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Sökning: LAR1:cth > Tsigas Philippas 1967

  • Resultat 1-10 av 189
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  • Almgren, Magnus, 1972-, et al. (författare)
  • Mapping Systems Security Research at Chalmers
  • 2011
  • Ingår i: First SysSec Workshop (SysSec 2011). - 978-0-7695-4530-1 ; s. 67-70
  • Konferensbidrag (refereegranskat)abstract
    • The department of Computer Science and Engineering at Chalmers University has a long tradition of research in systems security, including security metrics, attack detection, and mitigation. We focus on security issues arising in four specific environments: (1) backbone links, (2) sensor networks, (3) the connected car, and (4) the smart grid. In this short summary we describe recent results as well as open research questions we are exploring.
  • Assarsson, Ulf, 1972-, et al. (författare)
  • Image-Space Dynamic Transparency for Improved Object Discovery in 3D Environments
  • 2006
  • Rapport (övrigt vetenskapligt)abstract
    • We present an image-space algorithm for dynamic transparency with the purpose of supporting object discovery and access in information-rich 3D visualization environments. The algorithm is based on multiple rendering passes and detects instances of object occlusion in the image-space using the fragment shader capabilities of modern programmable graphics hardware, creating alpha maps of opacity gradients around the occluded objects. In essence, the effect is somewhat similar to “X-ray vision” of a superhero. We have implemented a prototype version of our algorithm with real-time rendering performance using a number of optimizations and speedups on current graphics hardware. To evaluate its use, we have also implemented three different example applications portraying different scenarios, including abstract visualization, virtual walkthrough, and gaming. Preliminary results from an empirical user study comparing our technique to standard viewpoint controls indicate that our technique is superior in regards to object discovery efficiency. These results hold over both completion times as well as correctness of a visual search task.
  • Atalar, Aras, 1985-, et al. (författare)
  • Analyzing the Performance of Lock-Free Data Structures: A Conflict-Based Model
  • 2015
  • Ingår i: Lecture Notes in Computer Science - 29th International Symposium on Distributed Computing (DISC). - 0302-9743. - 978-3-662-48653-5 ; 9363, s. 341-355
  • Konferensbidrag (refereegranskat)abstract
    • This paper considers the modeling and the analysis of the performance of lock-free concurrent data structures that can be represented as linear combinations of fixed size retry loops. Our main contribution is a new way of modeling and analyzing a general class of lock-free algorithms, achieving predictions of throughput that are close to what we observe in practice. We emphasize two kinds of conflicts that shape the performance: (i) hardware conflicts, due to concurrent calls to atomic primitives; (ii) logical conflicts, caused by concurrent operations on the shared data structure. We propose also a common framework that enables a fair comparison between lock-free implementations by covering the whole contention domain, and comes with a method for calculating a good back-off strategy. Our experimental results, based on a set of widely used concurrent data structures and on abstract lock-free designs, show that our analysis follows closely the actual code behavior.(1)
  • Atalar, Aras, 1985-, et al. (författare)
  • How lock-free data structures perform in dynamic environments: Models and analyses
  • 2017
  • Ingår i: Leibniz International Proceedings in Informatics, LIPIcs. - 1868-8969. - 978-395977031-6 ; 70, s. 231-2317
  • Konferensbidrag (refereegranskat)abstract
    • © Aras Atalar, Paul Renaud-Goud, and Philippas Tsigas.In this paper we present two analytical frameworks for calculating the performance of lock-free data structures. Lock-free data structures are based on retry loops and are called by application-specific routines. In contrast to previous work, we consider in this paper lock-free data structures in dynamic environments. The size of each of the retry loops, and the size of the application routines invoked in between, are not constant but may change dynamically. The new frameworks follow two different approaches. The first framework, the simplest one, is based on queuing theory. It introduces an average-based approach that facilitates a more coarse-grained analysis, with the benefit of being ignorant of size distributions. Because of this independence from the distribution nature it covers a set of complicated designs. The second approach, instantiated with an exponential distribution for the size of the application routines, uses Markov chains, and is tighter because it constructs stochastically the execution, step by step. Both frameworks provide a performance estimate which is close to what we observe in practice. We have validated our analysis on (i) several fundamental lock-free data structures such as stacks, queues, deques and counters, some of them employing helping mechanisms, and (ii) synthetic tests covering a wide range of possible lock-free designs. We show the applicability of our results by introducing new back-off mechanisms, tested in application contexts, and by designing an efficient memory management scheme that typical lock-free algorithms can utilize.
  • Atalar, Aras, 1985-, et al. (författare)
  • Modeling and Analyzing the Performance of Lock-Free Data Structures
  • 2014
  • Rapport (övrigt vetenskapligt)abstract
    • This paper considers the modeling and the analysis of the performance of lock-free concurrent data structures. Lock-free designs employ an optimistic conflict control approach, allowing several processes to access the shared data object at the same time. The operations on these data structures are typically designed as compositions of retry loops. Our main contribution is a new way of modeling and analyzing a general class of lock-free algorithms, achieving predictions of throughput that are close to what we observe in practice. In our model we introduce two key metrics that shape the performance of lock-free algorithms: (i) expansion in execution time of a retry due to memory congestion and (ii) number of wasted retries. We show how to compute these two metrics, and how to combine them, to calculate the throughput of an arguably large class of lock-free algorithms. Our analysis also captures the throughput performance of a lock-free algorithm when executed as part of a larger parallel application. This part of our analysis leads to an analytical method for calculating a good back-off strategy to finely tune the performance of a lock-free application. Our experimental results, based on a set of widely used concurrent data structures and on abstract lock-free designs, show that our analysis follows closely the actual code behavior. To the best of our knowledge, this is the first attempt to make ends meet between theoretical bounds on performance and actual measured throughput.
  • Atalar, Aras, 1985-, et al. (författare)
  • Modeling Energy Consumption of Lock-Free Queue Implementations
  • 2015
  • Rapport (övrigt vetenskapligt)abstract
    • This paper considers the problem of modeling the energy behavior of lock-free concurrent queue data structures. Our main contribution is a way to model the energy behavior of lock-free queue implementations and parallel applications that use them. Focusing on steady state behavior we decompose energy behavior into throughput and power dissipation which can be modeled separately and later recombined into several useful metrics, such as energy per operation. Based on our models, instantiated from synthetic benchmark data, and using only a small amount of additional application specific information, energy and throughput predictions can be made for parallel applications that use the respective data structure implementation. To model throughput we propose a generic model for lock-free queue throughput behavior, based on a combination of the dequeuers’ throughput and enqueuers’ throughput. To model power dissipation we commonly split the contributions from the various computer components into static, activation and dynamic parts, where only the dynamic part depends on the actual instructions being executed. To instantiate the models a synthetic benchmark explores each queue implementation over the dimensions of processor frequency and number of threads. Finally, we show how to make predictions of application throughput and power dissipation for a parallel application using a lock-free queue requiring only a limited amount of information about the application work done between queue operations. Our case study on a Mandelbrot application shows convincing prediction results.
  • Benkner, S., et al. (författare)
  • The PEPPHER approach to programmability and performance portability for heterogeneous many-core architectures
  • 2012
  • Ingår i: Advances in Parallel Computing. - 9781614990406
  • Bokkapitel (övrigt vetenskapligt)abstract
    • The European FP7 project PEPPHER is addressing programmability and performance portability for current and emerging heterogeneous many-core architectures. As its main idea, the project proposes a multi-level parallel execution model comprised of potentially parallelized components existing in variants suitable for different types of cores, memory configurations, input characteristics, optimization criteria, and couples this with dynamic and static resource and architecture aware scheduling mechanisms. Crucial to PEPPHER is that components can be made performance aware, allowing for more efficient dynamic and static scheduling on the concrete, available resources. The flexibility provided in the software model, combined with a customizable, heterogeneous, memory and topology aware run-time system is key to efficiently exploiting the resources of each concrete hardware configuration. The project takes a holistic approach, relying on existing paradigms, interfaces, and languages for the parallelization of components, and develops a prototype framework, a methodology for extending the framework, and guidelines for constructing performance portable software and systems-including paths to migration of existing software-for heterogeneous many-core processors. This paper gives a high-level project overview, and presents a specific example showing how the PEPPHER component variant model and resource-aware run-time system enable performance portability of a numerical kernel. © 2012 The authors and IOS Press. All rights reserved.
  • Casado, Lander, 1985-, et al. (författare)
  • ContikiSec: A Secure Network Layer for Wireless Sensor Networks under the Contiki Operating System
  • 2009
  • Ingår i: Proceedings of the 14th Nordic Conference on Secure IT Systems (NordSec 2009), Lecture Notes in Computer Science. - 1611-3349. - 978-3-642-04765-7 ; 5838, s. 133-147
  • Konferensbidrag (refereegranskat)abstract
    • In this paper we introduce ContikiSec, a secure network layer for wireless sensor networks, designed for the Contiki Operating System. ContikiSec has a configurable design, providing three security modes starting from confidentiality and integrity, and expanding to confidentiality, authentication, and integrity. ContikiSec has been designed to balance low energy consumption and security while conforming to a small memory footprint. Our design was based on performance evaluation of existing security primitives and is part of the contribution of this paper. Our evaluation was performed in the Modular Sensor Board hardware platform for wireless sensor networks, running Contiki. Contiki is an open source, highly portable operating system for wireless sensor networks (WSN) that is widely used in WSNs.
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