arXiv subjects

Martin Wimmer

Publications and source records attributed to Martin Wimmer.

15 recordsLinked to original sources

arXiv2026

An Ontology-Based Approach to Security Risk Identification of Container Deployments in OT Contexts

In operational technology (OT) contexts, containerised applications often require elevated privileges to access low-level network interfaces or perform administrative tasks such as application monitoring. These privileges reduce the default isolation provided by containers and introduce significant security risks. Security risk identification for OT container deployments is challenged by hybrid IT/OT architectures, fragmented stakeholder knowledge, and continuous system changes. Existing approaches lack reproducibility, interpretability across contexts, and technical integration with deployment artefacts. We propose a model-based approach, implemented as the Container Security Risk Ontology (CSRO), which integrates five key domains: adversarial behaviour, contextual assumptions, attack scenarios, risk assessment rules, and container security artefacts. Our evaluation of CSRO in a case study demonstrates that the end-to-end formalisation of risk calculation, from artefact to risk level, enables automated and reproducible risk identification. While CSRO currently focuses on technical, container-level treatment measures, its modular and flexible design provides a solid foundation for extending the approach to host-level and organisational risk factors.

cs.SE↗

arXiv2024

Selective laser etching of displays: Closing the gap between optical simulations and fabrication

Simulations and measurements on selective laser etching of display glasses are reported. By means of a holographic 3D beam splitter, ultrashort laser pulses are focused inside the volume of a glass sample creating type III modifications along a specific trajectory like pearls on a string. Superimposed by a feed of the glass sample a full 3D area of modifications is achieved building the cornerstone for subsequent etch processes. Based on KOH the modifications are selectively etched at a much higher rate compared to unmodified regions resulting in a separation of the glass along the trajectory of modifications. For gaining further insight into the etch process, we perform simulations on this wet chemical process and compare it to our experimental results.

physics.optics↗

arXiv2022

Theory of hydrodynamic phenomena in optical mesh lattices

Signatures of superfluid-like behaviour have recently been observed experimentally in a nonlinear optical mesh lattice, where the arrival time of optical pulses propagating in a pair of coupled optical fiber loops is interpreted as a synthetic spatial dimension. Here, we develop a general theory of the fluid of light in such optical mesh lattices. On the one hand, this theory provides a solid framework for an analytical and numerical interpretation of the experimental observations. On the other hand it anticipates new physical effects stemming from the specific spatio-temporally periodic geometry of our set-up. Our work opens the way towards the full exploitation of optical mesh lattices system as a promising platform for studies of hydrodynamics phenomena in fluids of light in novel configurations.

physics.optics↗

arXiv2020

Superfluidity of Light and its Break-Down in Optical Mesh Lattices

Hydrodynamic phenomena can be observed with light thanks to the analogy between quantum gases and nonlinear optics. In this Letter, we report an experimental study of the superfluid-like properties of light in a (1+1)-dimensional nonlinear optical mesh lattice, where the arrival time of optical pulses plays the role of a synthetic spatial dimension. A spatially narrow defect at rest is used to excite sound waves in the fluid of light and measure the sound speed. The critical velocity for superfluidity is probed by looking at the threshold in the deposited energy by a moving defect, above which the apparent superfluid behaviour breaks down. Our observations establish optical mesh lattices as a promising platform to study fluids of light in novel regimes of interdisciplinary interest, including non-Hermitian and/or topological physics.

physics.optics↗

arXiv2020

Predicting the orbifold origin of the MSSM

MSSM-like string models from the compactification of the heterotic string on toroidal orbifolds (of the kind $T^6/P$) have distinct phenomenological properties, like the spectrum of vector-like exotics, the scale of supersymmetry breaking, and the existence of non-Abelian flavor symmetries. We show that these characteristics depend crucially on the choice of the underlying orbifold point group $P$. In detail, we use boosted decision trees to predict $P$ from phenomenological properties of MSSM-like orbifold models. As this works astonishingly well, we can utilize machine learning to predict the orbifold origin of the MSSM.

hep-th↗

arXiv2019

Collapse on the line -- how synthetic dimensions influence nonlinear effects

Power induced wave collapse is one of the most fascinating phenomena in optics as it provides extremely high intensities, thus stimulating a range of nonlinear processes. For low power levels, propagation of beams in bulk media is dominated by diffraction, while above a certain threshold self-focusing is steadily enhanced by the action of a positive nonlinearity. An autocatalytic blow-up occurs, which is only stopped by saturation of the nonlinearity, material damage or the inherent medium discreteness. In the latter case, this leads to energy localization on a single site. It is commonly believed that for cubic nonlinearities, this intriguing effect requires at least two transverse dimensions to occur and is thus out of reach in fiber optics. Following the concept of synthetic dimensions, we demonstrate that mixing short and long-range interaction resembles a two-dimensional mesh lattice and features wave collapse at mW-power levels in a genuine 1D system formed by coupled fiber loops.

physics.optics↗

arXiv2019

2D Solitons in PT-symmetric photonic lattices

Parity-time (PT) symmetry has attracted a lot of attention since the concept of pseudo-Hermitian dynamics of open quantum systems was first demonstrated two decades ago. Contrary to their Hermitian counterparts, non-conservative environments a priori do not show real energy eigenvalues and unitary evolution. However, if PT-symmetry requirements are satisfied, even dissipative systems can exhibit real energy eigenvalues, thus ensuring energy conservation in the temporal average. In optics, PT-symmetry can be readily introduced by incorporating, in a balanced way, regions having optical gain and loss. However, all optical realizations have been restricted so far to a single transverse dimension (1D) such as optical waveguide arrays. In many cases, only losses were modulated relying on a scaling argument being valid for linear systems only. Both restrictions crucially limit potential applications. Here, we present an experimental platform for investigating the interplay of PT-symmetry and nonlinearity in two dimensions (2D) and observe nonlinear localization and soliton formation. Contrary to the typical dissipative solitons, we find a one-parametric family of solitons which exhibit properties similar to its conservative counterpart. In the limit of high optical power, the solitons collapse on a discrete network and give rise to an amplified, self-accelerating field.

physics.optics↗

arXiv2016

Experimental Measurement of the Berry Curvature from Anomalous Transport

Geometrical properties of energy bands underlie fascinating phenomena in a wide-range of systems, including solid-state materials, ultracold gases and photonics. Most famously, local geometrical characteristics like the Berry curvature can be related to global topological invariants such as those classifying quantum Hall states or topological insulators. Regardless of the band topology, however, any non-zero Berry curvature can have important consequences, such as in the semi-classical evolution of a wave packet. Here, we experimentally demonstrate for the first time that wave packet dynamics can be used to directly map out the Berry curvature. To this end, we use optical pulses in two coupled fibre loops to study the discrete time-evolution of a wave packet in a 1D geometrical "charge" pump, where the Berry curvature leads to an anomalous displacement of the wave packet under pumping. This is both the first direct observation of Berry curvature effects in an optical system, and, more generally, the proof-of-principle demonstration that semi-classical dynamics can serve as a high-resolution tool for mapping out geometrical properties.

physics.optics↗

arXiv2016

Observation of Bloch oscillations in complex PT-symmetric photonic lattices

Light propagation in periodic environments is often associated with a number of interesting and potentially useful processes. If a crystalline optical potential is also linearly ramped, light can undergo periodic Bloch oscillations, a direct outcome of localized Wannier-Stark states and their equidistant eigenvalue spectrum. Even though these effects have been extensively explored in conservative settings, this is by no means the case in non-Hermitian photonic lattices encompassing both amplification and attenuation. Quite recently, Bloch oscillations have been predicted in parity-time-symmetric structures involving gain and loss in a balanced fashion. While in a complex bulk medium, one intuitively expects that light will typically follow the path of highest amplification, in a periodic system this behavior can be substantially altered by the underlying band structure. Here, we report the first experimental observation of Bloch oscillations in parity-time-symmetric mesh lattices. We show that these revivals exhibit unusual properties like secondary emissions and resonant restoration of PT symmetry. In addition, we present a versatile method for reconstructing the real and imaginary components of the band structure by directly monitoring the light evolution during a cycle of these oscillations.

physics.optics↗

arXiv2016

Benchmarking Concurrent Priority Queues: Performance of k-LSM and Related Data Structures

A number of concurrent, relaxed priority queues have recently been proposed and implemented. Results are commonly reported for a throughput benchmark that uses a uniform distribution of keys drawn from a large integer range, and mostly for single systems. We have conducted more extensive benchmarking of three recent, relaxed priority queues on four different types of systems with different key ranges and distributions. While we can show superior throughput and scalability for our own k-LSM priority queue for the uniform key distribution, the picture changes drastically for other distributions, both with respect to achieved throughput and relative merit of the priority queues. The throughput benchmark alone is thus not sufficient to characterize the performance of concurrent priority queues. Our benchmark code and k-LSM priority queue are publicly available to foster future comparison.

cs.DS↗

arXiv2015

The Lock-free $k$-LSM Relaxed Priority Queue

Priority queues are data structures which store keys in an ordered fashion to allow efficient access to the minimal (maximal) key. Priority queues are essential for many applications, e.g., Dijkstra's single-source shortest path algorithm, branch-and-bound algorithms, and prioritized schedulers. Efficient multiprocessor computing requires implementations of basic data structures that can be used concurrently and scale to large numbers of threads and cores. Lock-free data structures promise superior scalability by avoiding blocking synchronization primitives, but the \emph{delete-min} operation is an inherent scalability bottleneck in concurrent priority queues. Recent work has focused on alleviating this obstacle either by batching operations, or by relaxing the requirements to the \emph{delete-min} operation. We present a new, lock-free priority queue that relaxes the \emph{delete-min} operation so that it is allowed to delete \emph{any} of the $ρ+1$ smallest keys, where $ρ$ is a runtime configurable parameter. Additionally, the behavior is identical to a non-relaxed priority queue for items added and removed by the same thread. The priority queue is built from a logarithmic number of sorted arrays in a way similar to log-structured merge-trees. We experimentally compare our priority queue to recent state-of-the-art lock-free priority queues, both with relaxed and non-relaxed semantics, showing high performance and good scalability of our approach.

cs.DS↗

arXiv2014

An improved, easily computable combinatorial lower bound for weighted graph bipartitioning

There has recently been much progress on exact algorithms for the (un)weighted graph (bi)partitioning problem using branch-and-bound and related methods. In this note we present and improve an easily computable, purely combinatorial lower bound for the weighted bipartitioning problem. The bound is computable in $O(n\log n+m)$ time steps for weighted graphs with $n$ vertices and $m$ edges. In the branch-and-bound setting, the bound for each new subproblem can be updated in $O(n+(m/n)\log n)$ time steps amortized over a series of $n$ branching steps; a rarely triggered tightening of the bound requires search on the graph of unassigned vertices and can take from $O(n+m)$ to $O(nm+n^2\log n)$ steps depending on implementation and possible bound quality. Representing a subproblem uses $O(n)$ space. Although the bound is weak, we believe that it can be advantageous in a parallel setting to be able to generate many subproblems fast, possibly out-weighting the advantages of tighter, but much more expensive (algebraic, spectral, flow) lower bounds. We use a recent priority task-scheduling framework for giving a parallel implementation, and show the relative improvements in bound quality and solution speed by the different contributions of the lower bound. A detailed comparison with standardized input graphs to other lower bounds and frameworks is pending. Detailed investigations of branching and subproblem selection rules are likewise not the focus here, but various options are discussed.

cs.DS↗

arXiv2013

Data Structures for Task-based Priority Scheduling

Many task-parallel applications can benefit from attempting to execute tasks in a specific order, as for instance indicated by priorities associated with the tasks. We present three lock-free data structures for priority scheduling with different trade-offs on scalability and ordering guarantees. First we propose a basic extension to work-stealing that provides good scalability, but cannot provide any guarantees for task-ordering in-between threads. Next, we present a centralized priority data structure based on $k$-fifo queues, which provides strong (but still relaxed with regard to a sequential specification) guarantees. The parameter $k$ allows to dynamically configure the trade-off between scalability and the required ordering guarantee. Third, and finally, we combine both data structures into a hybrid, $k$-priority data structure, which provides scalability similar to the work-stealing based approach for larger $k$, while giving strong ordering guarantees for smaller $k$. We argue for using the hybrid data structure as the best compromise for generic, priority-based task-scheduling. We analyze the behavior and trade-offs of our data structures in the context of a simple parallelization of Dijkstra's single-source shortest path algorithm. Our theoretical analysis and simulations show that both the centralized and the hybrid $k$-priority based data structures can give strong guarantees on the useful work performed by the parallel Dijkstra algorithm. We support our results with experimental evidence on an 80-core Intel Xeon system.

cs.DC↗

arXiv2013

Configurable Strategies for Work-stealing

Work-stealing systems are typically oblivious to the nature of the tasks they are scheduling. For instance, they do not know or take into account how long a task will take to execute or how many subtasks it will spawn. Moreover, the actual task execution order is typically determined by the underlying task storage data structure, and cannot be changed. There are thus possibilities for optimizing task parallel executions by providing information on specific tasks and their preferred execution order to the scheduling system. We introduce scheduling strategies to enable applications to dynamically provide hints to the task-scheduling system on the nature of specific tasks. Scheduling strategies can be used to independently control both local task execution order as well as steal order. In contrast to conventional scheduling policies that are normally global in scope, strategies allow the scheduler to apply optimizations on individual tasks. This flexibility greatly improves composability as it allows the scheduler to apply different, specific scheduling choices for different parts of applications simultaneously. We present a number of benchmarks that highlight diverse, beneficial effects that can be achieved with scheduling strategies. Some benchmarks (branch-and-bound, single-source shortest path) show that prioritization of tasks can reduce the total amount of work compared to standard work-stealing execution order. For other benchmarks (triangle strip generation) qualitatively better results can be achieved in shorter time. Other optimizations, such as dynamic merging of tasks or stealing of half the work, instead of half the tasks, are also shown to improve performance. Composability is demonstrated by examples that combine different strategies, both within the same kernel (prefix sum) as well as when scheduling multiple kernels (prefix sum and unbalanced tree search).

cs.DC↗

arXiv2010

Work-stealing for mixed-mode parallelism by deterministic team-building

We show how to extend classical work-stealing to deal also with data parallel tasks that can require any number of threads r >= 1 for their execution. We explain in detail the so introduced idea of work-stealing with deterministic team-building which in a natural way generalizes classical work-stealing. A prototype C++ implementation of the generalized work-stealing algorithm has been given and is briefly described. Building on this, a serious, well-known contender for a best parallel Quicksort algorithm has been implemented, which naturally relies on both task and data parallelism. For instance, sorting 2^27-1 randomly generated integers we could improve the speed-up from 5.1 to 8.7 on a 32-core Intel Nehalem EX system, being consistently better than the tuned, task-parallel Cilk++ system.

cs.DC↗