would-be user of crowdsourcing effectively and efficiently employ monetary incentives? To solve this problem, designers need two things: (1) a theoretical model that predicts how people will respond to different price/task scenarios and (2) data-driven refinements of that model. The refinements should allow the model to account for behavioral biases, such as those identified by experimental economics, and the idiosyncrasies of particular applications. To borrow an analogy from the economist and market designer Al Roth, just as builders of suspension bridges must be informed by both the elegant theory of mechanics and the messy details of metallurgy and geology, builders of social systems must possess both a general theory of behavior and detailed contextual knowledge [18] .. An appropriate theoretical model for the labor supply aspect of crowdsourcing design should tell the designer (1) how workers decide whether or not to participate in a crowdsourcing project and (2) how workers decide the amount to produce, conditional upon participating. In the language of economics, the designer must be able to predict the labor supply on both the (1) extensive and (2) intensive margins.. These two aspects of labor supply have intrigued economists from Adam Smith onward, but research in conventional labor economics is unlikely to directly apply to crowdsourcing "jobs" that last seconds and pay pennies. This caveat aside, labor economics offers a theoretical framework for understanding decision-making in paid crowdsourcing scenarios, and it potentially provides the predictive model that designers need. There are several papers on the design of incentives in crowdsourcing [5] and on obtaining work from AMT [19, 20] , but our paper is most closely related to work by Watts and Mason [17] , who also conducted labor supply experiments. They found that workers respond to prices in a way that is at least consistent with rational behavior. However, their findings also suggest that the relationship between incentives and output is complex: higher pay rates did not improve work quality-a result the authors attribute to high wages affecting worker beliefs. They also found that the structure of incentives (i.e., whether workers faced a piece-rate or a quota system) affected output.. Motivated by the evidence from [17] that AMT workers respond to cash incentives, in Section 2.1 we develop a simple rational model of crowdsourcing labor supply. We also present a novel method for estimating the reservation wage of each worker (Section 2.3) that makes use of a highly concave earnings function rather than a simple piece-rate. The reservation wage is the minimum wage a worker is willing to accept as compensation in exchange for performing some task; it is the key parameter in models of labor supply.. With subjects recruited from AMT, we tested the predictions of the model in two separate experiments. In both experiments, subjects could choose how much output to produce. We tested whether output fell when we made the task more difficult (Experiment A, Section 4) and whether output fell when we lowered the price (Experiment B, Section 5). We find mixed evidence for the rational model: workers are clearly sensitive to price, but they are insensitive to variations in the amount of time it takes to complete a task.. According to the model, reservation wages are fixed and thus should not be affected by our experimental manipulations. Although we find that the imputed wage distributions are quite similar in Experiment A, large differences appear in Experiment B. The cause seems to be that some workers are "target earners" who focus on reaching salient earnings targets. This stands in sharp contrast to the rational model that predicts that workers should consider only the offered wage. These findings have important implications for the design of incentives and are discussed in Section 6.. The rational model clearly misses important elements of reality, but there are no immediate replacements for the model, and it makes reasonable predictions in some cases. For these reasons, we demonstrate in Section 8 how the model can be used to predict labor supply for any price/task scenario, using calibration data pooled from both experiments. We conclude with a discussion of the contributions and limitations of the paper and our thoughts on directions for future research. We find some agreement with a simple rational model, as well as important anomalies; we find fairly strong evidence that at least some workers work to targets. Designers should consider this propensity when designing incentives schemes and give people natural targets that will increase output, though they should also consider that such schemes might seem manipulative and could backfire (and potentially be unethical).. In the following section, we demonstrate how our calibrated model can be used in applied work. While we find only partial agreement between the model's predictions and reality, we believe it still offers a useful approximation. We conclude by discussing how our paper fits into a larger research program and laying out some directions for future investigations. The experimental results are theoretically ambiguous but are internally consistent. Given that workers in HARD took longer per task but had essentially the same output pattern as subjects in EASY, we would expect subjects in HARD to have relatively higher reservation wages, which is what we find, albeit the effect has a t-statistic of only 1.41.. Perhaps the simplest explanation for the lack of treatment effects is that our treatment was not strong enough and that workers are not attuned to fairly small differences in time. Lord Robbins' notion of the "marginal utility of not bothering with marginal utility" seems particular apt, especially if workers have a heuristic task-based (as opposed to timebased) reservation wage [16] .. As per the model's prediction, lower pay reduces output, but the finding that being in LOW lowers a worker's reservation wage implies that the lower output in LOW is not as low as it should be. Because the actual tasks are identical across groups, the difference in imputed reservation wages cannot be due to uncontrolled-for differences in amenities. There are several other possible explanations for the data, including worker error, increasing but non-time-based marginal costs and target earning behavior; we believe target earning provides the most compelling explanation for our findings.. Worker error could explain our results: we would find lower reservation wages in LOW if (1) subjects in LOW falsely believe that they are being paid more than they actually are, (2) subjects in HIGH falsely believe that they are being paid less than they actually are or (3) some combination of (1) and (2). Liebman and Zeckhauser [15] argue that people systematically misinterpret schedules, usually by letting early marginal payments "bleed over" to affect perceptions of future marginal payments. However, with our concave schedules, this bleed-over would occur in both groups. While we have no evidence on this question, it seems likely that any bleed-over effects would be greater in HIGH, which would lead to effects in the opposite direction of what we do find. Furthermore, because subjects were informed of their precise marginal payment after each block, this bleed-over explanation seems unlikely.. Constant marginal costs are a key assumption in our reservation wage estimation method. If marginal costs are rapidly increasing, then we would incorrectly infer that subjects in LOW had reservation wages too low: the additional output that subjects in HIGH should produce because of their higher wages is moderated by the higher costs of high output. Although an explanation involving increasing marginal cost is possible, it seems unlikely, given that subjects could rest for as much time as they liked between blocks. Furthermore, even subjects producing lots of output spent less than 15 minutes total, including resting time (recall that per block time for the 100-pixel group was less than 7 seconds). Rapid increases in marginal costs over such a short time seem rather unlikely. 8 An additional explanation for the results is that some workers may be target earners. Target earners try to obtain some self-imposed earnings goal rather than respond to the current offered wage. If at least some workers are target earners, then the results are easy to explain: because their wages are lower, subjects in LOW must produce more output to achieve their earnings targets than they would have to produce if they were instead assigned to HIGH. .

O O O O O B O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


A New Caching Technique to Support Conjunctive Queries in P2P DHT SUMMARY P2P DHT (Peer-to-Peer Distributed Hash Table) is one of typical techniques for realizing an efficient management of shared resources distributed over a network and a keyword search over such networks in a fully distributed manner. In this paper, we propose a new method for supporting conjunctive queries in P2P DHT. The basic idea of the proposed technique is to share a global information on past trials by conducting a local caching of search results for conjunctive queries and by registering the fact to the global DHT. Such a result caching is expected to significantly reduce the amount of transmitted data compared with conventional schemes. The effect of the proposed method is experimentally evaluated by simulation. The result of experiments indicates that by using the proposed method, the amount of returned data is reduced by 60% compared with conventional P2P DHT which does not support conjunctive queries. key words: P2P DHT, conjunctive query, result cache An explosive spread of the information technology rapidly increases the number of users who acquire various information and resources distributed over the network. Such an increase of the number of users significantly enlarges data flows over the network, which severely concentrates the load of specific servers under conventional client/server (C/S) systems, with respect to the data storage to store the contents of shared objects and the CPU time for processing queries issued by the clients. As a means to overcome such serious situations, peer-to-peer systems (P2P) attention in recent years [1] , [8] .. P2P is a distributed system consisting of several computers called nodes or peers, which can communicate with each other through an underlying interconnection network. In P2P, contents of objects are distributed over the network, and managed by each peer participating in the system in a distributed manner. Hence, before accessing an object in P2P, each peer must learn about the name of peers who manages the contents of the object. Previous schemes to realize such an indexing to the objects could be classified into the following three types: hybrid type, flooding type, and P2P distributed hash table (P2P DHT).. In the hybrid type, all indexes are maintained by a centralized server called index server. Thus, each peer can easily acquire the index of the requested object by simply referring to the index server [10] , while it does not resolve the problem of overloading of specific servers in C/S systems. In the flooding type, the search of an object is realized by flooding a query message over an overlay network; i.e., upon receiving a query from an adjacent peer in the overlay; it notifies the search result to the originator of the query if it holds a matching object, and otherwise, it forwards a copy of the query to the other adjacent peers. The number of hops taken by a query is generally bounded by an appropriate value in order to prevent an unnecessary increase of the message traffic. Examples of the flooding type P2P include Gnutella [7] and Freenet , [5] . In contrast to the above two types, in P2P DHT, indexes to objects are stored in a logical space constructed over an overlay network, and the forwarding of query is systematically controlled to prevent an increase in the message traffic (a detailed explanation of P2P DHT will be given in the next section). Examples of P2P DHT include Chord [14] , CAN (Content-Addressable Network) [11] , Pastry [13] , and Tapestry [15] .. As will be described later, in P2P DHT, objects managed by the system are identified by an identifier (ID) given to the objects. Thus, we could surely obtain the contents of an object if we accurately specified the ID of the object. Although an ID may be the file name of the object, we can associate several keywords to each object as ID's, and use them to realize a keyword search (in the literature, such method is referred to as the reversed DHT [12] ), and in the following, we will focus our attention to P2P DHT supporting keyword search which is one of general applications for P2P DHTs. The main goal of our research is to realize an advanced keyword search engine for distributed resources (e.g., WWW resources) in a P2P fashion. A critical problem in realizing keyword search in an efficient manner is how to bound the number of objects matching the given keyword. The number of matching objects seriously increases as the number of peers participating in the system grows, which will make it difficult to find target objects that we're looking for. This problem could be (partially) solved by executing a conditional search such as the processing of conjunctive queries consisting of several keywords because the search result can be narrowed down by specifying multiple keywords. The technique for supporting such conjunctive queries is indispensable in keyword search systems and it has been normally supported in existing WWW search engines. Unfortunately however, most conventional P2P DHT approaches do not support conjunctive (or con- support conjunctive queries in P2P DHT. The basic idea of the method is to extend the original DHT in such a way that it keeps the index of peers who cache the result of conjunctive queries. Furthermore, we introduce the following two key techniques to increase the efficiency of the caching scheme:. • We will limit the maximum conjunction length in order to increase the memory utilization ratio. • We will introduce the notion of index navigator to urge the sharing of cached information.. In the following, we explain the above two points in detail. This section surveys conventional techniques to realize efficient keyword search in P2P DHT. An extension of the hash function to the function from the set of keywords to the hash table was originally proposed by Vahdat [12] . This method is referred to as the reversed DHT method in the literature. In the reversed DHT, each keyword associated with objects are mapped to a point in the hash table by an appropriate hash function, and a list of indexes associated with the keyword is stored at the calculated point. Thus, each user can acquire the index of objects associated with a keyword by applying hash function to the keyword, and by sending a query message to the calculated point. Note that P2P DHT considered in this paper is based on the revered DHT as was described in Sect. 2.. Bhattacharjee proposed a data structure called view tree to support an efficient processing of conjunctive queries in P2P DHT [2] . View tree realizes a result cache on P2P DHT, and efficiently supports the processing of conjunctive queries by maintaining cached conjunctions in a treestructured overlay network in a hierarchical manner. Each vertex in the tree keeps the search result for a conjunctive query similar to our proposed method. The tree is maintained in such a way that the parent vertex corresponds to a conjunction that is a prefix of the conjunction corresponding to a child vertex. When a conjunctive query is submitted to a view tree, it first examines if the result for the query is cached in the tree by conducting a tree traversal in a depth first manner. If it is cached, the requester could obtain the search result by simply refer to the corresponding vertex, and otherwise, the requester inserts a new vertex corresponding to the query to the view tree after obtaining the search result for the query (in a similar manner to the proposed scheme). View tree seems to be a natural realization of the result cache on P2P DHT. However, this method has the following serious drawbacks:. 1. It does not provide a means to directly access a vertex corresponding to a given conjunction. Thus, the view tree may contain several candidate vertices for a given conjunction, which causes frequent backtrackings during the depth first search. 2. To realize a depth first search, each vertex must keep the information about all children in the view tree. However, it is not practical, since the number of children of a vertex could be as large as the number of all keywords issued by the peers.

O O O O O O O O O B I O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O B I B I B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O B I O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


A Fully Conservative Second-Order Finite Difference Scheme for Incompressible Flow on Nonuniform Grids A second-order-accurate finite difference discretization of the incompressible Navier-Stokes is presented that discretely conserves mass, momentum, and kinetic energy (in the inviscid limit) in space and time. The method is thus completely free of numerical dissipation and potentially well suited to the direct numerical simulation or large-eddy simulation of turbulent flow. The method uses a staggered arrangement of velocity and pressure on a structured Cartesian grid and retains its discrete conservation properties for both uniform and nonuniform gird spacing. The predicted conservation properties are confirmed by inviscid simulations on both uniform and nonuniform grids. The capability of the method to resolve turbulent flow is demonstrated by repeating the turbulent channel flow simulations of H. Choi and P. Moin The direct numerical simulation (DNS) and large-eddy simulation (LES) of turbulent flow require accurate numerical methods to resolve the wide range of spatial and temporal scales inherent to turbulence. Experience has shown that it is not only the order of accuracy of the discrete approximation that determines the overall accuracy of a given method. Of equal or perhaps even greater importance are the discrete conservation properties of the method. Numerical schemes with good discrete conservation properties in terms of mass, momentum, and kinetic energy have been shown to produce superior results when compared to their nonconservative counterparts [2] . In a recent investigation, Morinishi et al. [3] analyzed several finite difference schemes for their conservation properties and derived discretely conservative fourth-order schemes for uniform meshes. The schemes were extended to nonuniform meshes by Vasilyev [4] , although the simultaneous discrete conservation of mass, momentum, and kinetic energy was not possible. Only schemes that discretely conserved mass and momentum but not kinetic energy or discretely conserved mass and kinetic energy but not momentum could be developed. This was true even for second-order-accurate schemes on nonuniform meshes. Vasilyev identified the source of the nonconservation as the commutation error between the discrete differencing and averaging operators on the nonuniform mesh.. The concept of discrete conservation can be applied to time as well as space. Morinishi et al. did not, however, consider discrete conservation in time as an analytical requirement for a proper set of discretization equations. In the analyses of both Morinishi et al. and Vasilyev, time advancement was accomplished by a third-order Runge-Kutta time-stepping method. The authors justified the nonconservation by observing that the associated errors appeared to be dissipative and decreased with the cube of the computational time step. The recent work of Perot [5] on two-dimensional unstructured staggered schemes extends the discrete conservation analysis to include time.. The objective of the present work is to develop a second-order-accurate finite difference scheme for structured nonuniform Cartesian meshes that discretely conserve mass, momentum, and kinetic energy in both space and time and then test its suitability for DNS. To our knowledge, this is the first fully conservative scheme for nonuniform meshes reported in the literature; although we thank a reviewer for bringing to our attention the work of Kajishima [6] , who already proposed some elements of the scheme.. The rest of the paper is organized as follows. In Section 2, the concepts of differential and discrete conservation are reviewed. In Section 3, the discrete equations for incompressible flow are presented and their conservation properties derived. In Section 4, the predicted conservation properties are confirmed by inviscid simulations on both uniform and nonuniform grids. Section 4 also includes several "thought" experiments that investigate the computed energy spectra for the inviscid simulations and the reversibility of the scheme in the inviscid limit. In Section 5, the method is applied to the DNS of turbulent plane channel flow to investigate the effect of the computational time step on the computed turbulence. Section 6 presents a summary. A second-order-accurate finite difference discretization of the incompressible NavierStokes was presented that discretely conserves mass, momentum, and kinetic energy (in the inviscid limit) in both space and time. The method uses a staggered arrangement of velocity and pressure on a structured Cartesian grid and retains its discrete conservation properties for both uniform and nonuniform grid spacing. The predicted conservation properties were confirmed by inviscid simulations on both uniform and nonuniform grids. The capability of the method to resolve turbulent flow was demonstrated by repeating the turbulent channel flow simulations of Choi and Moin [1] , where the effect of computational time step on the computed turbulence was investigated. With the present fully conservative scheme, turbulent flow solutions were achieved over the entire range of computational time steps investigated ( t + = tu 2 τ /ν = 0.4 to 5.0). Little variation in statistical turbulence quantities was observed up to t + = 1.6, corresponding to CFL = 4. The present results differ significantly from those reported by Choi and Moin, suggesting that the cause of their observed sensitivity to computational time step may have been the splitting errors associated with their fractional-step time advancement scheme, rather than the more physical explanation they proposed.

O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O B O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O B O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Multivoltage Aware Resistive Open Fault Model Abstract-Resistive open faults (ROFs) represent common interconnect manufacturing defects in VLSI designs causing delay failures and reliability-related concerns. The widespread utilization of multiple supply voltages in contemporary VLSI designs and emerging test methods poses a critical concern as to whether conventional models for resistive opens will still be effective. Conventional models do not explicitly model the V DD effect on fault behavior and detectability. We have empirically observed that a sensitized ROF could exhibit multiple behaviors across its resistance continuum. We also observe that the detectable resistance range versus V DD varies with test speed. We consequently propose a voltage-aware model that divides the full range of open resistances into continuous behavioral intervals and three detectability ranges. The presented model is expected to substantially enhance multivoltage test generation and fault distinction.Index Terms-Open resistance intervals, resistive open faults (ROFs), small delay faults, voltage-aware modeling. R ESISTIVE opens are common manufacturing failures that induce voltage-dependent delay faults and pose potential reliability risks due to their partial break-like nature, [1] - [4] . Distinguishing such faults is important in reducing infant mortality failures. Therefore, promising techniques for distinguishing delay failures induced by resistive open faults (ROFs) from those induced by other mechanisms have been proposed [5] , [6] . These techniques have relied on empirical observations of delay pattern versus V DD for resistive opens compared to other fault mechanisms. However, inductive fault analysis on ROFs shows that the voltage-delay dependencies are significantly sensitive to other test conditions [7] . Due to the absence of a comprehensive behavioral fault model considering these critical dependencies, such techniques could be less reliable and less effective in distinguishing the cause of detected delay faults.. On the other hand, due to the supply voltage-dependent delay behavior, previous work has demonstrated that fault coverage could be improved using selective testing at multiple V DD values [8] - [13] . With the emergence of low power designs that utilize multiple supply voltage levels [14] , such multi-V DD testing could become necessary for effective fault testing. Nevertheless, the delay fault detectability is also sensitive to the speed of the test [15] . The unavailability of a comprehensive model describing the fault detectability with test conditions, including test speed, has impeded the development of effective test generation algorithms.. The behavior and detectability of ROFs depend on the technological and electrical characteristics of the design and test conditions, as well as the supply voltage [7] , [8] , [13] , [16] . Given such dependencies, comprehensive modeling of the fault behavior and detectability becomes challenging. Previous models [17] - [21] for ROFs did not explicitly account for V DD and have not considered the behavioral aspect of the fault. Therefore for efficient multi-V DD fault detection and testing, a voltage-aware model which considers these issues has to be developed.. The aim of this paper is to present a parametric, voltageaware model, including both the behavioral and detectability aspects of the delay caused by ROF. The premise of the technique is that the full resistance continuum can be divided into intervals, each having a unique behavior with respect to V DD , and can have detectability ranges for each V DD depending on the test speed. Additionally we explain a methodology to determine the various behavioral and detectability parameters of the model. The model assumes a limited number of V DD values. The behavioral information revealed by this model assists in defining the exact test pattern that can be used to excite a particular fault behavior. The detection information helps to identify the right test conditions for a given test speed that maximizes the fault coverage. Hence, this model improves the test pattern generation and fault detection algorithms.. A preliminary version of this paper was published in ETS 2012 [22] . The work presented in this paper brings the following extensions. Preliminary verifications on benchmark circuits, computation of the critical resistance for behavior intervals, and an algorithm for identifying the behavioral parameters for each fault and test pattern. The organization of this paper is as follows. Background and preliminary work are given in Section II. In Section III, the motivation for developing the voltage-aware model based on delay-voltage dependencies of resistive opens is explained. An illustration of the model followed by a methodology used to identify the behavior intervals and detection ranges is discussed in Section IV. Simulation results on benchmark circuits using different technology models are discussed and analyzed in Section VI. Concluding remarks are given in Section VIII. An ROF is a parametric model representing a partial-open failure, in which the size of the open (break) is represented by a resistance. In previous work, the resistances of opens (RO) have been experimentally characterized for an aluminum process and it was reported that for ROFs, RO can vary in the range of Ohms to Megohms [2] . The distribution of resistance values can be used statistically to compute the fault coverage [23] . According to the resistance of the partial-open fault and the relative length of the sensitized faulty path to the longest path, an ROF can cause extra delays over several orders of magnitude. A gross delay fault is defined to have a cumulative propagation delay exceeding the longest path delay. Other faults are defined as small delay faults [3] . Gross delay faults are detected by at-speed transition delay fault testing, whereas small delay faults are tested using faster-thanat-speed testing. Generally delay faults due to ROF show some sensitivity to test patterns and to V DD , as will be discussed later in this section.. Kruseman and Heiligers [7] conducted an inductive fault analysis for resistive opens on several standard cells using a wide range of technologies. They highlighted different voltagedelay dependencies with different fault locations and test patterns. In that work, the additional delay caused by ROFs was characterized and several delay behaviors were identified. An increasing delay pattern corresponds to an increase in delay for reduced voltage; a constant delay pattern means an almost constant delay for high voltages, but an increase in delay with smaller voltages; and a constant inverse delay means a decrease in delay for reduced voltage. The listed behaviors are sensitive to the transition test pattern and intra-gate location.. The effect of fault resistance on the additional delay at different V DD can generally be assumed to be relative [7] . This means at each V DD , the larger the resistance, the larger the additional delay. A more accurate relationship is, however, reported in [24] . Arumi et al. [24] assessed the behavior (delay) of ROFs at a single V DD using an experimental chip and reported that high and low resistance values have different timing responses and have to be modeled differently. This is attributed to the ON resistance of MOS transistors driving a faulty node as well as the capacitance of the driven gates [24] . This shows that the range of open resistances can exhibit different behaviors with V DD according to the relative value of RO with respect to the transistor ON resistance (R ON ).. Other factors affecting the behavior include: the location of an ROF within the interconnect [24] , the threshold voltage [25] , capacitance of driven gates [26] , and the technology used [16] . Each of the named factors affects the delay differently, and therefore questions whether assuming a fixed behavior for a fault is correct.. Li et al. [8] demonstrated that resistive opens in wire delaydominant (WDD) paths are better detected at the highest V DD . On the other hand, for gate delay dominant (GDD) paths, a low V DD gives better detection. It may be noted that WDD paths have a high wire resistance, while GDD paths have a low wire resistance. This implies that the delay-voltage dependency can be different for different RO values.. It can be concluded that the delay behavior due to ROF differs with V DD and also depends on the fault location and the applied test pattern, as well as the actual resistance. On the other hand, the intrinsic delay of a fault-free path decreases with V DD and its magnitude depends on the length of the path [14] , [27] . Consequently this makes prediction of the observable behavior of a faulty path difficult. This in turn could cause different fault detectability at different V DD values. Therefore, two hypotheses can be proposed. First, if the behavior of an ROF is a function of the resistance, then the ROF behavior can be modeled by dividing the range of open resistance values into intervals, each having its own observable fault behavior. Second, if the detectable resistance range varies with the supply voltage, the ROF detectability can be modeled by identifying the minimum detectable resistance value at each V DD . In the next section, these hypotheses are investigated using a set of benchmark circuits.

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I I B I I B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Chunks and Tasks: a programming model for parallelization of dynamic algorithms We propose Chunks and Tasks, a parallel programming model built on abstractions for both data and work. The application programmer specifies how data and work can be split into smaller pieces, chunks and tasks, respectively. The Chunks and Tasks library maps the chunks and tasks to physical resources. In this way we seek to combine user friendliness with high performance. An application programmer can express a parallel algorithm using a few simple building blocks, defining data and work objects and their relationships. No explicit communication calls are needed; the distribution of both work and data is handled by the Chunks and Tasks library. This makes efficient implementation of complex applications that require dynamic distribution of work and data easier. At the same time, Chunks and Tasks imposes restrictions on data access and task dependencies that facilitates the development of high performance parallel back ends. We discuss the fundamental abstractions underlying the programming model, as well as performance and fault resilience considerations. We also present a pilot C++ library implementation for clusters of multicore machines and demonstrate its performance for sparse blocked matrix-matrix multiplication.Keywords: distributed memory parallelization, dynamic distribution, dynamic load balancing, fault tolerance, parallel programming model, work stealingThis manuscript was submitted to Parallel Computing (Elsevier) for the special issue devoted to the PMAA 2012 conference. Parallel computing can be difficult. In order to have a functioning parallel program, one first needs to find algorithms that can be executed in parallel.. Then, the algorithms must be expressed in such a way that the parallelism is exposed so that separate, independent parts can be identified. Furthermore, the work must be distributed among the available processors/nodes and data required by each processor must be communicated as needed. We present in this article a parallel programming model intended to facilitate the above process of developing parallel programs.. Our aim is to achieve scalability up to any number of processors, on distributed memory machines and possibly on heterogeneous computers, to handle not only algorithms with static distribution of work and data but also hierarchic, recursive algorithms where communication patterns are not known beforehand, and may change dynamically. Fault tolerance is also important, especially considering the trends in modern supercomputers, where hardware errors are unavoidable in any large parallel calculation. We have presented the Chunks and Tasks programming model, our answer to the question of how to draw the line between the concerns of an application programmer and the concerns of a parallel library or language. Our philosophy is that application programmers should focus on parallel algorithm development and on exposing parallelism in both data and work.. This makes the development of parallel programs easier, in particular for applications that require dynamic distribution of both work and data. At the same time, Chunks and Tasks imposes restrictions on data access and task dependencies that make it possible to implement Chunks and Tasks libraries with high performance. Furthermore, it is possible to achieve fault resilience at the Chunks and Tasks library level, which derives from the fact that the library is responsible for mapping of both work and data to physical resources.. We see before us that parallelization tools building on the Chunks and Tasks programming model will expand the applicability of high performance parallel computing to an important class of applications that require dynamic handling of work and data.

O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O B I I B I B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Complex Wavelet Bases, Steerability, and the Marr-Like Pyramid Abstract-Our aim in this paper is to tighten the link between wavelets, some classical image-processing operators, and David Marr's theory of early vision. The cornerstone of our approach is a new complex wavelet basis that behaves like a smoothed version of the Gradient-Laplace operator. Starting from first principles, we show that a single-generator wavelet can be defined analytically and that it yields a semi-orthogonal complex basis of 2 ( 2 ), irrespective of the dilation matrix used. We also provide an efficient FFT-based filterbank implementation. We then propose a slightly redundant version of the transform that is nearly translation-invariant and that is optimized for better steerability (Gaussian-like smoothing kernel). We call it the Marr-like wavelet pyramid because it essentially replicates the processing steps in Marr's theory of early vision. We use it to derive a primal wavelet sketch which is a compact description of the image by a multiscale, subsampled edge map. Finally, we provide an efficient iterative algorithm for the reconstruction of an image from its primal wavelet sketch.Index Terms-Feature extraction, primal sketch, steerable filters, wavelet design.ULTISCALE transforms are powerful tools for signal and image processing, computer vision, and for modeling biological vision. A prominent example is the 1-D wavelet transform, which acts as a multiscale version of an th-order derivative operator, where is the number of vanishing moments of the wavelet [1] . Its extension to multiple dimensions and to 2-D, in particular, is typically achieved by forming tensorproduct basis functions. However, such separable wavelets are not well matched to the singularities occuring in images such as lines and edges which can be arbitrarily oriented and even curved. Consequently, there has been a considerable research effort in developing alternative multiscale transforms that are better tuned to the geometry of natural images. Notable examples of these "geometrical x-lets" include biologically-inspired 2-D Gabor transforms [2] [24] . The derivation of the filters is not based on wavelets, but rather obtained through a numerical optimization process. Special constraints are imposed to ensure that the frequency response of the filters is essentially polar-separable and that the decomposition is simple to invert numerically (approximate tight-frame property).In this paper, we present an alternative approach based on an explicit analytical and spline-based formulation of complex wavelet bases of . Special care is given to design basis functions that best match the properties of the visual system, in accordance with Marr's theory of early vision [25] . To motivate our construction, we specify a number of properties that are highly desirable and that are fulfilled, sometimes implicitly, by a number of classical image-processing algorithms.• Invariance. We aim at invariance with respect to elementary geometric operations such as translation, scaling, and rotation. Traditional wavelet transforms only satisfy these properties to some extent, since a trade-off needs to be found between the conciseness of the representation-its (non)redundancy-and the degree of scale and translation invariance. Rotation invariance is probably the most challenging property for wavelets, especially if one insists on having basis functions (nonredundant representation).• Feature detection. Image differentials are valuable clues for feature detection. Most classical edge-detection algorithms are gradient-or Laplacian-based, and, sometimes, a combination of both. A wavelet transform that behaves like a multiscale version of these fundamental operators could, therefore, be of great use for image analysis.• Steerability (a.k.a. rotation covariance). This property is satisfied when all rotated versions of a basis function remain in the span of the wavelet at that particular scale [23] . It is one of the keys for efficient directional analysis and, more importantly, for designing image-processing algorithms that are truly rotation-invariant.• Localization. A wavelet can always be expressed as a multiple-order derivative of some smoothing function. The smoothing function should be isotropic, for good 1057-7149/$25.00

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O