Author: David COUDERT

Concurrent Disjoint Set Union

Robert E. Tarjan

  • Speaker: Robert E. Tarjan, Department of Computer Science, Princeton University and Intertrust Technologies
  • Title: Concurrent Disjoint Set Union — (watch the video)
  • When: December 9, 2016 — 10:30
  • Where: Inria Sophia Antipolis, amphi Kahn

  • Abstract: The disjoint set union problem is a classical problem in data structures with a simple and efficient sequential solution that has a notoriously complicated analysis. One application is to find strongly connected components in huge, implicitly defined graphs arising in model checking. In this application, the use of multiprocessors has the potential to produce significant speedups. We explore this possibility. We devise and analyze concurrent versions of standard sequential algorithms that use single and double compare-and-swap primitives for synchronization, making them wait-free. We obtain work bounds that grow logarithmically with the number of processors, suggesting the possibility of significant speedup in practice. This is ongoing joint work with Siddhartha Jayanti, an undergraduate at Princeton.

  • Short Bio: Since 1985, Robert E. Tarjan has been the James S. McDonnell Distinguished University Professor of Computer Science at Princeton University. He previously held academic positions at Cornell, Berkeley, Stanford, and NYU, and industrial research positions at Bell Labs, NEC, Intertrust Technologies, HP, and Microsoft. Among other honors, he received the Nevanlina Prize in Informatics, given by the International Mathematical Union, in 1982, and the A.C.M. Turing Award in 1986. He is a member of the National Academy of Sciences and the National Academy of Engineering, and a Fellow of the American Philosophical Society and the American Academy of Arts and Sciences. He has published over 250 papers, mostly in the areas of the design and analysis of data structures and graph and network algorithms.

Winner of the FHCP Challenge

, ,

FHCP Challenge Set

The team composed of Nathann Cohen (CNRS, LRI, Paris XI) and David Coudert won the Flinders Hamiltonian Cycle Problem (FHCP) Challenge organized by the Flinders Hamiltonian Cycle Project (Flinders University, Adelaide, Australia).

The challenge consisted in solving 1001 instances of the Hamiltonian Cycle Problem over a one year period (September 30 2015 till September 30 2016). The FHCP Challenge Set is a collection of 1001 instances of the Hamiltonian Cycle Problem, ranging in size from 66 vertices up to 9528 vertices, with an average size of just over 3000 vertices.

We were able to solve 985 instances!

PhD defense of Fatima Zahra Moataz

PhD Fatima Zahra Moataz

  • Title: “Towards Efficient and Fault-Tolerant Optical Networks: Complexity and Algorithms”
  • When: October 30, 2015 — 14:30
  • Where: Inria Sophia Antipolis, salle Euler Violet
  • Committee:

  • Abstract: We study in this thesis optimization problems with application in optical networks. The problems we consider are related to fault-tolerance and efficient resource allocation and the results we obtain are mainly related to the computational complexity of these problems.
    The first part of this thesis is devoted to finding paths and disjoint paths. Finding a path is crucial in all types of networks in order to set up connections and finding disjoint paths is a common approach used to provide some degree of protection against failures in networks. We study these problems under different settings. We first focus on finding paths and vertex or link-disjoint paths in networks with asymmetric nodes, which are nodes with restrictions on their internal connectivity. Afterwards, we consider networks with star Shared Risk Link Groups (SRLGs) which are groups of links that might fail simultaneously due to a localized event. In these networks, we investigate the problem of finding SRLG-disjoint paths.
    The second part of this thesis focuses on the problem of Routing and Spectrum Assignment (RSA) in Elastic Optical Networks (EONs). EONs are proposed as the new generation of optical networks and they aim at an efficient and flexible use of the optical resources. RSA is the key problem in EONs and it deals with allocating resources to requests under multiple constraints. We first study the static version of RSA in tree networks. Afterwards, we examine a dynamic version of RSA in which a non-disruptive spectrum defragmentation technique is used.
    Finally, we present in the appendix another problem that has been studied during this thesis. It is a graph-theoretic problem referred to as minimum size tree-decomposition and it deals with the decomposition of graphs in a tree-like manner with the objective of minimizing the size of the tree.

  • Keywords: Asymmetric nodes, forbidden transitions, shared risk link group, routing and spectrum assignment, tree-decomposition, complexity.

PhD defense of Mohamed Bergach

PhD Mohamed Bergach

  • Title: “Adaptation of the Fast Fourier Transform processing on hybride integrated CPU/GPU architecture”
  • When: October 2, 2015 — 10:30
  • Where: Inria Sophia Antipolis, salle Euler Violet
  • Committee:

  • Abstract: Multicore architectures Intel Core (IvyBridge, Haswell, etc.) contain both general purpose CPU cores (4) and dedicated GPU cores embedded on the same chip (16 and 40 respectively). As part of the activity of Kontron (the company partially funding this CIFRE scholarship), an important objective is to efficiently compute arrays and sequences of fast Fourier transforms (FFT) such as one finds in radar applications, on this architecture. While native (but proprietary) libraries exist for Intel CPU, nothing is currently available for the GPU part.
    The aim of the thesis was to define the efficient placement of FFT modules, and to study theoretically the optimal form for grouping computing stages of such FFT according to data locality on a single computing core. This choice should allow processing efficiency, by adjusting the memory size available to the required application data size.
    Then the multiplicity of cores is exploitable to compute several FFT in parallel, without interference (except for possible bus contention between the CPU and the GPU). We have achieved significant results, both in the implementation of an FFT (1024 points) on a SIMD CPU core, expressed in C, and in the implementation of a FFT of the same size on a GPU SIMT core, then expressed in OpenCL.
    In addition, our results allow to define rules to automatically synthesize such solutions, based solely on the size of the FFT (more specifically its number of stages), and the size of the local memory for a given computing core. The performances obtained are better than the native Intel library for CPU, and demonstrate a significant gain in consumption on GPU. All these points are detailed in the thesis document.
    These results should lead to exploitation of the code as library by the Kontron company.

  • Keywords: FFT, GPU, multicores.

Best student paper award

, ,

Fatima Zahra Moataz

Fatima Zahra Moataz received the best student paper award of the conference ALGOTEL 2015 for her paper entitled “On Spectrum Assignment in Elastic Optical Tree-Networks“.
 
 

  • Title: “On Spectrum Assignment in Elastic Optical Tree-Networks“
  • Author: Fatima Zahra Moataz
  • Event: ALGOTEL 2015 — 17èmes Rencontres Francophones sur les Aspects Algorithmiques des Télécommunications, June 2015, Beaune, France.

  • Abstract: To face the explosion of the Internet traffic, a new generation of optical networks is being developed; the Elastic optical Networks (EONs). The aim with EONs is to use the optical spectrum efficiently and flexibly. The benefit of the flexibility is accompanied by more difficulty in the resource allocation problems. In this report, we study the problem of Spectrum Allocation in Elastic Optical Tree-Networks. In trees, even though the routing is fixed, the spectrum allocation is NP-hard. We survey the complexity and approximability results that have been established for the SA in trees and prove new results for stars and binary trees.

  • Keywords: Interval coloring; Optical networks; Routing and Spectrum Assignment; Approximation algorithms.

PhD defense of Alvinice Kodjo

PhD Alvinice Kodjo

  • Title: “Design and Optimization of Wireless Backhaul Networks”
  • When: December 18, 2014 — 14:30
  • Where: Polytech-Nice Sophia Antipolis, salle E+131 (Amphi Gradiné Templiers 2)
  • Committee:

  • Abstract: The main work of this thesis focuses on the wireless backhaul networks. This type of network has the advantage of allowing rapid and easy deployment at relatively low costs while offering capacity of up to 1Gbps over links of a hundred of kilometers. We studied different optimization problems in such networks that represent real challenges for industrial sector.
    The first issue addressed in this thesis concerns the capacity allocation on the links at minimum cost. It was solved by a linear programming approach with column generation. Our method solves the problems on large size networks. In addition, we quickly obtain very good quality solutions.
    We then studied the problem of network infrastructure sharing between virtual operators. The objective is to maximize the revenue of the operator of the physical infrastructure while satisfying the demands (constraints of quality of service) of virtual operators customers of the network. In this context, we proposed a model to the problem using mixed integer linear programming. Our formulation is robust to changes in traffic demands of virtual operators.
    Another operational expenditure in this type of network is the energy consumption. Many operators are now seeking to reduce network energy consumption, both by using more efficient equipments and by using more appropriate routing solutions. We proposed a robust energy-aware routing solution for the network. The proposed model is based on backbone networks, but it can easily be adapted to wireless back- haul networks. Our solution was formulated using a mixed integer linear program. We also proposed heuristics to find efficient solutions for large networks.
    The last work of this thesis focuses on cognitive radio networks and more specifically on the problem of bandwidth sharing. We formalized it using a linear program with a different approach to robust optimization. This model differs from other cases of robust optimization discussed above by the position of the uncertain terms in the model. We based our solution on the 2-stage linear robust formulation proposed by Michel Minoux.
    This thesis was carried out in partnership with the SME 3ROAM (http://www.3roam.com) and in collaboration with various researchers from different universities in the world. It was co-funded by the PACA province and the SME 3ROAM.

  • Keywords: Backhaul networks, Wireless networks, Operations Research, Robust optimization.