Jack Dennis, an influential MIT professor emeritus of computer science and engineering, died on March 14 at age 94. The original leader of the Computation Structures Group within the MIT Computer Science and Artificial Intelligence Laboratory (CSAIL), he pioneered the development of dataflow models of computation, and, subsequently, many novel principles of computer architecture inspired by dataflow models.
The second child of an engineer and a textile designer, Dennis showed early interest in both engineering and music, rewriting Gilbert and Sullivan lyrics with his parents and playing piano with the Norwalk Symphony Orchestra in Connecticut as a teen, while building a canoe at home with his father. As an undergraduate at MIT, he developed his wide array of interests further, joining the VI-A Cooperative Program in Electrical Engineering; working at the Air Force Cambridge Research Laboratories on projects in speech processing and novel radar systems; participating in the model railroad club; and joining the MIT Symphony Orchestra, where he met his first wife, Jane Hodgson ’55, SM ’56, PhD ’61. (The two later separated when she went to study medicine in Florida.)
Dennis earned his BS (1953), MS (1954), and ScD (1958) from MIT before joining the then-Department of Electrical Engineering as a faculty member. He was promoted to full professor status in 1969. His doctoral thesis, entitled, “Mathematical Programming and electrical networks,” explored analogies between electric circuit theory and quadratic programming problems. Ideas he developed in that paper further crystallized in his 1964 paper, “Distributed solution of network programming problems,” which created an important early class of digital distributed optimization solvers.
In a 2003 piece that Dennis wrote for his undergraduate class’s 50th reunion, he remembered his earliest encounters with computers at the Institute: “I prepared programs written in assembly language on punched paper tape using Frieden 'Flexowriters,' and stood aside watching the myriad lights blink and flash while operator Mike Solamita fed the tapes [...] That was 1954. Fifty years later, much has changed: A room full of vacuum tubes has become a tiny chip with millions of transistors. A phenomenon once limited to research laboratories has become an industry producing commodity products that anyone can own and use beneficially.”
Dennis’ influence in steering that change was profound. As a collaborator with the teams behind both Project MAC and Multics, the earliest attempts to allow multiple users to work with a single computer seemingly simultaneously (i.e., a time-shared operating system), Dennis helped to specify the unique segment addressing and paging mechanisms that became a fundamental part of the General Electric Model 645 computer. His insights stemmed from a tendency to pay equal attention to both hard- and software when others considered themselves specialists in one or the other.
“I formed the Computation Structures Group [within CSAIL] and focused on architectural concepts that could narrow the acknowledged gap between programming concepts and the organization of computer hardware,” Dennis explained in his 2003 recollection. “I found myself dismayed that people would consider themselves to be either hardware or software experts, but paid little heed to how joint advances in programming and architecture could lead to a synergistic outcome that might revolutionize computing practice.”
Dennis’ emphasis on synergy did not go unnoticed. Gerald Sussman, the Panasonic Professor of Electrical Engineering, points out “the relationship of [Dennis’] dataflow architecture to single-assignment programs, and thus to pure functional programs. This coupled the virtue of referential transparency in programming to the effective use of hardware parallelism. Dennis also pioneered the use of self-timed circuits in digital systems. The ideas from that work generalize to much of the work on highly distributed systems.”
The Computation Structures Group attracted multiple scholars interested in developing asynchronous computing and dataflow architecture, many of whom became lifelong friends and collaborators. These included Peter Denning, with whom Dennis and Joseph Qualitz co-authored the textbook “Machines, Languages, and Computation” (1978); the late Arvind, who became faculty head of computer science for the Department of Electrical Engineering and Computer Science (EECS), and the late Guang R. Gao, who became distinguished professor of electrical and computer engineering at the University of Delaware.
In recognition of his contributions to the Multics project, Dennis was elected fellow of the Institute of Electrical and Electronics Engineers (IEEE). Many additional honors would follow: He received the Association for Computing Machinery (ACM)/IEEE Eckert-Mauchly Award in 1984; was inducted as a fellow of the ACM (1994); was named to the National Academy of Engineering (2009); was elected to the (ACM) Special Interest Group on Operating Systems (SIGOPS) Hall of Fame (2012); and was awarded the IEEE John von Neumann Medal (2013).
A successful researcher, Dennis was perhaps equally influential in the development of EECS’ curriculum, developing six subjects in areas of computer theory and systems: Theoretical Models for Computation; Computation Structures; Structure of Computer Systems; Semantic Theory for Computer Systems; Semantics of Parallel Computation; and Computer System Architecture (taught in collaboration with Arvind.) Several of the courses that Dennis developed continue to be taught, in updated form, to this day.
Following his retirement from teaching in 1987, he consulted on projects relating to parallel computer hardware and software for such varied groups as NASA Research Institute for Advanced Computer Science; Boeing Aerospace; McGill University; the Architecture Group of Carlstedt Elektronik in Gothenburg, Sweden; and Acorn Networks, Inc. His fruitful relationship with former student Guang Gao continued in the form of a lecture tour through China, as well as co-authorship of a book, “Dataflow Architecture,” currently in progress at MIT Press.
A voracious lifelong learner, Dennis was fond of repeating a friend’s observation that “a scholar is just a book’s way of making another book.” In a full and active retirement, he still made room for music, trying his hand at composing; performing at Tanglewood as a tenor in Chorus Pro Musica; playing piano at the marriage of Guang Gao’s son Nick; and joining the chorus at the First Church in Belmont, Massachusetts, where his celebration of life (with concurrent livestreaming) will be held on Monday, June 8, at 2 p.m.
Dennis is survived by his wife Therese Smith ’75; children David Hodgson Dennis of North Miami, Florida; Randall Dennis of Connecticut; and Galen Dennis, a resident of Australia.
