926 research outputs found
Caladan: a distributed meta-OS for data center disaggregation
Data center resource disaggregation promises cost savings by pool-ing compute, storage and memory resources into separate, net-worked nodes. The benefits of this model are clear, but a closer lookshows that its full performance and efficiency potential cannot beeasily realized. Existing systems use CPUs pervasively to interface ar-bitrary devices with the network and to orchestrate communicationamong them, reducing the benefits of disaggregation.In this paper we presentCaladan, a novel system with a trusteduni-versal resource fabricthat interconnects all resources and efficientlyoffloads the system and application control planes to SmartNICs,freeing server CPUs to execute application logic. Caladan offersthree core services: capability-driven distributed name space, virtualdevices, and direct inter-device communications. These servicesare implemented in a trustedmeta-kernelthat executes in per-nodeSmartNICs. Low-level device drivers running on the commodity hostOS are used for setting up accelerators and I/O devices, and exposingthem to Caladan. Applications run in a distributed fashion acrossCPUs and multiple accelerators, which in turn can directly performI/O, i.e., access files, other accelerators or host services. Our dis-tributed dataflow runtime runs on top of this substrate. It orchestratesthe distributed execution, connecting disaggregated resources usingdata transfers and inter-device communication, while eliminatingthe performance bottlenecks of the traditional CPU-centric design
'I like money, I like many things'. The relationship between drugs and crime from the perspective of young people in contact with criminal justice systems
Based on research undertaken as part of the EU funded EPPIC project, this paper aims to update and elaborate on the relationship between drug use and offending behaviours by exploring variations within a cross-national sample of drug-experienced young people in touch with criminal justice systems. Adopting a trajectory-based approach, interviews were undertaken with 198 young people aged 15–25 in six European countries (Austria, Denmark, Germany, Italy, Poland, and UK). Data were analysed by applying the Bennett and Holloway categorization of the drugs-crime link, with a focus on the concept of social exclusion as developed by Seddon. Three main types of mechanisms (economic, pharmaceutical, and lifestyles) are used to interpret the data, showing how the relationship between drugs and offending can vary according to type of substances and over time. Furthermore, it can be associated with very different degrees of social exclusion and needs. The results suggest that while economic inequalities still play key roles in explaining drug use and offending, both behaviours can originate from a state of relative deprivation, resulting from the contradictions inherent in ‘bulimic societies’ that raise aspirations and desires while providing young people scarce opportunities for self-realisation and social recognition
Distribution of the time at which the deviation of a Brownian motion is maximum before its first-passage time
We calculate analytically the probability density of the time
at which a continuous-time Brownian motion (with and without drift) attains its
maximum before passing through the origin for the first time. We also compute
the joint probability density of the maximum and . In the
driftless case, we find that has power-law tails: for large and for small . In
presence of a drift towards the origin, decays exponentially for large
. The results from numerical simulations are in excellent agreement with
our analytical predictions.Comment: 13 pages, 5 figures. Published in Journal of Statistical Mechanics:
Theory and Experiment (J. Stat. Mech. (2007) P10008,
doi:10.1088/1742-5468/2007/10/P10008
Area distribution and the average shape of a L\'evy bridge
We consider a one dimensional L\'evy bridge x_B of length n and index 0 <
\alpha < 2, i.e. a L\'evy random walk constrained to start and end at the
origin after n time steps, x_B(0) = x_B(n)=0. We compute the distribution
P_B(A,n) of the area A = \sum_{m=1}^n x_B(m) under such a L\'evy bridge and
show that, for large n, it has the scaling form P_B(A,n) \sim n^{-1-1/\alpha}
F_\alpha(A/n^{1+1/\alpha}), with the asymptotic behavior F_\alpha(Y) \sim
Y^{-2(1+\alpha)} for large Y. For \alpha=1, we obtain an explicit expression of
F_1(Y) in terms of elementary functions. We also compute the average profile <
\tilde x_B (m) > at time m of a L\'evy bridge with fixed area A. For large n
and large m and A, one finds the scaling form = n^{1/\alpha}
H_\alpha({m}/{n},{A}/{n^{1+1/\alpha}}), where at variance with Brownian bridge,
H_\alpha(X,Y) is a non trivial function of the rescaled time m/n and rescaled
area Y = A/n^{1+1/\alpha}. Our analytical results are verified by numerical
simulations.Comment: 21 pages, 4 Figure
Single-crossover dynamics: finite versus infinite populations
Populations evolving under the joint influence of recombination and
resampling (traditionally known as genetic drift) are investigated. First, we
summarise and adapt a deterministic approach, as valid for infinite
populations, which assumes continuous time and single crossover events. The
corresponding nonlinear system of differential equations permits a closed
solution, both in terms of the type frequencies and via linkage disequilibria
of all orders. To include stochastic effects, we then consider the
corresponding finite-population model, the Moran model with single crossovers,
and examine it both analytically and by means of simulations. Particular
emphasis is on the connection with the deterministic solution. If there is only
recombination and every pair of recombined offspring replaces their pair of
parents (i.e., there is no resampling), then the {\em expected} type
frequencies in the finite population, of arbitrary size, equal the type
frequencies in the infinite population. If resampling is included, the
stochastic process converges, in the infinite-population limit, to the
deterministic dynamics, which turns out to be a good approximation already for
populations of moderate size.Comment: 21 pages, 4 figure
Observation of interatomic Coulombic decay induced by double excitation of helium in nanodroplets
Interatomic Coulombic decay (ICD) plays a crucial role in weakly bound
complexes exposed to intense or high-energy radiation. So far, neutral or ionic
atoms or molecules have been prepared in singly excited electron or hole states
which can transfer energy to neighboring centers and cause ionization and
radiation damage. Here we demonstrate that a doubly excited atom, despite its
extremely short lifetime, can decay by ICD; evidenced by high-resolution
photoelectron spectra of He nanodroplets excited to the 2s2p+ state. We find
that ICD proceeds by relaxation into excited HeHe atom-pair states, in
agreement with calculations. The ability of inducing ICD by resonant excitation
far above the single-ionization threshold opens opportunities for controlling
radiation damage to a high degree of element specificity and spectral
selectivity.Comment: 6 pages, 4 figures, to be submitted to PR
Universal Order Statistics of Random Walks
We study analytically the order statistics of a time series generated by the
successive positions of a symmetric random walk of n steps with step lengths of
finite variance \sigma^2. We show that the statistics of the gap
d_{k,n}=M_{k,n} -M_{k+1,n} between the k-th and the (k+1)-th maximum of the
time series becomes stationary, i.e, independent of n as n\to \infty and
exhibits a rich, universal behavior. The mean stationary gap (in units of
\sigma) exhibits a universal algebraic decay for large k,
/\sigma\sim 1/\sqrt{2\pi k}, independent of the details of the
jump distribution. Moreover, the probability density (pdf) of the stationary
gap exhibits scaling, Proba.(d_{k,\infty}=\delta)\simeq (\sqrt{k}/\sigma)
P(\delta \sqrt{k}/\sigma), in the scaling regime when \delta\sim
\simeq \sigma/\sqrt{2\pi k}. The scaling function P(x) is
universal and has an unexpected power law tail, P(x) \sim x^{-4} for large x.
For \delta \gg the scaling breaks down and the pdf gets cut-off
in a nonuniversal way. Consequently, the moments of the gap exhibit an unusual
multi-scaling behavior.Comment: 5 pages, 3 figures. Revised version, typos corrected. Accepted for
publication in Physical Review Letter
Universal Record Statistics of Random Walks and L\'evy Flights
It is shown that statistics of records for time series generated by random
walks are independent of the details of the jump distribution, as long as the
latter is continuous and symmetric. In N steps, the mean of the record
distribution grows as the sqrt(4N/pi) while the standard deviation grows as
sqrt((2-4/pi) N), so the distribution is non-self-averaging. The mean shortest
and longest duration records grow as sqrt(N/pi) and 0.626508... N,
respectively. The case of a discrete random walker is also studied, and similar
asymptotic behavior is found.Comment: 4 pages, 3 figures. Added journal ref. and made small changes.
Compatible with published versio
- …