| 1. |
- Alava, Mikko, et al.
(författare)
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Circumspect descent prevails in solving random constraint satisfaction problems
- 2008
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 105:40, s. 15253-15257
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Tidskriftsartikel (refereegranskat)abstract
- We study the performance of stochastic local search algorithms for random instances of the K-satisfiability (K-SAT) problem. We present a stochastic local search algorithm, ChainSAT, which moves in the energy landscape of a problem instance by never going upwards in energy. ChainSAT is a focused algorithm in the sense that it focuses on variables occurring in unsatisfied clauses. We show by extensive numerical investigations that ChainSAT and other focused algorithms solve large K-SAT instances almost surely in linear time, up to high clause-to-variable ratios a; for example, for K = 4 we observe linear-time performance well beyond the recently postulated clustering and condensation transitions in the solution space. The performance of ChainSAT is a surprise given that by design the algorithm gets trapped into the first local energy minimum it encounters, yet no such minima are encountered. We also study the geometry of the solution space as accessed by stochastic local search algorithms.
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| 2. |
- Ardelius, John, et al.
(författare)
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Behavior of heuristics on large and hard satisfiability problems
- 2006
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 74:3, s. 037702-
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Tidskriftsartikel (refereegranskat)abstract
- We study the behavior of a heuristic for solving random satisfiability problems by stochastic local search near the satisfiability threshold. The heuristic for average satisfiability (ASAT), is similar to the Focused Metropolis Search heuristic, and shares the property of being focused, i.e., only variables in unsatisfied clauses are updated in each step. It is significantly simpler than the benchmark WALKSAT heuristic. We show that ASAT solves instances as large as N=10(6) in linear time, on average, up to a ratio of 4.21 clauses per variable in random three-satisfiability. For K higher than 3, ASAT appears to solve instances of K-satisfiability up to the Montanari-Ricci-Tersenghi-Parisi full replica symmetry breaking (FSRB) threshold denoted alpha(s)(K) in linear time.
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| 3. |
- Ardelius, John, et al.
(författare)
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Clustering of solutions in hard satisfiability problems
- 2007
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Ingår i: Journal of Statistical Mechanics: Theory and Experiment. - 1742-5468. ; :10, s. P10012-
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Tidskriftsartikel (refereegranskat)abstract
- We study numerically the solution space structure of random 3-SAT problems close to the SAT/UNSAT transition. This is done by considering chains of satisfiability problems, where clauses are added sequentially to a problem instance. Using the overlap measure of similarity between different solutions found on the same problem instance, we examine geometrical changes as a function of α. In each chain, the overlap distribution is first smooth, but then develops a tiered structure, indicating that the solutions are found in well separated clusters. On chains of not too large instances, all remaining solutions are eventually observed to be found in only one small cluster before vanishing. This condensation transition point is estimated by finite size scaling to be αc = 4.26 with an apparent critical exponent of about 1.7. The average overlap value is also observed to increase with α up to the transition, indicating a reduction in solutions space size, in accordance with theoretical predictions. The solutions are generated by a local heuristic, ASAT, and compared to those found by the Survey Propagation algorithm up to αc.
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| 4. |
- Ardelius, John, et al.
(författare)
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Exhaustive enumeration unveils clustering and freezing in the random 3-satisfiability problem
- 2008
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Ingår i: Physical Review E - Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755. ; 78:4, s. 040101-
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Tidskriftsartikel (refereegranskat)abstract
- We study geometrical properties of the complete set of solutions of the random 3-satisfiability problem. We show that even for moderate system sizes the number of clusters corresponds surprisingly well with the theoretic asymptotic prediction. We locate the freezing transition in the space of solutions, which has been conjectured to be relevant in explaining the onset of computational hardness in random constraint satisfaction problems.
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| 5. |
- Ardelius, John, et al.
(författare)
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Modeling the performance of ring based DHTs in the presence of network address translators
- 2011
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Ingår i: Lecture Notes in Computer Science. - Berlin, Heidelberg : Springer Nature. - 0302-9743 .- 1611-3349. ; 6723, s. 15-28
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Tidskriftsartikel (refereegranskat)abstract
- Dealing with Network Address Translators (NATs) is a central problem in many peer-to-peer applications on the Internet today. However, most analytical models of overlay networks assume the underlying network to be a complete graph, an assumption that might hold in evaluation environments such as PlanetLab but turns out to be simplistic in practice. In this work we introduce an analytical network model where a fraction of the communication links are unavailable due to NATs. We investigate how the topology induced by the model affects the performance of ring based DHTs. We quantify two main performance issues induced by NATs namely large lookup inconsistencies and increased break-up probability, and suggest how theses issues can be addressed. The model is evaluated using discrete based simulation for a wide range of parameters.
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| 6. |
- Ardelius, John, 1978-
(författare)
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On state space structure and average case complexity in random K-SAT problems
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis gives an introduction to a currently active area in the cross-section between theoretical computer science and theoretical physics. In the last ten years it has been suggested that critical behaviour, usually seen in models from condensed matter physics, may be responsible for the intractability of NP complete computation problems. This would suggest a very deep connection between the two fields on the most fundamental level. How deep this connection really is is subject to ongoing research as well as the topic of this thesis. Some of the conjectrues from the physics community regarding computational hardness in certain problem classes has turned out to be wrong or misinterpreted but the gained interest in both fields has promising potiential in moving the research frontier forward. The material presented in this thesis is the result of nearly two years work in trying to clearify how the results from physics can be interpreted in the language of actuall computation problems.
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| 7. |
- Ardelius, John, et al.
(författare)
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On the effects of caching in access aggregation networks
- 2012
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Ingår i: ICN 2012, Helsinki, Finland. - : Association for Computing Machinery (ACM). - 9781450314794
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Konferensbidrag (refereegranskat)abstract
- All forecasts of Internet trac point at a substantial growth over the next few years. From a network operator perspective, efficient in-network caching of data is and will be a key component in trying to cope with and profit from this increasing demand. One problem, however, is to evaluate the performance of different caching policies as the number of available data items as well as the distribution networks grows very large.In this work, we develop an analytical model of an aggregation access network receiving a continuous flow of requests from external clients. We provide exact analytical solutions for cache hit rates, data availability and more. This enables us to provide guidelines and rules of thumb for operators and Information-Centric Network designers.Finally, we apply our analytical results to a real VoD trace from a network operator and show that substantial bandwidth savings can be expected when using in-network caching in a realistic setting.
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| 8. |
- Ardelius, John, et al.
(författare)
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On the effects of caching in access aggregation networks
- 2012. - 6
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Konferensbidrag (refereegranskat)abstract
- All forecasts of Internet traffic point at a substantial growth over the next few years. From a network operator perspective, efficient in-network caching of data is and will be a key component in trying to cope with and profit from this increasing demand. One problem, however, is to evaluate the performance of different caching policies as the number of available data items as well as the distribution networks grows very large. In this work, we develop an analytical model of an aggregation access network receiving a continuous flow of requests from external clients. We provide exact analytical solutions for cache hit rates, data availability and more. This enables us to provide guidelines and rules of thumb for operators and Information-Centric Network designers. Finally, we apply our analytical results to a real VoD trace from a network operator and show that substantial bandwidth savings can be expected when using in-network caching in a realistic setting.
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| 9. |
- Ardelius, John, 1978-
(författare)
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On the Performance Analysis of Large Scale, Dynamic, Distributed and Parallel Systems.
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Evaluating the performance of large distributed applications is an important and non-trivial task. With the onset of Internet wide applications there is an increasing need to quantify reliability, dependability and performance of these systems, both as a guide in system design as well as a means to understand the fundamental properties of large-scale distributed systems. Previous research has mainly focused on either formalised models where system properties can be deduced and verified using rigorous mathematics or on measurements and experiments on deployed applications. Our aim in this thesis is to study models on an abstraction level lying between the two ends of this spectrum. We adopt a model of distributed systems inspired by methods used in the study of large scale system of particles in physics and model the application nodes as a set of interacting particles each with an internal state whose actions are specified by the application program. We apply our modeling and performance evaluation methodology to four different distributed and parallel systems. The first system is the distributed hash table (DHT) Chord running in a dynamic environment. We study the system under two scenarios. First we study how performance (in terms of lookup latency) is affectedon a network with finite communication latency. We show that an average delay in conjunction with other parameters describing changes in the network (such as timescales for network repair and join and leave processes)induces fundamentally different system performance. We also verify our analytical predictions via simulations.In the second scenario we introduce network address translators (NATs) to the network model. This makes the overlay topology non-transitive and we explore the implications of this fact to various performance metrics such as lookup latency, consistency and load balance. The latter analysis is mainly simulation based.Even though these two studies focus on a specific DHT, many of our results can easily be translated to other similar ring-based DHTs with long-range links, and the same methodology can be applied evento DHT's based on other geometries.The second type of system studied is an unstructured gossip protocol running a distributed version of the famous Belman-Ford algorithm. The algorithm, called GAP, generates a spanning tree over the participating nodes and the question we set out to study is how reliable this structure is(in terms of generating accurate aggregate values at the root) in the presence of node churn. All our analytical results are also verified using simulations.The third system studied is a content distribution network (CDN) of interconnected caches in an aggregation access network. In this model, content which sits at the leaves of the cache hierarchy tree, is requested by end users. Requests can then either be served by the first cache level or sent further up the tree. We study the performance of the whole system under two cache eviction policies namely LRU and LFU. We compare our analytical results with traces from related caching systems.The last system is a work stealing heuristic for task distribution in the TileraPro64 chip. This system has access to a shared memory and is therefore classified as a parallel system. We create a model for the dynamic generation of tasks as well as how they are executed and distributed among the participating nodes. We study how the heuristic scales when the number of nodes exceeds the number of processors on the chip as well as how different work stealing policies compare with each other. The work on this model is mainly simulation-based.
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| 10. |
- Ardelius, John
(författare)
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Temperature dependent sampling in hard satisfiability problems
- 2008
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The solution sets of constraint satisfaction problems (CSPs)have been conjectured to to split up into clusters (connectedcomponents) when they are close to critically constrained,and this has been assumed to be related to computationalhardness. Simple stochastic local search (SLS) heuristics hashowever shown to be very efficient for many of these problems,and it has been unclear if this is related to clustering.In this work an SLS is used to sample the space of solutionsand the results are compared to the actual solution space generatedwith a complete solver. We show that different heuristicsfind different types of clusters. An increased greedinessresults in a sampling more uniform over the set of clusters,rather than over the set of solutions and the fastest solver visitsthe smaller cluster much more frequently than its solutionsdensity would imply. We also show that the level of randomness(temperature) is related in a non-trivial way to the abilityto escape from local minimas. This approach confirms thatclusters are important in understanding computational hardnessand may begin to explain the efficiency of SLSs in fragmentedsolution spaces.
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