9628178
Uniform Interpretations of Grammar Forms
Encouraged by positive experiences with so-called uniform-interpretations of L-forms, we investigate in this paper a suitable analogous definition of uniform interpretations of grammar forms. Concerning CF grammar forms it is shown that a rich variety of language families can be obtained. CF grammar forms with a single variable are extensively examined both with regard to generative capacity (including a characterization of subregular, sublinear and subfinite index families) and with regard to the notion of goodness and badness. Concerning non-CF grammar forms it is shown that each of the families of EOL-, ETOL-, matrix-, scattered context-, context sensitive, type 0-languages (and many others) can be obtained by using interpretations of one form specific to this family.
30874363
Synchronized EOL Forms Under Uniform Interpretation
— The aim ofthis paper is thejurther study oj EOLjorms under uniform interprétation. We are naturally led to the considération oj synchronized EOLjorms, which become the central thème of this paper. We prove: a ""strongest"" possible normaljorm resuit, the spanning normalJorm theorem; the non-existence oj uni-generatorsjor "reasonable" language amilies, which include thefinite, regular and context-jree j amilies; and a char act erization oj those ynchronized jorms which, under uniform interprétation, yield all regular languages. In closing we remark on three topics which are worthy oj separate investigation. Résumé. — Le but de cet article est de poursuivre l'étude des EOL-jormes sous des interprétations unijormes. Nous sommes conduits naturellement à considérer les EOL-jormes synchronisées qui deviennent le thème central de cet article. Nous établissons : un résultat sur la forme normale la plus « serrée » possible, le théorème sur la forme normale recouvrante, la non-existence d'uni-générateurs pour des jamilles « raisonnables » de langages qui comprennent les langages finis, rationnels et algébriques; et une caractérisation des formes synchronisées qui donnent, par interprétation unijorme, tous les langages rationnels. Pour terminer, nous effleurons trois domaines qui méritent une étude séparée.
7533318
Completeness of Context-Free Grammar Forms
The notions of a grammar form and g-interpretation were first introduced in [2]. However, it quickly became apparent, particularly for EOL forms [5], that (strict) interpretations had a wider interest as well as being better motivated mathematically. The study of parsing of grammar forms detailed in [4], the basic investigation in [8] on linear completeness amongst other results, the study of density in [9, lo] and the connections with graph theory shown in [ 1 l] have affirmed this position. The present paper continues the approach of [8] focussing on the characterization of complete grammar forms, that is grammar forms which generate all’ context-free languages. Some decidability problems will also be discussed. For further morivation and background material we refer the reader to [8, 141, while for all unexplained concepts in language theory, see [ 131. After giving the necessary definitions in the remainder of this section we characterize complete grammar forms in Section 2, 3 and 4. In Section 2 we introduce the central concept of expansion spectrum and in Section 3 the recently proved super normal form theorem [ 121, while in Section 4 the characterization is completed and generators and hierarchies are briefly mentioned. Consider context-free grammars G = (V, C, P, S), where Z is the alphabet of terminals, VZ the alphabet of nonterminals, P is the set of productions and
21847551
Bounded Delay L Codes
Abstract Three natural notions of bounded delay for L codes are investigated. All interrelations, many of them rather surprising, between ordinary codes of bounded delay and L codes of bounded delay of various types are established.
33163861
Finitary and infinitary interpretations of languages
The main result of the paper can be viewed both as a generalization of a classical results concerning colorings of graphs, as well as a very useful over-all theorem from which various specific results concerning grammar and language families can be deduced. Essentially our main result says that, when considering infinitary families obtainable by inverse morphisms, it suffices to consider their finitary parts. Various applications are given, in particular, to grammar forms and context-free languages over infinite alphabets.
12582971
Stabbing line segments
An algorithm for the geometric problem of determining a line (called a stabbing line) which intersects each ofn given line segments in the plane is presented. As a matter of fact, the algorithm computes a description of all stabbing lines. A purely geometric fact is proved which infers that this description requiresO(n) space to be specified. Our algorithm computes it inO(n logn) time which is optimal in the worst case.Using the description of the stabbing lines, we are able to decide inO(logn) time whether or not a specified line is a stabbing line. Finally, the problem of maintaining the description of all stabbing lines while inserting and deleting line segments is addressed.
2475557
Three-Dimensional Grid Drawings with Sub-quadratic Volume
A three-dimensional grid drawing of a graph is a placement of the vertices at distinct points with integer coordinates, such that the straight line-segments representing the edges are pairwise non-crossing. A \(\mathcal{O}(n^{3/2})\) volume bound is proved for three-dimensional grid drawings of graphs with bounded degree, graphs with bounded genus, and graphs with no bounded complete graph as a minor. The previous best bound for these graph families was \(\mathcal{O}(n^{2})\). These results (partially) solve open problems due to Pach, Thiele, and Toth [Graph Drawing 1997] and Felsner, Liotta, and Wismath [Graph Drawing 2001].
1943361
Modeling cost/performance of a parallel computer simulator
This article examines the cost/performance of simulating a hypothetical target parallel computer using a commercial host parallel computer. We address the question of whether parallel simulation is simply faster than sequential simulation, or if it is also more cost-effective. To answer this, we develop a performance model of the Wisconsin Wind Tunnel (WWT), a system that simulates cache-coherent shared-memory machines on a message-passing Thinking Machines CM-5. The performance model uses Kruskal and Weiss's fork-join model to account for the effect of event processing time variability on WWT's conservative fixed-window simulation algorithm. A generalization of Thiebaut and Stone's footprint model accurately predicts the effect of cache interference on the CM-5. The model is calibrated using parameters extracted from a fully parallel simulation (p = N), and validated by measuring the speedup as the number of processors (p) ranges from 1 to the number of target nodes (N). Together with simple cost models, the performance model indicates that for target system sizes of 32 nodes and larger, parallel simulation is more cost-effective than sequential simulation. The key intuition behind this result is that large simulations require large memories, which dominate the cost of a uniprocessor; parallel computers allow multiple processors to simultaneously access this large memory.
14349700
Normal form algorithms for extended context-free grammars
We investigate the complexity of a variety of normal-form transformations for extended context-free grammars, where by extended we mean that the set of right-hand sides for each nonterminal in such a grammar is a regular set. The study is motivated by the implementation project GraMa which will provide a C++ toolkit for the symbolic manipulation of context-free objects just as Grail does for regular objects. Our results generalize known complexity bounds for context-free grammars but do so in nontrivial ways. Specifically, we introduce a new representation scheme for extended context-free grammars (the symbol-threaded expression forest), a new normal form for these grammars (dot normal form) and new regular expression algorithms. Copyright 2001 Elsevier Science B.V.
8648546
Variability in architectural simulations of multi-threaded workloads
Multi-threaded commercial workloads implement many important Internet services. Consequently, these workloads are increasingly used to evaluate the performance of uniprocessor and multiprocessor system designs. This paper identifies performance variability as a potentially major challenge for architectural simulation studies using these workloads. Variability refers to the differences between multiple estimates of a workload's performance. Time variability occurs when a workload exhibits different characteristics during different phases of a single run. Space variability occurs when small variations in timing cause runs starting from the same initial condition to follow widely different execution paths. Variability is a well-known phenomenon in real systems, but is nearly universally ignored in simulation experiments. In a central result of this paper we show that variability in multi-threaded commercial workloads can lead to incorrect architectural conclusions (e.g., 31% of the time in one experiment). We propose a methodology, based on multiple simulations and standard statistical techniques, to compensate for variability. Our methodology greatly reduces the probability of reaching incorrect conclusions, while enabling simulations to finish within reasonable time limits.
13905466
Supporting nested transactional memory in logTM
Nested transactional memory (TM) facilitates software composition by letting one module invoke another without either knowing whether the other uses transactions. Closed nested transactions extend isolation of an inner transaction until the toplevel transaction commits. Implementations may flatten nested transactions into the top-level one, resulting in a complete abort on conflict, or allow partial abort of inner transactions. Open nested transactions allow a committing inner transaction to immediately release isolation, which increases parallelism and expressiveness at the cost of both software and hardware complexity.This paper extends the recently-proposed flat Log-based Transactional Memory (LogTM) with nested transactions. Flat LogTM saves pre-transaction values in a log, detects conflicts with read (R) and write (W) bits per cache block, and, on abort, invokes a software handler to unroll the log. Nested LogTM supports nesting by segmenting the log into a stack of activation records and modestly replicating R/W bits. To facilitate composition with nontransactional code, such as language runtime and operating system services, we propose escape actions that allow trusted code to run outside the confines of the transactional memory system.