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21
Lower bounds for the noisy broadcast problem
 In Proceedings of the 46 th IEEE Symposium on Foundations of Computer Science (FOCS 2005
, 2005
"... We prove the first nontrivial (super linear) lower bound in the noisy broadcast model, defined by El Gamal in [6]. In this model there are n + 1 processors P0, P1,..., Pn, each of which is initially given a private input bit xi. The goal is for P0 to learn the value of f(x1,..., xn), for some speci ..."
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We prove the first nontrivial (super linear) lower bound in the noisy broadcast model, defined by El Gamal in [6]. In this model there are n + 1 processors P0, P1,..., Pn, each of which is initially given a private input bit xi. The goal is for P0 to learn the value of f(x1,..., xn), for some specified function f, using a series of noisy broadcasts. At each step a designated processor broadcasts one bit to all of the other processors, and the bit received by each processor is flipped with fixed probability (independently for each recipient). In 1988, Gallager [16] gave a noiseresistant protocol that allows P0 to learn the entire input with constant probability in O(n log log n) broadcasts. We prove that Gallager’s protocol is optimal, up to a constant factor. Our lower bound follows by reduction from a lower bound for generalized noisy decision trees, a new model which may be of independent interest. For this new model we show a lower bound of Ω(n log n) on the depth of a tree that learns the entire input. We also show an Ω(n log log n) lower bound for the number of broadcasts required to compute certain explicit booleanvalued functions, when the correct output must be attained with probability at least 1 − n −α for a constant parameter α> 0 (this bound applies to all threshold functions, as well as any other booleanvalued function with linear sensitivity). This bound also follows by reduction from a lower bound of Ω(n log n) on the depth of generalized noisy decision trees that compute the same functions with the same error. We also show a (nontrivial) Ω(n) lower bound on the depth of generalized noisy decision trees that compute such functions with small constant error. Finally, we show the first protocol in the noisy broadcast model that computes the Hamming weight of the input using a linear number of broadcasts.
Optimal resilient sorting and searching in the presence of memory faults
 IN PROC. 33RD INTERNATIONAL COLLOQUIUM ON AUTOMATA, LANGUAGES AND PROGRAMMING, VOLUME 4051 OF LECTURE NOTES IN COMPUTER SCIENCE
, 2006
"... We investigate the problem of reliable computation in the presence of faults that may arbitrarily corrupt memory locations. In this framework, we consider the problems of sorting and searching in optimal time while tolerating the largest possible number of memory faults. In particular, we design an ..."
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Cited by 17 (5 self)
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We investigate the problem of reliable computation in the presence of faults that may arbitrarily corrupt memory locations. In this framework, we consider the problems of sorting and searching in optimal time while tolerating the largest possible number of memory faults. In particular, we design an O(n log n) time sorting algorithm that can optimally tolerate up to O ( √ n log n) memory faults. In the special case of integer sorting, we present an algorithm with linear expected running time that can tolerate O ( √ n) faults. We also present a randomized searching algorithm that can optimally tolerate up to O(log n) memory faults in O(log n) expected time, and an almost optimal deterministic searching algorithm that can tolerate O((log n) 1−ǫ) faults, for any small positive constant ǫ, in O(log n) worstcase time. All these results improve over previous bounds.
Max Algorithms in Crowdsourcing Environments
"... Our work investigates the problem of retrieving the maximum item from a set in crowdsourcing environments. We first develop parameterized families of max algorithms, that take as input a set of items and output an item from the set that is believed to be the maximum. Such max algorithms could, for i ..."
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Cited by 15 (1 self)
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Our work investigates the problem of retrieving the maximum item from a set in crowdsourcing environments. We first develop parameterized families of max algorithms, that take as input a set of items and output an item from the set that is believed to be the maximum. Such max algorithms could, for instance, select the best Facebook profile that matches a given person or the best photo that describes a given restaurant. Then, we propose strategies that select appropriatemaxalgorithmparameters. Ourframeworksupports various human error and cost models and we consider many of them for our experiments. We evaluate under many metrics, both analytically and via simulations, the tradeoff between three quantities: (1) quality, (2) monetary cost, and (3) execution time. Also, we provide insights on the effectiveness of the strategies in selecting appropriate max algorithm parameters and guidelines for choosing max algorithms and strategies for each application.
Resilient search trees
 IN PROCEEDINGS OF 18TH ACMSIAM SODA
, 2007
"... We investigate the problem of computing in a reliable fashion in the presence of faults that may arbitrarily corrupt memory locations. In this framework, we focus on the design of resilient data structures, i.e., data structures that, despite the corruption of some memory values during their lifetim ..."
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Cited by 15 (5 self)
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We investigate the problem of computing in a reliable fashion in the presence of faults that may arbitrarily corrupt memory locations. In this framework, we focus on the design of resilient data structures, i.e., data structures that, despite the corruption of some memory values during their lifetime, are nevertheless able to operate correctly (at least) on the set of uncorrupted values. In particular, we present resilient search trees which achieve optimal time and space bounds while tolerating up to O ( √ log n) memory faults, where n is the current number of items in the search tree. In more detail, our resilient search trees are able to insert, delete and search for a key in O(log n + δ 2) amortized time, where δ is an upper bound on the total number of faults. The space required is O(n + δ).
Priority queues resilient to memory faults
 IN: PROC. 10TH INTERNATIONAL WORKSHOP ON ALGORITHMS AND DATA STRUCTURES
, 2007
"... In the faultymemory RAM model, the content of memory cells can get corrupted at any time during the execution of an algorithm, and a constant number of uncorruptible registers are available. A resilient data structure in this model works correctly on the set of uncorrupted values. In this paper w ..."
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Cited by 13 (5 self)
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In the faultymemory RAM model, the content of memory cells can get corrupted at any time during the execution of an algorithm, and a constant number of uncorruptible registers are available. A resilient data structure in this model works correctly on the set of uncorrupted values. In this paper we introduce a resilient priority queue. The deletemin operation of a resilient priority queue returns either the minimum uncorrupted element or some corrupted element. Our resilient priority queue uses O(n) space to store n elements. Both insert and deletemin operations are performed in O(log n + δ) time amortized, where δ is the maximum amount of corruptions tolerated. Our priority queue matches the performance of classical optimal priority queues in the RAM model when the number of corruptions tolerated is O(log n). We prove matching worst case lower bounds for resilient priority queues storing only structural information in the uncorruptible registers between operations.
Optimal resilient dynamic dictionaries
 IN PROCEEDINGS OF 15TH EUROPEAN SYMPOSIUM ON ALGORITHMS (ESA
, 2007
"... We investigate the problem of computing in the presence of faults that may arbitrarily (i.e., adversarially) corrupt memory locations. In the faulty memory model, any memory cell can get corrupted at any time, and corrupted cells cannot be distinguished from uncorrupted ones. An upper bound δ on the ..."
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Cited by 12 (8 self)
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We investigate the problem of computing in the presence of faults that may arbitrarily (i.e., adversarially) corrupt memory locations. In the faulty memory model, any memory cell can get corrupted at any time, and corrupted cells cannot be distinguished from uncorrupted ones. An upper bound δ on the number of corruptions and O(1) reliable memory cells are provided. In this model, we focus on the design of resilient dictionaries, i.e., dictionaries which are able to operate correctly (at least) on the set of uncorrupted keys. We first present a simple resilient dynamic search tree, based on random sampling, with O(log n+δ) expected amortized cost per operation, and O(n) space complexity. We then propose an optimal deterministic static dictionary supporting searches in Θ(log n+δ) time in the worst case, and we show how to use it in a dynamic setting in order to support updates in O(log n + δ) amortized time. Our dynamic dictionary also supports range queries in O(log n+δ+t) worst case time, where t is the size of the output. Finally, we show that every resilient search tree (with some reasonable properties) must take Ω(log n + δ) worstcase time per search.
Designing Reliable Algorithms in Unreliable Memories
"... Some of today’s applications run on computer platforms with large and inexpensive memories, which are also errorprone. Unfortunately, the appearance of even very few memory faults may jeopardize the correctness of the computational results. An algorithm is resilient to memory faults if, despite t ..."
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Cited by 11 (3 self)
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Some of today’s applications run on computer platforms with large and inexpensive memories, which are also errorprone. Unfortunately, the appearance of even very few memory faults may jeopardize the correctness of the computational results. An algorithm is resilient to memory faults if, despite the corruption of some memory values before or during its execution, it is nevertheless able to get a correct output at least on the set of uncorrupted values. In this paper we will survey some recent work on reliable computation in the presence of memory faults.
Sorting and Selection with Imprecise Comparisons
"... Abstract. In experimental psychology, the method of paired comparisons was proposed as a means for ranking preferences amongst n elements of a human subject. The method requires performing all ( n 2 comparisons then sorting elements according to the number of wins. The large number of comparisons i ..."
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Cited by 9 (1 self)
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Abstract. In experimental psychology, the method of paired comparisons was proposed as a means for ranking preferences amongst n elements of a human subject. The method requires performing all ( n 2 comparisons then sorting elements according to the number of wins. The large number of comparisons is performed to counter the potentially faulty decisionmaking of the human subject, who acts as an imprecise comparator. We consider a simple model of the imprecise comparisons: there exists some δ> 0 such that when a subject is given two elements to compare, if the values of those elements (as perceived by the subject) differ by at least δ, then the comparison will be made correctly; when the two elements have values that are within δ, the outcome of the comparison is unpredictable. This δ corresponds to the just noticeable difference unit (JND) or difference threshold in the psychophysics literature, but does not require the statistical assumptions used to define this value. In this model, the standard method of paired comparisons minimizes the errors introduced by the imprecise comparisons at the cost of ( n 2 comparisons. We show that the same optimal guarantees can be achieved using 4n 3/2 comparisons, and we prove the optimality of our method. We then explore the general tradeoff between the guarantees on the error that can be made and number of comparisons for the problems of sorting, maxfinding, and selection. Our results provide closetooptimal solutions for each of these problems. 1
Counting in the Presence of Memory Faults
"... Abstract. The faulty memory RAM presented by Finocchi and Italiano [1] is a variant of the RAM model where the content of any memory cell can get corrupted at any time, and corrupted cells cannot be distinguished from uncorrupted cells. An upper bound, δ, on the number of corruptions and O(1) reliab ..."
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Cited by 6 (1 self)
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Abstract. The faulty memory RAM presented by Finocchi and Italiano [1] is a variant of the RAM model where the content of any memory cell can get corrupted at any time, and corrupted cells cannot be distinguished from uncorrupted cells. An upper bound, δ, on the number of corruptions and O(1) reliable memory cells are provided. In this paper we investigate the fundamental problem of counting in faulty memory. Keeping many reliable counters in the faulty memory is easily done by replicating the value of each counter Θ(δ) times and paying Θ(δ) time every time a counter is queried or incremented. In this paper we decrease the expensive increment cost to o(δ) and present upper and lower bound tradeoffs decreasing the increment time at the cost of the accuracy of the counters. 1