The Experts below are selected from a list of 5832 Experts worldwide ranked by ideXlab platform
Denis Trystram - One of the best experts on this subject based on the ideXlab platform.
-
on scheduling send graphs and receive graphs under the Logp model
Information Processing Letters, 2002Co-Authors: Wolf Zimmermann, Welf Lowe, Denis TrystramAbstract:In this paper we consider scheduling of send-graphs (i.e., a root sends a message to all its leaves) and receive-graphs (i.e., a root receives messages from all its leaves) under the Logp-model. It turns out that even for these simple task-graphs it is NP-complete to decide whether there is a schedule under the Logp-model with makespan at most D for any fixed number of processors. We present approximation algorithms for these problems.
-
list scheduling of general task graphs under Logp
Parallel Computing, 2000Co-Authors: Tomasz Kalinowski, Iskander Kort, Denis TrystramAbstract:List scheduling is the most frequently used scheduling technique. In this context worst case analysis as well as many experimental studies were performed for various computational models. However, many new models have been proposed during the last decade with the aim to provide a realistic but still simple and general model of parallel computation. Logp is one of the most popular models so far suggested. It takes into account the time a computation processor spends to manage a communication. Many experimental studies on current parallel architectures have shown that such a parameter cannot be neglected. The aim of this paper is to assess the applicability of the list scheduling approach to the Logp model. More precisely, we present two adaptations of the earliest task first (ETF) heuristic. Then, we establish an upper bound on list schedules under Logp. Finally, we present an extensive experimental study for diAerent graph classes and model instances. ” 2000 Elsevier Science B.V. All rights reserved.
-
some results on scheduling flat trees in Logp model
Infor, 1999Co-Authors: Iskander Kort, Denis TrystramAbstract:AbstractThis paper deals with the problem of scheduling a flat tree in the Logp model. A flat tree refers here, to either a Fork or to a Join graph. Logp is a computational model which allows to describe a wide range of parallel machines in a realistic but simple way.We first report some experiments on an IBM-SP machine to assess this model. Then the problem of scheduling a Fork graph with an unbounded number of processors is studied. This study is extended to the case of Join graphs by showing the equivalence of the corresponding scheduling problems.The purpose of this paper is to show that some scheduling problems remain tractable even in fine computational models like Logp.
-
scheduling fork graphs under Logp with an unbounded number of processors
European Conference on Parallel Processing, 1998Co-Authors: Iskander Kort, Denis TrystramAbstract:This paper deals with the problem of scheduling a specific precedence task graph, namely the Fork graph, under the Logp model. Logp is a computational model more sophisticated than the usual ones which was introduced to be closer to actual machines.
Danny Segev - One of the best experts on this subject based on the ideXlab platform.
-
Set Connectivity Problems in Undirected Graphs and the Directed Steiner Network Problem
2018Co-Authors: Chandra Chekuri, Anupam Gupta, Guy Even, Danny SegevAbstract:In the generalized connectivity problem, we are given an edge-weighted graph G = (V, E) and a collection D = {(S1,T1),…, (Sk,Tk)} of distinct demands; each demand (Si, Ti) is a pair of disjoint vertex subsets. We say that a subgraph F ⊆ G connects a demand (Si, Ti) when it contains a path with one endpoint in Si and the other in Ti. The goal is to identify a minimum weight subgraph that connects all demands in D. Alon et al. (SODA '04) introduced this problem to study online network formation settings and showed that it captures some well-studied problems such as Steiner forest, non-metric facility location, tree multicast, and group Steiner tree. Finding a non-trivial approximation ratio for generalized connectivity was left as an open problem. Our starting point is the first polylogarithmic approximation for generalized connectivity attaining a performance guarantee of O(log2 n log2 k). Here n is the number of vertices in G and k is the number of demands. We also prove that the cut-covering relaxation of this problem has an O(log3 n log2 k) integrality gap. Building upon the results for generalized connectivity we obtain improved approximation algorithms for two problems that contain generalized connectivity as a special case. For the directed Steiner network problem, we obtain an O(k1/2+ε) approximation, which improves on the currently best performance guarantee of O(k2/3) due to Charikar et al. (SODA '98). For the set connector problem, recently introduced by Fukunaga and Nagamochi (IPCO '07), we present a polylogarithmic approximation; this result improves on the previously known ratio which can be Ω(n) in the worst case
-
set connectivity problems in undirected graphs and the directed steiner network problem
ACM Transactions on Algorithms, 2011Co-Authors: Chandra Chekuri, Anupam Gupta, Guy Even, Danny SegevAbstract:In the generalized connectivity problem, we are given an edge-weighted graph G e (V,E) and a collection D e {(S1, T1), …, (Sk, Tk)} of distinct demands each demand (Si,Ti) is a pair of disjoint vertex subsets. We say that a subgraph F of G connects a demand (Si, Ti) when it contains a path with one endpoint in Si and the other in Ti. The goal is to identify a minimum weight subgraph that connects all demands in D. Alon et al. (SODA '04) introduced this problem to study online network formation settings and showed that it captures some well-studied problems such as Steiner forest, facility location with nonmetric costs, tree multicast, and group Steiner tree. Obtaining a nontrivial approximation ratio for generalized connectivity was left as an open problem. We describe the first poly-logarithmic approximation algorithm for generalized connectivity that has a performance guarantee of O(log2 n log2 k). Here, n is the number of vertices in G and k is the number of demands. We also prove that the cut-covering relaxation of this problem has an O(log3 n log2 k) integrality gap. Building upon the results for generalized connectivity, we obtain improved approximation algorithms for two problems that contain generalized connectivity as a special case. For the directed Steiner network problem, we obtain an O(k1/2 + e) approximation which improves on the currently best performance guarantee of O(k2/3) due to Charikar et al. (SODA '98). For the set connector problem, recently introduced by Fukunaga and Nagamochi (IPCO '07), we present a poly-logarithmic approximation; this result improves on the previously known ratio which can be Ω(n) in the worst case.
-
set connectivity problems in undirected graphs and the directed steiner network problem
Symposium on Discrete Algorithms, 2008Co-Authors: Chandra Chekuri, Anupam Gupta, Guy Even, Danny SegevAbstract:In the generalized connectivity problem, we are given an edge-weighted graph G = (V, E) and a collection D = {(S1,T1),…, (Sk,Tk)} of distinct demands; each demand (Si, Ti) is a pair of disjoint vertex subsets. We say that a subgraph F ⊆ G connects a demand (Si, Ti) when it contains a path with one endpoint in Si and the other in Ti. The goal is to identify a minimum weight subgraph that connects all demands in D. Alon et al. (SODA '04) introduced this problem to study online network formation settings and showed that it captures some well-studied problems such as Steiner forest, non-metric facility location, tree multicast, and group Steiner tree. Finding a non-trivial approximation ratio for generalized connectivity was left as an open problem. Our starting point is the first polylogarithmic approximation for generalized connectivity attaining a performance guarantee of O(log2 n log2 k). Here n is the number of vertices in G and k is the number of demands. We also prove that the cut-covering relaxation of this problem has an O(log3 n log2 k) integrality gap. Building upon the results for generalized connectivity we obtain improved approximation algorithms for two problems that contain generalized connectivity as a special case. For the directed Steiner network problem, we obtain an O(k1/2+e) approximation, which improves on the currently best performance guarantee of O(k2/3) due to Charikar et al. (SODA '98). For the set connector problem, recently introduced by Fukunaga and Nagamochi (IPCO '07), we present a polylogarithmic approximation; this result improves on the previously known ratio which can be Ω(n) in the worst case.
Shengang Yuan - One of the best experts on this subject based on the ideXlab platform.
-
svm approach for predicting Logp
Molecular Diversity, 2006Co-Authors: Quan Liao, Jianhua Yao, Shengang YuanAbstract:The logarithm of the partition coefficient between n-octanol and water (Logp) is an important parameter for drug discovery. Based upon the comparison of several prediction Logp models, i.e. Support Vector Machines (SVM), Partial Least Squares (PLS) and Multiple Linear Regression (MLR), the authors reported SVM model is the best one in this paper.
Anupam Gupta - One of the best experts on this subject based on the ideXlab platform.
-
Set Connectivity Problems in Undirected Graphs and the Directed Steiner Network Problem
2018Co-Authors: Chandra Chekuri, Anupam Gupta, Guy Even, Danny SegevAbstract:In the generalized connectivity problem, we are given an edge-weighted graph G = (V, E) and a collection D = {(S1,T1),…, (Sk,Tk)} of distinct demands; each demand (Si, Ti) is a pair of disjoint vertex subsets. We say that a subgraph F ⊆ G connects a demand (Si, Ti) when it contains a path with one endpoint in Si and the other in Ti. The goal is to identify a minimum weight subgraph that connects all demands in D. Alon et al. (SODA '04) introduced this problem to study online network formation settings and showed that it captures some well-studied problems such as Steiner forest, non-metric facility location, tree multicast, and group Steiner tree. Finding a non-trivial approximation ratio for generalized connectivity was left as an open problem. Our starting point is the first polylogarithmic approximation for generalized connectivity attaining a performance guarantee of O(log2 n log2 k). Here n is the number of vertices in G and k is the number of demands. We also prove that the cut-covering relaxation of this problem has an O(log3 n log2 k) integrality gap. Building upon the results for generalized connectivity we obtain improved approximation algorithms for two problems that contain generalized connectivity as a special case. For the directed Steiner network problem, we obtain an O(k1/2+ε) approximation, which improves on the currently best performance guarantee of O(k2/3) due to Charikar et al. (SODA '98). For the set connector problem, recently introduced by Fukunaga and Nagamochi (IPCO '07), we present a polylogarithmic approximation; this result improves on the previously known ratio which can be Ω(n) in the worst case
-
on the lovasz theta function for independent sets in sparse graphs
SIAM Journal on Computing, 2018Co-Authors: Nikhil Bansal, Anupam Gupta, Guru GuruganeshAbstract:We consider the maximum independent set problem on sparse graphs with maximum degree d. We show that the Lovasz ϑ-function based semidefinite program (SDP) has an integrality gap of O(d/log3/2 d), improving on the previous best result of O(d/log d). This improvement is based on a new Ramsey-theoretic bound on the independence number of Kr-free graphs for large values of r. We also show that for stronger SDPs, namely, those obtained using polylog(d) levels of the SA+ semidefinite hierarchy, the integrality gap reduces to O(d/log2 d). This matches the best unique-games-based hardness result up to lower-order poly(log log d) factors. Finally, we give an algorithmic version of this SA+-based integrality gap result, albeit using d levels of SA+, via a coloring algorithm of Johansson.
-
on the lovasz theta function for independent sets in sparse graphs
Symposium on the Theory of Computing, 2015Co-Authors: Nikhil Bansal, Anupam Gupta, Guru GuruganeshAbstract:We consider the maximum independent set problem on graphs with maximum degree d. We show that the integrality gap of the Lovasz Theta function-based SDP has an integrality gap of O~(d/log3/2 d). This improves on the previous best result of O~(d/log d), and narrows the gap of this basic SDP to the integrality gap of O~(d/log2 d) recently shown for stronger SDPs, namely those obtained using poly log(d) levels of the SA+ semidefinite hierarchy. The improvement comes from an improved Ramsey-theoretic bound on the independence number of Kr-free graphs for large values of r. We also show how to obtain an algorithmic version of the above-mentioned SAplus-based integrality gap result, via a coloring algorithm of Johansson. The resulting approximation guarantee of O~(d/log2 d) matches the best unique-games-based hardness result up to lower-order poly (log log d) factors.
-
set connectivity problems in undirected graphs and the directed steiner network problem
ACM Transactions on Algorithms, 2011Co-Authors: Chandra Chekuri, Anupam Gupta, Guy Even, Danny SegevAbstract:In the generalized connectivity problem, we are given an edge-weighted graph G e (V,E) and a collection D e {(S1, T1), …, (Sk, Tk)} of distinct demands each demand (Si,Ti) is a pair of disjoint vertex subsets. We say that a subgraph F of G connects a demand (Si, Ti) when it contains a path with one endpoint in Si and the other in Ti. The goal is to identify a minimum weight subgraph that connects all demands in D. Alon et al. (SODA '04) introduced this problem to study online network formation settings and showed that it captures some well-studied problems such as Steiner forest, facility location with nonmetric costs, tree multicast, and group Steiner tree. Obtaining a nontrivial approximation ratio for generalized connectivity was left as an open problem. We describe the first poly-logarithmic approximation algorithm for generalized connectivity that has a performance guarantee of O(log2 n log2 k). Here, n is the number of vertices in G and k is the number of demands. We also prove that the cut-covering relaxation of this problem has an O(log3 n log2 k) integrality gap. Building upon the results for generalized connectivity, we obtain improved approximation algorithms for two problems that contain generalized connectivity as a special case. For the directed Steiner network problem, we obtain an O(k1/2 + e) approximation which improves on the currently best performance guarantee of O(k2/3) due to Charikar et al. (SODA '98). For the set connector problem, recently introduced by Fukunaga and Nagamochi (IPCO '07), we present a poly-logarithmic approximation; this result improves on the previously known ratio which can be Ω(n) in the worst case.
-
set connectivity problems in undirected graphs and the directed steiner network problem
Symposium on Discrete Algorithms, 2008Co-Authors: Chandra Chekuri, Anupam Gupta, Guy Even, Danny SegevAbstract:In the generalized connectivity problem, we are given an edge-weighted graph G = (V, E) and a collection D = {(S1,T1),…, (Sk,Tk)} of distinct demands; each demand (Si, Ti) is a pair of disjoint vertex subsets. We say that a subgraph F ⊆ G connects a demand (Si, Ti) when it contains a path with one endpoint in Si and the other in Ti. The goal is to identify a minimum weight subgraph that connects all demands in D. Alon et al. (SODA '04) introduced this problem to study online network formation settings and showed that it captures some well-studied problems such as Steiner forest, non-metric facility location, tree multicast, and group Steiner tree. Finding a non-trivial approximation ratio for generalized connectivity was left as an open problem. Our starting point is the first polylogarithmic approximation for generalized connectivity attaining a performance guarantee of O(log2 n log2 k). Here n is the number of vertices in G and k is the number of demands. We also prove that the cut-covering relaxation of this problem has an O(log3 n log2 k) integrality gap. Building upon the results for generalized connectivity we obtain improved approximation algorithms for two problems that contain generalized connectivity as a special case. For the directed Steiner network problem, we obtain an O(k1/2+e) approximation, which improves on the currently best performance guarantee of O(k2/3) due to Charikar et al. (SODA '98). For the set connector problem, recently introduced by Fukunaga and Nagamochi (IPCO '07), we present a polylogarithmic approximation; this result improves on the previously known ratio which can be Ω(n) in the worst case.
Wolf Zimmermann - One of the best experts on this subject based on the ideXlab platform.
-
on scheduling task graphs to Logp machines with disturbances
European Conference on Parallel Processing, 2002Co-Authors: Welf Lowe, Wolf ZimmermannAbstract:We consider the problem of scheduling task-graphs to Logp-machines when the execution of the schedule may be delayed. If each time stepin the schedule is delayed with a certain probability, we show that under Logp the expected execution time for a schedule s is at most O(TIME(s)) where TIME(s) is the makespan of the schedule s.
-
on scheduling send graphs and receive graphs under the Logp model
Information Processing Letters, 2002Co-Authors: Wolf Zimmermann, Welf Lowe, Denis TrystramAbstract:In this paper we consider scheduling of send-graphs (i.e., a root sends a message to all its leaves) and receive-graphs (i.e., a root receives messages from all its leaves) under the Logp-model. It turns out that even for these simple task-graphs it is NP-complete to decide whether there is a schedule under the Logp-model with makespan at most D for any fixed number of processors. We present approximation algorithms for these problems.
-
bsp Logp and oblivious programs
European Conference on Parallel Processing, 1998Co-Authors: Jorn Eisenbiegler, Welf Lowe, Wolf ZimmermannAbstract:We compare the BSP and the Logp model from a practical point of view. Using compilation instead of interpretation improves the (best known) simulations of BSP programs on Logp machines by a factor of O(log P) for oblivious programs. We show that the runtime decreases for classes of oblivious BSP programs if they are compiled into Logp programs instead of executed directly using a BSP runtime library. Measurements support the statements above.
-
on linear schedules of task graphs for generalized Logp machines
European Conference on Parallel Processing, 1997Co-Authors: Welf Lowe, Wolf Zimmermann, Jorn EisenbieglerAbstract:We discuss linear schedules of task-graphs under the communication cost model of the Logp-machine. In addition to our previous work, we consider also non-constant parameters L, o and g, i.e. we introduce messages of different sizes into the Logp-model. The main results of this work are the following: (i) in the Logp-model, less communication in linear schedules does not necessarily imply a better performance. (ii) We give an upper time bound on the execution time of linear clusterings. (iii) We give an efficient algorithm which computes linear clusterings with a minimum number of clusters.
