Interconnect Optimization in Chip Design

In this thesis, we take a closer look at the buffering problem. We review the literature on the buffering problem and examine how different algorithms try to solve it. We present an overview over the different aspects that are relevant for buffering, explain how they can be modelled and what decisions we made in modelling. Based on these considerations, we present a new problem formulation that captures more aspects and allows more degrees of freedom than all the previous algorithms and formulations. We present an (exponential time) algorithm that is able to solve this problem either approximately, or even optimally, with higher running time. Both variants depend on a slew accuracy function. We prove that the algorithm finds an optimum solution if it starts with a certain set of initial slews. Then, we show how to guess slews that allow us to find an optimum solution. In this way, we solve the inherent problem of including slew in buffering algorithms. Then, we demonstrate that the algorithm can be modified to include higher order delay models or increase the degrees of freedom at the cost of ignoring the slew. We also present multiple new speed up techniques. Among them are a data structure that efficiently maintains the labels in our algorithm, a construction of a sparse graph representing the routing space and buffering positions that retains helpful properties for finding good solutions and a practical (almost) feasible lower bound function. Then we develop as additional speedups a fast way to restrict the topology of solutions and present a new iterative clustering heuristic that is timing aware. It uses the Capacitated Tree Cover Problem with Edge Loads as a black box. We present a new 3-approximation algorithm for this problem that runs in O(mlog n) time. An underlying LP-relaxation can be solved optimally using a greedy algorithm, despite its exponential number of inequalities. We round this LP and apply a wellknown splitting technique to get to a feasible solution. In a final step, we prove that the integrality gap of our LP-relaxation is 3, which means that our analysis was optimal. Finally, we test our algorithm both in a benchmark setting and a practical setting. We show that we gain significant speedups by augmenting our algorithm with heuristic speedup techniques that do not lead to a notable loss in result quality. Then we examine how the fastest variants of our algorithm perform in a practical setting and see that it can give us significant improvements at a high runtime cost. Unfortunately, it is too slow to use it as a default tool in a design flow, but it can be used to optimize a few hard instances that otherwise would have to be optimized manually by a designer. Our algorithm allows us for the first time to benchmark existing buffering algorithms and heuristics. We can find out if solutions are good or how to improve them by comparing to our results. BonnRouteBuffer has been improved this way multiple times

Multi-Sided Boundary Labeling

In the Boundary Labeling problem, we are given a set of $n$ points, referred to as sites, inside an axis-parallel rectangle $R$, and a set of $n$ pairwise disjoint rectangular labels that are attached to $R$ from the outside. The task is to connect the sites to the labels by non-intersecting rectilinear paths, so-called leaders, with at most one bend. In this paper, we study the Multi-Sided Boundary Labeling problem, with labels lying on at least two sides of the enclosing rectangle. We present a polynomial-time algorithm that computes a crossing-free leader layout if one exists. So far, such an algorithm has only been known for the cases in which labels lie on one side or on two opposite sides of $R$ (here a crossing-free solution always exists). The case where labels may lie on adjacent sides is more difficult. We present efficient algorithms for testing the existence of a crossing-free leader layout that labels all sites and also for maximizing the number of labeled sites in a crossing-free leader layout. For two-sided boundary labeling with adjacent sides, we further show how to minimize the total leader length in a crossing-free layout

Impact of nonlinear loss on Stimulated Brillouin Scattering

We study the impact of two-photon absorption (2PA) and fifth-order nonlinear loss such as 2PA-induced free-carrier absorption in semiconductors on the performance of Stimulated Brillouin Scattering devices. We formulate the equations of motion including effective loss coefficients, whose explicit expressions are provided for numerical evaluation in any waveguide geometry. We find that 2PA results in a monotonic, algebraic relationship between amplification, waveguide length and pump power, whereas fifth-order losses lead to a non-monotonic relationship. We define a figure of merit for materials and waveguide designs in the presence of fifth-order losses. From this, we determine the optimal waveguide length for the case of 2PA alone and upper bounds for the total Stokes amplification for the case of 2PA as well as fifth-order losses. The analysis is performed analytically using a small-signal approximation and is compared to numerical solutions of the full nonlinear modal equations

Power limits and a figure of merit for stimulated Brillouin scattering in the presence of third and fifth order loss

We derive a set of design guidelines and a figure of merit to aid the engineering process of on-chip waveguides for strong Stimulated Brillouin Scattering (SBS). To this end, we examine the impact of several types of loss on the total amplification of the Stokes wave that can be achieved via SBS. We account for linear loss and nonlinear loss of third order (two-photon absorption, 2PA) and fifth order, most notably 2PA-induced free carrier absorption (FCA). From this, we derive an upper bound for the output power of continuous-wave Brillouin-lasers and show that the optimal operating conditions and maximal realisable Stokes amplification of any given waveguide structure are determined by a dimensionless parameter $\mathcal{F}$ involving the SBS-gain and all loss parameters. We provide simple expressions for optimal pump power, waveguide length and realisable amplification and demonstrate their utility in two example systems. Notably, we find that 2PA-induced FCA is a serious limitation to SBS in silicon and germanium for wavelengths shorter than 2200nm and 3600nm, respectively. In contrast, three-photon absorption is of no practical significance

Toughening Financial Sanctions on Russia: Enforcing Energy Sanctions and Reducing Shadow Reserves Effectively

Financial sanctions are key in enforcing restrictions on Russian energy exports - in particular the G7/EU oil price cap regime -, due to financial institutions' critical role in cross-border transactions. While the energy sanctions regime is having an impact on export earnings and budget revenues, evidence for potentially widespread violations is also emerging. Moreover, favorable external dynamics have allowed Russia to accumulate substantial assets abroad - "shadow reserves" -, which need to be kept out of reach of the regime

Carbontracker: Tracking and Predicting the Carbon Footprint of Training Deep Learning Models

Deep learning (DL) can achieve impressive results across a wide variety of tasks, but this often comes at the cost of training models for extensive periods on specialized hardware accelerators. This energy-intensive workload has seen immense growth in recent years. Machine learning (ML) may become a significant contributor to climate change if this exponential trend continues. If practitioners are aware of their energy and carbon footprint, then they may actively take steps to reduce it whenever possible. In this work, we present Carbontracker, a tool for tracking and predicting the energy and carbon footprint of training DL models. We propose that energy and carbon footprint of model development and training is reported alongside performance metrics using tools like Carbontracker. We hope this will promote responsible computing in ML and encourage research into energy-efficient deep neural networks.Comment: Accepted to be presented at the ICML Workshop on "Challenges in Deploying and monitoring Machine Learning Systems", 2020. Source code at this link https://github.com/lfwa/carbontracker

Comparison of three methods of extravascular lung water volume measurement in patients after cardiac surgery

This research was supported by an Intensive Care Society (UK) Young Investigator Award and unrestricted research grants from Barts and The London NHS Trust and LiDCO, Lt

Flat Choroidal Nevus Inaccessible to Ultrasound Sonography Evaluated by Enhanced Depth Imaging Optical Coherence Tomography

Purpose: To demonstrate the usefulness of enhanced depth imaging optical coherence tomography (EDI-OCT) in investigating choroidal lesions inaccessible to ultrasound sonography. Methods: In a 60-year-old woman with an asymptomatic choroidal nevus, normal OCT was used to observe the macula and EDI-OCT to image the choroidal nevus that was inaccessible to ultrasound. The exact location of the lesion in the choroid and the dimensions of the nevus were measured. Results: The lesion was located in the superior macula, and the nevus was homogeneous in its reflectivity. We observed a thickened choroid delineated by the shadow cone behind it, measuring 1,376 × 325 µm in the larger vertical cut and 1,220 × 325 µm in the larger horizontal cut in an image with a 1:1 pixel mapping and automatic zoom. The macular profile and thickness were both normal. Conclusions: EDI-OCT appears to be an excellent technique for measuring choroidal nevi and all choroidal lesions accessible to OCT imaging by depicting their exact location in the choroid, their dimensions, and their demarcation from the surrounding healthy tissue, thus allowing for a more efficient and accurate follow-up