DOCSLIB.ORG

Internally Deterministic Parallel Algorithms Can Be Fast

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Internally Deterministic Parallel Algorithms Can Be Fast Internally Deterministic Parallel Algorithms Can Be Fast Guy E. Blelloch∗ Jeremy T. Finemany Phillip B. Gibbonsz Julian Shun∗ ∗Carnegie Mellon University yGeorgetown University zIntel Labs, Pittsburgh [email protected] jfi[email protected] [email protected] [email protected] Abstract 1. Introduction The virtues of deterministic parallelism have been argued for One of the key challenges of parallel programming is dealing with decades and many forms of deterministic parallelism have been nondeterminism. For many computational problems, there is no in- described and analyzed. Here we are concerned with one of the herent nondeterminism in the problem statement, and indeed a se- strongest forms, requiring that for any input there is a unique de- rial program would be deterministic—the nondeterminism arises pendence graph representing a trace of the computation annotated solely due to the parallel program and/or due to the parallel ma- with every operation and value. This has been referred to as internal chine and its runtime environment. The challenges of nondetermin- determinism, and implies a sequential semantics—i.e., considering ism have been recognized and studied for decades [23, 24, 37, 42]. any sequential traversal of the dependence graph is sufficient for Steele’s 1990 paper, for example, seeks “to prevent the behavior analyzing the correctness of the code. In addition to returning deter- of the program from depending on any accidents of execution or- ministic results, internal determinism has many advantages includ- der that can arise from the indeterminacy” of asynchronous pro- ing ease of reasoning about the code, ease of verifying correctness, grams [42]. More recently, there has been a surge of advocacy for ease of debugging, ease of defining invariants, ease of defining and research in determinism, seeking to remove sources of non- good coverage for testing, and ease of formally, informally and ex- determinism via specially-designed hardware mechanisms [19, 20, perimentally reasoning about performance. On the other hand one 28], runtime systems and compilers [3, 5, 36, 45], operating sys- needs to consider the possible downsides of determinism, which tems [4], and programming languages/frameworks [11]. might include making algorithms (i) more complicated, unnatural While there seems to be a growing consensus that determinism or special purpose and/or (ii) slower or less scalable. is important, there is disagreement as to what degree of determin- In this paper we study the effectiveness of this strong form of ism is desired (worth paying for). Popular options include: determinism through a broad set of benchmark problems. Our main • contribution is to demonstrate that for this wide body of prob- Data-race free [2, 22], which eliminate a particularly problem- lems, there exist efficient internally deterministic algorithms, and atic type of nondeterminism: the data race. Synchronization moreover that these algorithms are natural to reason about and not constructs such as locks or atomic transactions protect ordinary complicated to code. We leverage an approach to determinism sug- accesses to shared data, but nondeterminism among such con- gested by Steele (1990), which is to use nested parallelism with structs (e.g., the order of lock acquires) can lead to considerable commutative operations. Our algorithms apply several diverse pro- nondeterminism in the execution. gramming paradigms that fit within the model including (i) a strict • Determinate (or external determinism), which requires that the functional style (no shared state among concurrent operations), (ii) program always produces the same output when run on the an approach we refer to as deterministic reservations, and (iii) the same input. Program executions for a given input may vary use of commutative, linearizable operations on data structures. We widely, as long as the program “converges” to the same output describe algorithms for the benchmark problems that use these de- each time. terministic approaches and present performance results on a 32- • Internal determinism, in which key aspects of intermediate core machine. Perhaps surprisingly, for all problems, our internally steps of the program are also deterministic, as discussed in deterministic algorithms achieve good speedup and good perfor- this paper. mance even relative to prior nondeterministic solutions. • Functional determinism, where the absence of side-effects in Categories and Subject Descriptors D.1 [Concurrent Program- purely functional languages make all components independent ming]: Parallel programming and safe to run in parallel. General Terms Algorithms, Experimentation, Performance • Synchronous parallelism, where parallelism proceeds in lock Keywords Parallel algorithms, deterministic parallelism, parallel step (e.g., SIMD-style) and each step has a deterministic out- programming, commutative operations, graph algorithms, geome- come. try algorithms, sorting, string processing There are trade-offs among these options, with stronger forms of determinism often viewed as better for reasoning and debugging but worse for performance and perhaps programmability. Making the proper choice for an application requires understanding what Permission to make digital or hard copies of all or part of this work for personal or the trade-offs are. In particular, is there a “sweet spot” for deter- classroom use is granted without fee provided that copies are not made or distributed minism, which provides a particularly useful combination of de- for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute buggability, performance, and programmability? to lists, requires prior specific permission and/or a fee. In this paper, we advocate a particular form of internal de- PPoPP ’12 February 25–29, 2012, New Orleans, Louisiana, USA. terminism as providing such a sweet spot for a class of nested- Copyright c 2012 ACM 978-1-4503-1160-1/12/02. $10.00 parallel (i.e., nested fork-join) computations in which there is no inherent nondeterminism in the problem statement. An execution straints in transactional systems [26, 30], an approach that does of a nested-parallel program defines a dependence DAG (directed not guarantee determinism. In contrast, this paper identifies use- acyclic graph) that represents every operation executed by the com- ful applications of non-trivial commutativity that can be used in the putation (the nodes) along with the control dependencies among design of internally deterministic algorithms. them (the edges). These dependencies represent ordering within se- We describe, for example, an approach we refer to as determin- quential code sequences, dependencies from a fork operation to its istic reservations for parallelizing certain greedy algorithms. In the children, and dependencies from the end of such children to the approach the user implements a loop with potential loop carried join point of the forking parent. We refer to this DAG when an- dependencies by splitting each iteration into reserve and commit notated with the operations performed at each node (including ar- phases. The loop is then processed in rounds in which each round guments and return values, if any) as the trace. Informally, a pro- takes a prefix of the unprocessed iterates applying the reserve phase gram/algorithm is internally deterministic if for any input there is in parallel and then the commit phase in parallel. Some iterates can a unique trace. This definition depends on the level of abstraction fail during the commit due to conflicts with earlier iterates and need of the operations in the trace. At the most primitive level the op- to be retried in the next round, but as long as the operations com- erations could represent individual machine instructions, but more mute within the reserve and commit phases and the prefix size is generally, and as used in this paper, it is any abstraction level at selected deterministically, the computation is internally determin- which the implementation is hidden from the programmer. We note istic (the same iterates always fail). that internal determinism does not imply a fixed schedule since any We describe algorithms for the benchmark problems using these schedule that is consistent with the DAG is valid. approaches and present performance results for our Cilk++ [31] Internal determinism has many benefits. In addition to leading implementations on a 32-core machine. Perhaps surprisingly, for to external determinism [37] it implies a sequential semantics—i.e., all problems, our internally deterministic algorithms achieve good considering any sequential traversal of the dependence DAG is suf- speedup and good performance even relative to prior nondetermin- ficient for analyzing the correctness of the code. This in turn leads istic and externally deterministic solutions, implying that the per- to many advantages including ease of reasoning about the code, formance penalty of internal determinism is quite low. We achieve ease of verifying correctness, ease of debugging, ease of defin- speedups of up to 31.6 on 32 cores with 2-way hyperthreading (for ing invariants, ease of defining good coverage for testing, and ease sorting). Almost all our speedups are above 16. Compared to what of formally, informally and experimentally reasoning about perfor- we believe are quite good sequential implementations we range mance [3–5, 11, 19, 20, 28, 36, 45]. Two primary concerns
Load more

Recommended publications
  • Deterministic Execution of Multithreaded Applications
    DETERMINISTIC EXECUTION OF MULTITHREADED APPLICATIONS FOR RELIABILITY OF MULTICORE SYSTEMS DETERMINISTIC EXECUTION OF MULTITHREADED APPLICATIONS FOR RELIABILITY OF MULTICORE SYSTEMS Proefschrift ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus prof. ir. K. C. A. M. Luyben, voorzitter van het College voor Promoties, in het openbaar te verdedigen op vrijdag 19 juni 2015 om 10:00 uur door Hamid MUSHTAQ Master of Science in Embedded Systems geboren te Karachi, Pakistan Dit proefschrift is goedgekeurd door de promotor: Prof. dr. K. L. M. Bertels Copromotor: Dr. Z. Al-Ars Samenstelling promotiecommissie: Rector Magnificus, voorzitter Prof. dr. K. L. M. Bertels, Technische Universiteit Delft, promotor Dr. Z. Al-Ars, Technische Universiteit Delft, copromotor Independent members: Prof. dr. H. Sips, Technische Universiteit Delft Prof. dr. N. H. G. Baken, Technische Universiteit Delft Prof. dr. T. Basten, Technische Universiteit Eindhoven, Netherlands Prof. dr. L. J. M Rothkrantz, Netherlands Defence Academy Prof. dr. D. N. Pnevmatikatos, Technical University of Crete, Greece Keywords: Multicore, Fault Tolerance, Reliability, Deterministic Execution, WCET The work in this thesis was supported by Artemis through the SMECY project (grant 100230). Cover image: An Artist’s impression of a view of Saturn from its moon Titan. The image was taken from http://www.istockphoto.com and used with permission. ISBN 978-94-6186-487-1 Copyright © 2015 by H. Mushtaq All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without the prior written permission of the copyright owner.
    [Show full text]
  • CUDA Toolkit 4.2 CURAND Guide
    CUDA Toolkit 4.2 CURAND Guide PG-05328-041_v01 | March 2012 Published by NVIDIA Corporation 2701 San Tomas Expressway Santa Clara, CA 95050 Notice ALL NVIDIA DESIGN SPECIFICATIONS, REFERENCE BOARDS, FILES, DRAWINGS, DIAGNOSTICS, LISTS, AND OTHER DOCUMENTS (TOGETHER AND SEPARATELY, "MATERIALS") ARE BEING PROVIDED "AS IS". NVIDIA MAKES NO WARRANTIES, EXPRESSED, IMPLIED, STATUTORY, OR OTHERWISE WITH RESPECT TO THE MATERIALS, AND EXPRESSLY DISCLAIMS ALL IMPLIED WARRANTIES OF NONINFRINGEMENT, MERCHANTABILITY, AND FITNESS FOR A PARTICULAR PURPOSE. Information furnished is believed to be accurate and reliable. However, NVIDIA Corporation assumes no responsibility for the consequences of use of such information or for any infringement of patents or other rights of third parties that may result from its use. No license is granted by implication or otherwise under any patent or patent rights of NVIDIA Corporation. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. NVIDIA Corporation products are not authorized for use as critical components in life support devices or systems without express written approval of NVIDIA Corporation. Trademarks NVIDIA, CUDA, and the NVIDIA logo are trademarks or registered trademarks of NVIDIA Corporation in the United States and other countries. Other company and product names may be trademarks of the respective companies with which they are associated. Copyright Copyright ©2005-2012 by NVIDIA Corporation. All rights reserved. CUDA Toolkit 4.2 CURAND Guide PG-05328-041_v01 | 1 Portions of the MTGP32 (Mersenne Twister for GPU) library routines are subject to the following copyright: Copyright ©2009, 2010 Mutsuo Saito, Makoto Matsumoto and Hiroshima University.
    [Show full text]
  • Unit: 4 Processes and Threads in Distributed Systems
    Unit: 4 Processes and Threads in Distributed Systems Thread A program has one or more locus of execution. Each execution is called a thread of execution. In traditional operating systems, each process has an address space and a single thread of execution. It is the smallest unit of processing that can be scheduled by an operating system. A thread is a single sequence stream within in a process. Because threads have some of the properties of processes, they are sometimes called lightweight processes. In a process, threads allow multiple executions of streams. Thread Structure Process is used to group resources together and threads are the entities scheduled for execution on the CPU. The thread has a program counter that keeps track of which instruction to execute next. It has registers, which holds its current working variables. It has a stack, which contains the execution history, with one frame for each procedure called but not yet returned from. Although a thread must execute in some process, the thread and its process are different concepts and can be treated separately. What threads add to the process model is to allow multiple executions to take place in the same process environment, to a large degree independent of one another. Having multiple threads running in parallel in one process is similar to having multiple processes running in parallel in one computer. Figure: (a) Three processes each with one thread. (b) One process with three threads. In former case, the threads share an address space, open files, and other resources. In the latter case, process share physical memory, disks, printers and other resources.
    [Show full text]
  • A Short History of Computational Complexity
    The Computational Complexity Column by Lance FORTNOW NEC Laboratories America 4 Independence Way, Princeton, NJ 08540, USA [email protected] http://www.neci.nj.nec.com/homepages/fortnow/beatcs Every third year the Conference on Computational Complexity is held in Europe and this summer the University of Aarhus (Denmark) will host the meeting July 7-10. More details at the conference web page http://www.computationalcomplexity.org This month we present a historical view of computational complexity written by Steve Homer and myself. This is a preliminary version of a chapter to be included in an upcoming North-Holland Handbook of the History of Mathematical Logic edited by Dirk van Dalen, John Dawson and Aki Kanamori. A Short History of Computational Complexity Lance Fortnow1 Steve Homer2 NEC Research Institute Computer Science Department 4 Independence Way Boston University Princeton, NJ 08540 111 Cummington Street Boston, MA 02215 1 Introduction It all started with a machine. In 1936, Turing developed his theoretical com- putational model. He based his model on how he perceived mathematicians think. As digital computers were developed in the 40's and 50's, the Turing machine proved itself as the right theoretical model for computation. Quickly though we discovered that the basic Turing machine model fails to account for the amount of time or memory needed by a computer, a critical issue today but even more so in those early days of computing. The key idea to measure time and space as a function of the length of the input came in the early 1960's by Hartmanis and Stearns.
    [Show full text]
  • Deterministic Parallel Fixpoint Computation
    Deterministic Parallel Fixpoint Computation SUNG KOOK KIM, University of California, Davis, U.S.A. ARNAUD J. VENET, Facebook, Inc., U.S.A. ADITYA V. THAKUR, University of California, Davis, U.S.A. Abstract interpretation is a general framework for expressing static program analyses. It reduces the problem of extracting properties of a program to computing an approximation of the least fixpoint of a system of equations. The de facto approach for computing this approximation uses a sequential algorithm based on weak topological order (WTO). This paper presents a deterministic parallel algorithm for fixpoint computation by introducing the notion of weak partial order (WPO). We present an algorithm for constructing a WPO in almost-linear time. Finally, we describe Pikos, our deterministic parallel abstract interpreter, which extends the sequential abstract interpreter IKOS. We evaluate the performance and scalability of Pikos on a suite of 1017 C programs. When using 4 cores, Pikos achieves an average speedup of 2.06x over IKOS, with a maximum speedup of 3.63x. When using 16 cores, Pikos achieves a maximum speedup of 10.97x. CCS Concepts: • Software and its engineering → Automated static analysis; • Theory of computation → Program analysis. Additional Key Words and Phrases: Abstract interpretation, Program analysis, Concurrency ACM Reference Format: Sung Kook Kim, Arnaud J. Venet, and Aditya V. Thakur. 2020. Deterministic Parallel Fixpoint Computation. Proc. ACM Program. Lang. 4, POPL, Article 14 (January 2020), 33 pages. https://doi.org/10.1145/3371082 1 INTRODUCTION Program analysis is a widely adopted approach for automatically extracting properties of the dynamic behavior of programs [Balakrishnan et al.
    [Show full text]
  • Byzantine Fault Tolerance for Nondeterministic Applications Bo Chen
    BYZANTINE FAULT TOLERANCE FOR NONDETERMINISTIC APPLICATIONS BO CHEN Bachelor of Science in Computer Engineering Nanjing University of Science & Technology, China July, 2006 Submitted in partial fulfillment of the requirements for the degree MASTER OF SCIENCE IN COMPUTER ENGINEERING at the CLEVELAND STATE UNIVERSITY December, 2008 The thesis has been approved for the Department of ELECTRICAL AND COMPUTER ENGINEERING and the College of Graduate Studies by ______________________________________________ Thesis Committee Chairperson, Dr. Wenbing Zhao ____________________ Department/Date _____________________________________________ Dr. Yongjian Fu ______________________ Department/Date _____________________________________________ Dr. Ye Zhu ______________________ Department/Date ACKNOWLEDGEMENT First, I must thank my thesis supervisor, Dr. Wenbing Zhao, for his patience, careful thought, insightful commence and constant support during my two years graduate study and my career. I feel very fortunate for having had the chance to work closely with him and this thesis is as much a product of his guidance as it is of my effort. The other member of my thesis committee, Dr. Yongjian Fu and Dr. Ye Zhu suggested many important improvements to this thesis and interesting directions for future work. I greatly appreciate their suggestions. It has been a pleasure to be a graduate student in Secure and Dependable System Group. I want to thank all the group members: Honglei Zhang and Hua Chai for the discussion we had. I also wish to thank Dr. Fuqin Xiong for his help in advising the details of the thesis submission process. I am grateful to my parents for their support and understanding over the years, especially in the month leading up to this thesis. Above all, I want to thank all my friends who made my life great when I was preparing and writing this thesis.
    [Show full text]
  • Entropy Harvesting in GPU[1] CUDA Code for Collecting Entropy
    CATEGORY: DEVELOPER - ALGORITHMS - DA12 POSTER CONTACT NAME P5292 Yongjin Yeom: [email protected] Using GPU as Hardware Random Number Generator Taeill Yoo, Yongjin Yeom Department of Financial Information Security, Kookmin University, South Korea Abstract Entropy Harvesting in GPU[1] Experiments . Random number generator (RNG) is expected to provide unbiased, unpredictable Race Condition during parallel computation on GPU: GPU: GTX780 (We successfully run the same experiments on 610M & 690, too) bit sequences. RNG collects entropy from the environments and accumulates it . Array Size: 512 elements . When two or more cores try to update shared resources, a race condition into the pool. Then from the entropy pool, RNG generates seed with high entropy . The number of iterations: 1,000 occurs inevitably and brings about an unexpected result. as input to the cryptographic algorithm called pseudo-random bit generator. The number of Threads: 512 . In general, it is important to avoid race conditions in parallel computing. Since the lack of entropy sources leads the output random bits predictable, it is . The number of experiments: 5 (colored lines in the graph below) . However, we raise race condition intentionally so as to collecting entropy important to harvest enough entropy from physical noise sources. Here, we show from the uncertainty. Random noise on GTX780 that we can harvest sufficient entropy from the race condition in parallel 15600 computations on GPU. According to the entropy estimations in NIST SP 800-90B, we measure the entropy obtained from NVIDIA GPU such as GTX690 and GTX780. 15580 Our result can be combined with high speed random number generating library 15560 like cuRAND.
    [Show full text]
  • Deterministic Atomic Buffering
    2020 53rd Annual IEEE/ACM International Symposium on Microarchitecture (MICRO) Deterministic Atomic Buffering Yuan Hsi Chou∗¶, Christopher Ng∗¶, Shaylin Cattell∗, Jeremy Intan†, Matthew D. Sinclair†, Joseph Devietti‡, Timothy G. Rogers§ and Tor M. Aamodt∗ ∗University of British Columbia, †University of Wisconsin, ‡University of Pennsylvania, §Purdue University AMD Research {yuanhsi, chris.ng}@ece.ubc.ca, [email protected], {intan, sinclair}@cs.wisc.edu, [email protected], [email protected], [email protected] ¶equal contribution Abstract—Deterministic execution for GPUs is a desirable property as it helps with debuggability and reproducibility. It is also important for safety regulations, as safety critical workloads are starting to be deployed onto GPUs. Prior deterministic archi- tectures, such as GPUDet, attempt to provide strong determinism for all types of workloads, incurring significant performance overheads due to the many restrictions that are required to satisfy determinism. We observe that a class of reduction workloads, such as graph applications and neural architecture search for machine Fig. 1: Simplified non-deterministic reduction example: Base- learning, do not require such severe restrictions to preserve 10, 3-digit precision, rounding up (details in Section III-B). determinism. This motivates the design of our system, Deter- ministic Atomic Buffering (DAB), which provides deterministic execution with low area and performance overheads by focusing such as autonomous agents and medical diagnostics, require solely on ordering atomic instructions instead of all memory reproducibility to ensure that systems meet specifications or instructions. By scheduling atomic instructions deterministically that experimental results can be replicated for verification [7], with atomic buffering, the results of atomic operations are [8].
    [Show full text]
  • Complexity Theory Lectures 1–6
    Complexity Theory 1 Complexity Theory Lectures 1–6 Lecturer: Dr. Timothy G. Griffin Slides by Anuj Dawar Computer Laboratory University of Cambridge Easter Term 2009 http://www.cl.cam.ac.uk/teaching/0809/Complexity/ Cambridge Easter 2009 Complexity Theory 2 Texts The main texts for the course are: Computational Complexity. Christos H. Papadimitriou. Introduction to the Theory of Computation. Michael Sipser. Other useful references include: Computers and Intractability: A guide to the theory of NP-completeness. Michael R. Garey and David S. Johnson. Structural Complexity. Vols I and II. J.L. Balc´azar, J. D´ıaz and J. Gabarr´o. Computability and Complexity from a Programming Perspective. Neil Jones. Cambridge Easter 2009 Complexity Theory 3 Outline A rough lecture-by-lecture guide, with relevant sections from the text by Papadimitriou (or Sipser, where marked with an S). Algorithms and problems. 1.1–1.3. • Time and space. 2.1–2.5, 2.7. • Time Complexity classes. 7.1, S7.2. • Nondeterminism. 2.7, 9.1, S7.3. • NP-completeness. 8.1–8.2, 9.2. • Graph-theoretic problems. 9.3 • Cambridge Easter 2009 Complexity Theory 4 Outline - contd. Sets, numbers and scheduling. 9.4 • coNP. 10.1–10.2. • Cryptographic complexity. 12.1–12.2. • Space Complexity 7.1, 7.3, S8.1. • Hierarchy 7.2, S9.1. • Descriptive Complexity 5.6, 5.7. • Cambridge Easter 2009 Complexity Theory 5 Complexity Theory Complexity Theory seeks to understand what makes certain problems algorithmically difficult to solve. In Data Structures and Algorithms, we saw how to measure the complexity of specific algorithms, by asymptotic measures of number of steps.
    [Show full text]
  • Optimal Oblivious RAM*
    OptORAMa: Optimal Oblivious RAM* Gilad Asharov Ilan Komargodski Wei-Kai Lin Bar-Ilan University NTT Research and Cornell University Hebrew University Kartik Nayak Enoch Peserico Elaine Shi VMware and Duke University Univ. Padova Cornell University November 18, 2020 Abstract Oblivious RAM (ORAM), first introduced in the ground-breaking work of Goldreich and Ostrovsky (STOC '87 and J. ACM '96) is a technique for provably obfuscating programs' access patterns, such that the access patterns leak no information about the programs' secret inputs. To compile a general program to an oblivious counterpart, it is well-known that Ω(log N) amortized blowup is necessary, where N is the size of the logical memory. This was shown in Goldreich and Ostrovksy's original ORAM work for statistical security and in a somewhat restricted model (the so called balls-and-bins model), and recently by Larsen and Nielsen (CRYPTO '18) for computational security. A long standing open question is whether there exists an optimal ORAM construction that matches the aforementioned logarithmic lower bounds (without making large memory word assumptions, and assuming a constant number of CPU registers). In this paper, we resolve this problem and present the first secure ORAM with O(log N) amortized blowup, assuming one-way functions. Our result is inspired by and non-trivially improves on the recent beautiful work of Patel et al. (FOCS '18) who gave a construction with O(log N · log log N) amortized blowup, assuming one-way functions. One of our building blocks of independent interest is a linear-time deterministic oblivious algorithm for tight compaction: Given an array of n elements where some elements are marked, we permute the elements in the array so that all marked elements end up in the front of the array.
    [Show full text]
  • Semantics and Concurrence Module on Semantic of Programming Languages
    Semantics and Concurrence Module on Semantic of Programming Languages Pietro Di Gianantonio Universit`adi Udine Presentation Module of 6 credits (48 hours), Semantics of programming languages Describe the behavior of programs in a formal and rigorous way. • We present 3 (2) different approaches, methods to semantics. • Methods applied to a series of, more and more complex, programming languages. Some topics overlapping the Formal Methods course. Prerequisite for: • Module of Concurrence (Lenisa), • Abstract Interpretation (Comini), • any formal and rigorous treatment of programs. Semantics of programming languages Objective: to formally describe the behavior of programs, and programming constructors. As opposed to informal definitions, commonly used. Useful for: • avoid ambiguity in defining a programming language: • highlight the critical points of a programming language (e.g. static-dynamic environment, evaluation and parameter passing) • useful in building compilers, • to reason about single programs (define logic, reasoning principles, programs): to prove that a given program to satisfy a give property, specification, that is correct. Semantic approaches • Operational Semantics, describes how programs are executed, not on a real calculator, too complex, on a simple formal machine (Turing machines style). • Structured Operational Semantic (SOS). The formal machine consists of a rewriting system: system of rules, syntax. Semantic approaches • Denotational Semantics. The meaning of a program described by a mathematical object. Partial function, element in an ordered set. An element of an ordered set represents the meaning of a program, part of the program, program constructor. Alternative, action semantics, semantic games, categorical. • Axiomatic Semantics. The meaning of a program expressed in terms of pre-conditions and post-conditions. Several semantics because none completely satisfactory.
    [Show full text]
  • 00 Fast K-Selection Algorithms for Graphics Processing Units
    00 Fast K-selection Algorithms for Graphics Processing Units TOLU ALABI, JEFFREY D. BLANCHARD, BRADLEY GORDON, and RUSSEL STEINBACH, Grinnell College Finding the kth largest value in a list of n values is a well-studied problem for which many algorithms have been proposed. A naive approach is to sort the list and then simply select the kth term in the sorted list. However, when the sorted list is not needed, this method has done quite a bit of unnecessary work. Although sorting can be accomplished efficiently when working with a graphics processing unit (GPU), this article proposes two GPU algorithms, radixSelect and bucketSelect, which are several times faster than sorting the vector. As the problem size grows so does the time savings of these algorithms with a sixfold speedup for float vectors larger than 224 and for double vectors larger than 220, ultimately reaching a 19.1 times speed-up for double vectors of length 228. Categories and Subject Descriptors: D.1.3 [Concurrent Programming]: Parallel programming;; F.2.2 [Non-numerical Algorithms and Problems]: Sorting and searching;; G.4 [Mathematical Software]: Parallel and vector implementations;; I.3.1 [Hardware Architecture]: Graphics Processors General Terms: Algorithms, Design, Experimentation, Performance Additional Key Words and Phrases: K-selection, Order Statistics, Multi-core, Graphics Processing Units, GPGPU, CUDA 1. INTRODUCTION The k-selection problem, a well studied problem in computer science, asks one to find the kth largest value in a list of n elements. This problem is often referred to as finding the kth order statistic. This task appears in numerous applications and our motivating example is a thresholding operation, where only the k largest values in a list are retained while the remaining entries are set to zero.
    [Show full text]