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ACM SIGACT News Distributed Computing Column 28
ACM SIGACT News Distributed Computing Column 28 Idit Keidar Dept. of Electrical Engineering, Technion Haifa, 32000, Israel [email protected] Sergio Rajsbaum, who edited this column for seven years and established it as a relevant and popular venue, is stepping down. This issue is my first step in the big shoes he vacated. I would like to take this opportunity to thank Sergio for providing us with seven years’ worth of interesting columns. In producing these columns, Sergio has enjoyed the support of the community at-large and obtained material from many authors, who greatly contributed to the column’s success. I hope to enjoy a similar level of support; I warmly welcome your feedback and suggestions for material to include in this column! The main two conferences in the area of principles of distributed computing, PODC and DISC, took place this summer. This issue is centered around these conferences, and more broadly, distributed computing research as reflected therein, ranging from reviews of this year’s instantiations, through influential papers in past instantiations, to examining PODC’s place within the realm of computer science. I begin with a short review of PODC’07, and highlight some “hot” trends that have taken root in PODC, as reflected in this year’s program. Some of the forthcoming columns will be dedicated to these up-and- coming research topics. This is followed by a review of this year’s DISC, by Edward (Eddie) Bortnikov. For some perspective on long-running trends in the field, I next include the announcement of this year’s Edsger W. -
Edsger Dijkstra: the Man Who Carried Computer Science on His Shoulders
INFERENCE / Vol. 5, No. 3 Edsger Dijkstra The Man Who Carried Computer Science on His Shoulders Krzysztof Apt s it turned out, the train I had taken from dsger dijkstra was born in Rotterdam in 1930. Nijmegen to Eindhoven arrived late. To make He described his father, at one time the president matters worse, I was then unable to find the right of the Dutch Chemical Society, as “an excellent Aoffice in the university building. When I eventually arrived Echemist,” and his mother as “a brilliant mathematician for my appointment, I was more than half an hour behind who had no job.”1 In 1948, Dijkstra achieved remarkable schedule. The professor completely ignored my profuse results when he completed secondary school at the famous apologies and proceeded to take a full hour for the meet- Erasmiaans Gymnasium in Rotterdam. His school diploma ing. It was the first time I met Edsger Wybe Dijkstra. shows that he earned the highest possible grade in no less At the time of our meeting in 1975, Dijkstra was 45 than six out of thirteen subjects. He then enrolled at the years old. The most prestigious award in computer sci- University of Leiden to study physics. ence, the ACM Turing Award, had been conferred on In September 1951, Dijkstra’s father suggested he attend him three years earlier. Almost twenty years his junior, I a three-week course on programming in Cambridge. It knew very little about the field—I had only learned what turned out to be an idea with far-reaching consequences. a flowchart was a couple of weeks earlier. -
(Perry & Wolf 92) Software Architecture (Garlan & Shaw
ICS 221, Winter 2001 Software Architecture Software Architecture (Perry & Wolf 92) “Architecture is concerned with the selection of architectural elements, their interactions, and the Software Architecture constraints on those elements and their interactions necessary to provide a framework in which to satisfy the requirements and serve as a basis for the design.” David S. Rosenblum ICS 221 “Design is concerned with the modularization and detailed interfaces of the design elements, their Winter 2001 algorithms and procedures, and the data types needed to support the architecture and to satisfy the requirements.” Software Architecture Software Architecture (Garlan & Shaw 93) (Shaw & Garlan 96) “Software architecture is a level of design that goes “The architecture of a software system defines beyond the algorithms and data structures of the that system in terms of computational computation; designing and specifying the overall components and interactions among those system structure emerges as a new kind of problem. Structural issues include gross organization and components. … In addition to specifying the global control structure; protocols for communication, structure and topology of the system, the synchronization, and data access; assignment of architecture shows the correspondence functionality to design elements; physical between the requirements and elements of distribution; composition of design elements; scaling the constructed system, thereby providing and performance; and selection among design some rationale for the design decisions.” alternatives.” Analogies with Differences Between Civil and Civil Architecture Software Architecture Civil Engineering and Civil Architecture “Software systems are like cathedrals—first we are concerned with the engineering and design of build them and then we pray.” civic structures (roads, buildings, bridges, etc.) — Sam Redwine ! Multiple views ! Civil: Artist renderings, elevations, floor plans, blueprints ! Physical vs. -
Synchronization Spinlocks - Semaphores
CS 4410 Operating Systems Synchronization Spinlocks - Semaphores Summer 2013 Cornell University 1 Today ● How can I synchronize the execution of multiple threads of the same process? ● Example ● Race condition ● Critical-Section Problem ● Spinlocks ● Semaphors ● Usage 2 Problem Context ● Multiple threads of the same process have: ● Private registers and stack memory ● Shared access to the remainder of the process “state” ● Preemptive CPU Scheduling: ● The execution of a thread is interrupted unexpectedly. ● Multiple cores executing multiple threads of the same process. 3 Share Counting ● Mr Skroutz wants to count his $1-bills. ● Initially, he uses one thread that increases a variable bills_counter for every $1-bill. ● Then he thought to accelerate the counting by using two threads and keeping the variable bills_counter shared. 4 Share Counting bills_counter = 0 ● Thread A ● Thread B while (machine_A_has_bills) while (machine_B_has_bills) bills_counter++ bills_counter++ print bills_counter ● What it might go wrong? 5 Share Counting ● Thread A ● Thread B r1 = bills_counter r2 = bills_counter r1 = r1 +1 r2 = r2 +1 bills_counter = r1 bills_counter = r2 ● If bills_counter = 42, what are its possible values after the execution of one A/B loop ? 6 Shared counters ● One possible result: everything works! ● Another possible result: lost update! ● Called a “race condition”. 7 Race conditions ● Def: a timing dependent error involving shared state ● It depends on how threads are scheduled. ● Hard to detect 8 Critical-Section Problem bills_counter = 0 ● Thread A ● Thread B while (my_machine_has_bills) while (my_machine_has_bills) – enter critical section – enter critical section bills_counter++ bills_counter++ – exit critical section – exit critical section print bills_counter 9 Critical-Section Problem ● The solution should ● enter section satisfy: ● critical section ● Mutual exclusion ● exit section ● Progress ● remainder section ● Bounded waiting 10 General Solution ● LOCK ● A process must acquire a lock to enter a critical section. -
Arxiv:1901.04709V1 [Cs.GT]
Self-Stabilization Through the Lens of Game Theory Krzysztof R. Apt1,2 and Ehsan Shoja3 1 CWI, Amsterdam, The Netherlands 2 MIMUW, University of Warsaw, Warsaw, Poland 3 Sharif University of Technology, Tehran, Iran Abstract. In 1974 E.W. Dijkstra introduced the seminal concept of self- stabilization that turned out to be one of the main approaches to fault- tolerant computing. We show here how his three solutions can be for- malized and reasoned about using the concepts of game theory. We also determine the precise number of steps needed to reach self-stabilization in his first solution. 1 Introduction In 1974 Edsger W. Dijkstra introduced in a two-page article [10] the notion of self-stabilization. The paper was completely ignored until 1983, when Leslie Lamport stressed its importance in his invited talk at the ACM Symposium on Principles of Distributed Computing (PODC), published a year later as [21]. Things have changed since then. According to Google Scholar Dijkstra’s paper has been by now cited more than 2300 times. It became one of the main ap- proaches to fault tolerant computing. An early survey was published in 1993 as [26], while the research on the subject until 2000 was summarized in the book [13]. In 2002 Dijkstra’s paper won the PODC influential paper award (renamed in 2003 to Dijkstra Prize). The literature on the subject initiated by it continues to grow. There are annual Self-Stabilizing Systems Workshops, the 18th edition of which took part in 2016. The idea proposed by Dijkstra is very simple. Consider a distributed system arXiv:1901.04709v1 [cs.GT] 15 Jan 2019 viewed as a network of machines. -
Distributed Algorithms for Wireless Networks
A THEORETICAL VIEW OF DISTRIBUTED SYSTEMS Nancy Lynch, MIT EECS, CSAIL National Science Foundation, April 1, 2021 Theory for Distributed Systems • We have worked on theory for distributed systems, trying to understand (mathematically) their capabilities and limitations. • This has included: • Defining abstract, mathematical models for problems solved by distributed systems, and for the algorithms used to solve them. • Developing new algorithms. • Producing rigorous proofs, of correctness, performance, fault-tolerance. • Proving impossibility results and lower bounds, expressing inherent limitations of distributed systems for solving problems. • Developing foundations for modeling, analyzing distributed systems. • Kinds of systems: • Distributed data-management systems. • Wired, wireless communication systems. • Biological systems: Insect colonies, developmental biology, brains. This talk: 1. Algorithms for Traditional Distributed Systems 2. Impossibility Results 3. Foundations 4. Algorithms for New Distributed Systems 1. Algorithms for Traditional Distributed Systems • Mutual exclusion in shared-memory systems, resource allocation: Fischer, Burns,…late 70s and early 80s. • Dolev, Lynch, Pinter, Stark, Weihl. Reaching approximate agreement in the presence of faults. JACM,1986. • Lundelius, Lynch. A new fault-tolerant algorithm for clock synchronization. Information and Computation,1988. • Dwork, Lynch, Stockmeyer. Consensus in the presence of partial synchrony. JACM,1988. Dijkstra Prize, 2007. 1A. Dwork, Lynch, Stockmeyer [DLS] “This paper introduces a number of practically motivated partial synchrony models that lie between the completely synchronous and the completely asynchronous models and in which consensus is solvable. It gave practitioners the right tool for building fault-tolerant systems and contributed to the understanding that safety can be maintained at all times, despite the impossibility of consensus, and progress is facilitated during periods of stability. -
Edsger W. Dijkstra: a Commemoration
Edsger W. Dijkstra: a Commemoration Krzysztof R. Apt1 and Tony Hoare2 (editors) 1 CWI, Amsterdam, The Netherlands and MIMUW, University of Warsaw, Poland 2 Department of Computer Science and Technology, University of Cambridge and Microsoft Research Ltd, Cambridge, UK Abstract This article is a multiauthored portrait of Edsger Wybe Dijkstra that consists of testimo- nials written by several friends, colleagues, and students of his. It provides unique insights into his personality, working style and habits, and his influence on other computer scientists, as a researcher, teacher, and mentor. Contents Preface 3 Tony Hoare 4 Donald Knuth 9 Christian Lengauer 11 K. Mani Chandy 13 Eric C.R. Hehner 15 Mark Scheevel 17 Krzysztof R. Apt 18 arXiv:2104.03392v1 [cs.GL] 7 Apr 2021 Niklaus Wirth 20 Lex Bijlsma 23 Manfred Broy 24 David Gries 26 Ted Herman 28 Alain J. Martin 29 J Strother Moore 31 Vladimir Lifschitz 33 Wim H. Hesselink 34 1 Hamilton Richards 36 Ken Calvert 38 David Naumann 40 David Turner 42 J.R. Rao 44 Jayadev Misra 47 Rajeev Joshi 50 Maarten van Emden 52 Two Tuesday Afternoon Clubs 54 2 Preface Edsger Dijkstra was perhaps the best known, and certainly the most discussed, computer scientist of the seventies and eighties. We both knew Dijkstra |though each of us in different ways| and we both were aware that his influence on computer science was not limited to his pioneering software projects and research articles. He interacted with his colleagues by way of numerous discussions, extensive letter correspondence, and hundreds of so-called EWD reports that he used to send to a select group of researchers. -
2020 SIGACT REPORT SIGACT EC – Eric Allender, Shuchi Chawla, Nicole Immorlica, Samir Khuller (Chair), Bobby Kleinberg September 14Th, 2020
2020 SIGACT REPORT SIGACT EC – Eric Allender, Shuchi Chawla, Nicole Immorlica, Samir Khuller (chair), Bobby Kleinberg September 14th, 2020 SIGACT Mission Statement: The primary mission of ACM SIGACT (Association for Computing Machinery Special Interest Group on Algorithms and Computation Theory) is to foster and promote the discovery and dissemination of high quality research in the domain of theoretical computer science. The field of theoretical computer science is the rigorous study of all computational phenomena - natural, artificial or man-made. This includes the diverse areas of algorithms, data structures, complexity theory, distributed computation, parallel computation, VLSI, machine learning, computational biology, computational geometry, information theory, cryptography, quantum computation, computational number theory and algebra, program semantics and verification, automata theory, and the study of randomness. Work in this field is often distinguished by its emphasis on mathematical technique and rigor. 1. Awards ▪ 2020 Gödel Prize: This was awarded to Robin A. Moser and Gábor Tardos for their paper “A constructive proof of the general Lovász Local Lemma”, Journal of the ACM, Vol 57 (2), 2010. The Lovász Local Lemma (LLL) is a fundamental tool of the probabilistic method. It enables one to show the existence of certain objects even though they occur with exponentially small probability. The original proof was not algorithmic, and subsequent algorithmic versions had significant losses in parameters. This paper provides a simple, powerful algorithmic paradigm that converts almost all known applications of the LLL into randomized algorithms matching the bounds of the existence proof. The paper further gives a derandomized algorithm, a parallel algorithm, and an extension to the “lopsided” LLL. -
Glossary of Technical and Scientific Terms (French / English
Glossary of Technical and Scientific Terms (French / English) Version 11 (Alphabetical sorting) Jean-Luc JOULIN October 12, 2015 Foreword Aim of this handbook This glossary is made to help for translation of technical words english-french. This document is not made to be exhaustive but aim to understand and remember the translation of some words used in various technical domains. Words are sorted alphabetically. This glossary is distributed under the licence Creative Common (BY NC ND) and can be freely redistributed. For further informations about the licence Creative Com- mon (BY NC ND), browse the site: http://creativecommons.org If you are interested by updates of this glossary, send me an email to be on my diffusion list : • [email protected] • [email protected] Warning Translations given in this glossary are for information only and their use is under the responsability of the reader. If you see a mistake in this document, thank in advance to report it to me so i can correct it in further versions of this document. Notes Sorting of words Words are sorted by alphabetical order and are associated to their main category. Difference of terms Some words are different between the "British" and the "American" english. This glossary show the most used terms and are generally coming from the "British" english Words coming from the "American" english are indicated with the suffix : US . Difference of spelling There are some difference of spelling in accordance to the country, companies, universities, ... Words finishing by -ise, -isation and -yse can also be written with -ize, Jean-Luc JOULIN1 Glossary of Technical and Scientific Terms -ization and -yze. -
The Deadlock Problem
The deadlock problem More deadlocks mutex_t m1, m2; Same problem with condition variables void p1 (void *ignored) { • lock (m1); - Suppose resource 1 managed by c1, resource 2 by c2 lock (m2); - A has 1, waits on 2, B has 2, waits on 1 /* critical section */ c c unlock (m2); Or have combined mutex/condition variable unlock (m1); • } deadlock: - lock (a); lock (b); while (!ready) wait (b, c); void p2 (void *ignored) { unlock (b); unlock (a); lock (m2); - lock (a); lock (b); ready = true; signal (c); lock (m1); unlock (b); unlock (a); /* critical section */ unlock (m1); One lesson: Dangerous to hold locks when crossing unlock (m2); • } abstraction barriers! This program can cease to make progress – how? - I.e., lock (a) then call function that uses condition variable • Can you have deadlock w/o mutexes? • 1/15 2/15 Deadlocks w/o computers Deadlock conditions 1. Limited access (mutual exclusion): - Resource can only be shared with finite users. 2. No preemption: - once resource granted, cannot be taken away. Real issue is resources & how required • 3. Multiple independent requests (hold and wait): E.g., bridge only allows traffic in one direction - don’t ask all at once (wait for next resource while holding • - Each section of a bridge can be viewed as a resource. current one) - If a deadlock occurs, it can be resolved if one car backs up 4. Circularity in graph of requests (preempt resources and rollback). All of 1–4 necessary for deadlock to occur - Several cars may have to be backed up if a deadlock occurs. • Two approaches to dealing with deadlock: - Starvation is possible. -
Designing Software Architecture to Support Continuous Delivery and Devops: a Systematic Literature Review
Designing Software Architecture to Support Continuous Delivery and DevOps: A Systematic Literature Review Robin Bolscher and Maya Daneva University of Twente, Drienerlolaan 5, Enschede, The Netherlands [email protected], [email protected] Keywords: Software Architecture, Continuous Delivery, Continuous Integration, DevOps, Deployability, Systematic Literature Review, Micro-services. Abstract: This paper presents a systematic literature review of software architecture approaches that support the implementation of Continuous Delivery (CD) and DevOps. Its goal is to provide an understanding of the state- of-the-art on the topic, which is informative for both researchers and practitioners. We found 17 characteristics of a software architecture that are beneficial for CD and DevOps adoption and identified ten potential software architecture obstacles in adopting CD and DevOps in the case of an existing software system. Moreover, our review indicated that micro-services are a dominant architectural style in this context. Our literature review has some implications: for researchers, it provides a map of the recent research efforts on software architecture in the CD and DevOps domain. For practitioners, it describes a set of software architecture principles that possibly can guide the process of creating or adapting software systems to fit in the CD and DevOps context. 1 INTRODUCTION designing new software architectures tailored for CD and DevOps practices. The practice of releasing software early and often has For clarity, before elaborating on the subject of been increasingly more adopted by software this SLR, we present the definitions of the concepts organizations (Fox et al., 2014) in order to stay that we will address: Software architecture of a competitive in the software market. -
Fair K Mutual Exclusion Algorithm for Peer to Peer Systems ∗
Fair K Mutual Exclusion Algorithm for Peer To Peer Systems ∗ Vijay Anand Reddy, Prateek Mittal, and Indranil Gupta University of Illinois, Urbana Champaign, USA {vkortho2, mittal2, indy}@uiuc.edu Abstract is when multiple clients are simultaneously downloading a large multimedia file, e.g., [3, 12]. Finally applications k-mutual exclusion is an important problem for resource- where a service has limited computational resources will intensive peer-to-peer applications ranging from aggrega- also benefit from a k mutual exclusion primitive, preventing tion to file downloads. In order to be practically useful, scenarios where all the nodes of the p2p system simultane- k-mutual exclusion algorithms not only need to be safe and ously overwhelm the server. live, but they also need to be fair across hosts. We pro- The k mutual exclusion problem involves a group of pro- pose a new solution to the k-mutual exclusion problem that cesses, each of which intermittently requires access to an provides a notion of time-based fairness. Specifically, our identical resource called the critical section (CS). A k mu- algorithm attempts to minimize the spread of access time tual exclusion algorithm must satisfy both safety and live- for the critical resource. While a client’s access time is ness. Safety means that at most k processes, 1 ≤ k ≤ n, the time between it requesting and accessing the resource, may be in the CS at any given time. Liveness means that the spread is defined as a system-wide metric that measures every request for critical section access is satisfied in finite some notion of the variance of access times across a homo- time.