- Zeh, Norbert

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# Zeh, Norbert

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Zeh, Norbert

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- PublicationGeometric Algorithms for Private-Cache Chip MultiprocessorsWe study techniques for obtaining efficient algorithms for geometric problems on private-cache chip multiprocessors.
359ScopusÂ© Citations 11 - PublicationI/O-Optimal Distribution Sweeping on Private-Cache Chip MultiprocessorsThe parallel external memory (PEM) model has been used as a basis for the design and analysis of a wide range of algorithms for private-cache multi-core architectures. As a tool for developing geometric algorithms in this model, a parallel version of the I/O-efficient distribution sweeping framework was introduced recently, and a number of algorithms for problems on axis-aligned objects were obtained using this framework. The obtained algorithms were efficient but not optimal. In this paper, we improve the framework to obtain algorithms with the optimal I/O complexity of O(sort P(N) + K/PB) for a number of problems on axis-aligned objects, P denotes the number of cores/processors, B denotes the number of elements that fit in a cache line, N and K denote the sizes of the input and output, respectively, and sort P(N) denotes the I/O complexity of sorting N items using P processors in the PEM model. To obtain the above improvement, we present a new one-dimensional batched range counting algorithm on a sorted list of ranges and points that achieves an I/O complexity of O((N + K)/PB), where K is the sum of the counts of all the ranges. The key to achieving efficient load balancing among the processors in this algorithm is a new method to count the output without enumerating it, which might be of independent interest.
453ScopusÂ© Citations 4 - PublicationEngineering a Topological Sorting Algorithm for Massive Graphs(Society for Industrial and Applied Mathematics, 2011-12-01)
; ; We present an I/O-efficient algorithm for topologically sorting directed acyclic graphs (DAGs). No provably I/O-efficient algorithm for this problem is known. Similarly, the performance of our algorithm, which we call IterTS, may be poor in the worst case. However, our experiments show that IterTS achieves good performance in practise. The strategy of IterTS can be summarized as follows. We call an edge satisfied if its tail has a smaller number than its head. A numbering satisfying at least half the edges in the DAG is easy to find: A random numbering is expected to have this property. IterTS starts with such a numbering and then iteratively corrects the numbering to satisfy more and more edges until all edges are satisfied. To evaluate IterTS, we compared its running time to those of three competitors: PeelTS, an I/O-efficient implementation of the standard strategy of iteratively removing sources and sinks; ReachTS, an I/O-efficient implementation of a recent parallel divide-and-conquer algorithm based on reachability queries; and SeTS, standard DFS-based topological sorting built on top of a semi-external DFS algorithm. In our evaluation on various types of input graphs, IterTS consistently outperformed PeelTS and ReachTS, by at least an order of magnitude in most cases. SeTS outperformed IterTS on most graphs whose vertex sets fit in memory. However, IterTS often came close to the running time of SeTS on these inputs and, more importantly, SeTS was not able to process graphs whose vertex sets were beyond the size of main memory, while IterTS was able to process such inputs efficiently.386ScopusÂ© Citations 3