Lock-free dynamic hash tables with open addressing

We present an efficient lock-free algorithm for parallel accessible hash tables with open addressing, which promises more robust performance and reliability than conventional lock-based implementations. “Lock-free” means that it is guaranteed that always at least one process completes its operation within a bounded number of steps. For a single processor architecture our solution is as efficient as sequential hash tables. On a multiprocessor architecture this is also the case when all processors have comparable speeds. The algorithm allows processors that have widely different speeds or come to a halt. It can easily be implemented using C-like languages and requires on average only constant time for insertion, deletion or accessing of elements. The algorithm allows the hash tables to grow and shrink when needed. Lock-free algorithms are hard to design correctly, even when apparently straightforward. Ensuring the correctness of the design at the earliest possible stage is a major challenge in any responsible system development. In view of the complexity of the algorithm, we turned to the interactive theorem prover PVS for mechanical support. We employ standard deductive verification techniques to prove around 200 invariance properties of our algorithm, and describe how this is achieved with the theorem prover PVS.

Efficient almost wait-free parallel accesible dynamic hashtables

Abstract In multiprogrammed systems, synchronization often turns out to be a performance bottleneck and the source of poor fault-tolerance. Wait-free and lock-free algorithms can do without locking mechanisms, and therefore do not suffer from these problems. We present an efficient almost wait-free algorithm for parallel accessible hashtables, which promises more robust performance and reliability than conventional lock-based implementations. Our solution is as efficient as sequential hashtables. It can easily be implemented using C-like languages and requires on average only constant time for insertion, deletion or accessing of elements. Apart from that, our new algorithm allows the hashtables to grow and shrink dynamically when needed. A true problem of lock-free algorithms is that they are hard to design correctly, even when apparently straightforward. Ensuring the correctness of the design at the earliest possible stage is a major challenge in any responsible system development. Our algorithm contains 81 atomic statements. In view of the complexity of the algorithm and its correctness properties, we turned to the interactive theorem prover PVS for mechanical support. We employ standard deductive verification techniques to prove around 200 invariance properties of our almost wait-free algorithm, and describe how this is achieved using the theorem prover PVS. CR Subject Classification (1991): D.1 Programming techniques AMS Subject Classification (1991): 68Q22 Distributed algorithms, 68P20 Information storage and retrieval Keywords & Phrases: Hashtables, Distributed algorithms, Lock-free, Wait-fre

The missing massive satellites of the Milky Way

Recent studies suggest that only three of the 12 brightest satellites of the Milky Way (MW) inhabit dark matter haloes with maximum circular velocity, Vmax, exceeding ∼30 km s−1. This is in apparent contradiction with the Λ cold dark matter (CDM) simulations of the Aquarius Project, which suggest that MW-sized haloes should have at least eight subhaloes with Vmax > 30 km s−1. The absence of luminous satellites in such massive subhaloes is thus puzzling and may present a challenge to the ΛCDM paradigm. We note, however, that the number of massive subhaloes depends sensitively on the (poorly known) virial mass of the MW, and that their scarcity makes estimates of their abundance from a small simulation set like Aquarius uncertain. We use the Millennium Simulation series and the invariance of the scaled subhalo velocity function (i.e. the number of subhaloes as a function of ν, the ratio of the subhalo Vmax to the host halo virial velocity, V200) to secure improved estimates of the abundance of rare massive subsystems. In the range 0.1 ν) is approximately Poisson distributed about an average given by 〈Nsub〉 = 10.2 (ν/0.15)−3.11. This is slightly lower than that in Aquarius haloes, but consistent with recent results from the Phoenix Project. The probability that a ΛCDM halo has three or fewer subhaloes with Vmax above some threshold value, Vth, is then straightforward to compute. It decreases steeply both with decreasing Vth and with increasing halo mass. For Vth = 30 km s−1, ∼40 per cent of Mhalo = 1012 M⊙ haloes pass the test; fewer than ∼5 per cent do so for Mhalo ≳ 2 × 1012 M⊙ and the probability effectively vanishes for Mhalo ≳ 3 × 1012 M⊙. Rather than a failure of ΛCDM, the absence of massive subhaloes might simply indicate that the MW is less massive than is commonly thought

New Physics and CP Violation in Hyperon Nonleptonic Decays

The sum of the CP-violating asymmetries A(Lambda_-^0) and A(Xi_-^-) in hyperon nonleptonic decays is presently being measured by the E871 experiment. We evaluate contributions to the asymmetries induced by chromomagnetic-penguin operators, whose coefficients can be enhanced in certain models of new physics. Incorporating recent information on the strong phases in Xi->Lambda pi decay, we show that new-physics contributions to the two asymmetries can be comparable. We explore how the upcoming results of E871 may constrain the coefficients of the operators. We find that its preliminary measurement is already better than the epsilon parameter of K-Kbar mixing in bounding the parity-conserving contributions.Comment: 12 pages, 2 figure

Drag force in a string model dual to large-N QCD

We compute the drag force exerted on a quark and a di-quark systems in a background dual to large-N QCD at finite temperature. We find that appears a drag force in the former setup with flow of energy proportional to the mass of the quark while in the latter there is no dragging as in other studies. We also review the screening length.Comment: 15 pages, typos removed, error corrected, refs adde

Width distribution of contact lines on a disordered substrate

We have studied the roughness of a contact line of a liquid meniscus on a disordered substrate by measuring its width distribution. The comparison between the measured width distribution and the width distribution calculated in previous works, extended here to the case of open boundary conditions, confirms that the Joanny-de Gennes model is not sufficient to describe the dynamics of contact lines at the depinning threshold. This conclusion is in agreement with recent measurements which determine the roughness exponent by extrapolation to large system sizes.Comment: 4 pages, 3 figure

Four-point correlator constraints on electromagnetic chiral parameters and resonance effective Lagrangians

We pursue the analysis of a set of generalized DGMLY sum rules for the electromagnetic chiral parameters at order $e^2p^2$ and discuss implications for effective Lagrangians with resonances. We exploit a formalism in which charge spurions are introduced and treated as sources. We show that no inconsistency arises from anomalies up to quadratic order in the spurions. We focus on the sum rules associated with QCD 4-point correlators which were not analyzed in detail before. Convergence properties of the sum rules are deduced from a general analysis of the form of the counterterms in the presence of electromagnetic spurions. Following the approach in which vector and axial-vector resonances are described with antisymmetric tensor fields and have a chiral order, we show that the convergence constraints are violated at chiral order four and can be satisfied by introducing a set of terms of order six. The relevant couplings get completely and uniquely determined from a set of generalized Weinberg sum-rule relations. An update on the corrections to Dashen's low-energy theorem is given.Comment: 42 pages, 1 figure. v2: references adde

Spatiotemporal patterns of Sentinel-2 observations at image- and pixel-level of the Mexican territory between 2015 and 2019

[ES] Actualmente, las imágenes Sentinel-2 son uno de los acervos multiespectrales y gratuitos de mayor resolución temporal, espectral y espacial para monitorear la superficie terrestre. Sin embargo, la posibilidad de utilizar este acervo para distintas aplicaciones está condicionada por el número de observaciones sin nubes disponibles para una ventana espacio-temporal determinada. Por ello, este artículo tuvo el objetivo de analizar el número de observaciones de Sentinel-2 disponibles para el territorio mexicano a nivel de imagen y de pixel. En el primer caso, se contabilizó el total de imágenes disponibles por año y su porcentaje de nubosidad; mientras que, en el segundo, se calculó el número de observaciones despejadas por pixel. Además, para tomar en cuenta la diversidad del territorio, se evaluó el promedio mensual de las observaciones por pixel de cada una de las siete ecorregiones del país, así como la proporción de su superficie con por lo menos una observación despejada en intervalos mensuales, bimestrales, trimestrales y anuales. Los resultados mostraron que el número de observaciones válidas por pixel variaron entre 0 y 121 observaciones al año y entre 0 y 6.58 al mes. Adicionalmente, se observó que en el periodo 2017 – 2019 se pueden obtener observaciones de todo el país en ventanas anuales, mientras que en el periodo 2018 – 2019, se pueden obtener observaciones en intervalos mensuales o trimestrales, dependiendo de la ecorregión. Finalmente, consideramos que los resultados de este trabajo servirán de guía para los usuarios interesados en utilizar estas imágenes para distintos estudios.[EN] Sentinel-2 imagery has the highest temporal, spectral and spatial resolution to monitor land surface among the freely available multispectral collections. However, the possibility to use these images in different applications is conditioned by the number of cloudless observations available for a certain spatiotemporal window. Thus, the objective of this article is to analyze the number of Sentinel-2 observations available for the Mexican territory at image and pixel level. In the first case, the total number of available images and its cloud cover percentage was calculated; while in the second case, the number of cloudless observations was estimated for each pixel. Additionally, in order to take into account the territory diversity, the monthly mean number of cloudless observations, as well as the proportion of its surface with at least one cloudless observation in monthly, bimonthly, trimonthly and annual intervals, was computed for each one of the seven ecoregions of the country. The results show that annually, the number of valid observations per pixel is between 0 and 121 observations, while in monthly evaluations, between 0 and 6.58 observations. Additionally, in the 2017-2019 period annual observations can be obtained for the entire Mexican land surface, while in 2018-2019, monthly or trimonthly evaluations can be achieved, depending on the ecoregion. We consider that these results will provide useful information for researchers that are interested in using Sentinel-2 imagery for different applications.El primer autor agradece al CONACyT por la beca otorgada para realizar sus estudios de posgrado. Agradecemos a dos revisores anónimos por sus comentarios que nos ayudaron a mejorar significativamente el manuscrito de este artículoSolórzano, J.; Mas, J.; Gao, Y.; Gallardo-Cruz, J. (2020). Patrones espaciotemporales de las observaciones de Sentinel-2 a nivel de imagen y píxel sobre el territorio mexicano entre 2015 y 2019. Revista de Teledetección. 0(56):103-115. https://doi.org/10.4995/raet.2020.14044OJS10311505