ACM Turing Centenary Celebration

This past weekend, I was very fortunate to have a chance to attend the ACM‘s Turing Centenary Celebration in honor of the 100th anniversary of the birth of Alan Turing. The event brought together nearly every living Turing Award winner for a series of talks and panel discussions on subjects like AI, theory of computation, computer architecture, and the role of computation in other fields. A webcast of the entire event is already available online. Below are some of my own notes on the conference.

For many of us in the audience, this weekend was about seeing in person the people we’d learned so much from both academically and professionally. This awe-inspiring feeling was best articulated by Vint Cerf in closing yesterday: “it’s like the history books opened up and the people walked out”. For me, by far, the highlight was the panel on Computer Architecture, moderated by David Patterson (yes, that Patterson) and featuring Frederick Brooks (of OS/360 and Mythical Man-Month fame), Ivan Sutherland, and Chuck Thacker. More than the other panels, I found all of the speakers’ prepared remarks accessible (perhaps because my work and interests most closely align with theirs), but at the same time very instructional. Sutherland began with the “Tyranny of the Clock”, an eloquent articulation of an important barrier in modern chip design and a call to action for designers and researchers. Then, in a sort of reverential but thoughtful engineering-style postmortem, Brooks discussed why the machine that Turing actually built, unlike so much of his other work, was not very influential. Thacker discussed the nature of computer architecture research and the modern developments that have made it more accessible for students today. In the subsequent discussion, Patterson referenced a prophetic quote by Maurice Wilkes at the dawn of modern computing (that Bryan also cited in his QCon talk last year) in which Wilkes suddenly “realized that a large part of my life from then on was going to be spent in ļ¬nding mistakes in my own programs”.


Complexity was a prominent theme in several sessions. Ken Thompson expressed his disappointment at the complexity of modern Linux, pointing to its 1000+ syscalls compared to the original Unix’s 200. In his prepared remarks, he also shared some grim reflections on how Turing would feel about the state of computing today: Windows, phishing, botnets, C++, and so on. He compared his feeling to that of an early television pioneer visiting our time and seeing people watching Maury Povich. On a more positive note in the same session, Fernando Corbato (leader on Multics) gave a brief but fascinating account of what it was like to work on computers in the early days. He called Unix actually one the greatest results of Multics, Unix being a “distillation” of the better ideas of Multics without all the complexity. (It’s well worth watching Fernando’s and Ken’s remarks from the “Systems Architecture, Design, Engineering, and Verification” session.) Later, Thacker too called complexity “the enemy”, suggesting that we seriously reconsider many of the held-over assumptions in today’s systems that are costing us enormously today. (I’m sure that’s a good idea, and I’d have loved to hear some examples of these things that he had in mind.)

In the Programming Languages panel, Barbara Liskov lamented that the languages people use today for production systems are pretty complex for introducing new students to computer programming, but also admitted that building languages simple enough to be thoroughly understood in an introductory course and rich enough to support what professional engineers want is a serious challenge. She suggested starting from scratch with only the essential language features of modularity and encapsulation. In the same session, Nicklaus Wirth (in an entertaining, light presentation) explained how he sought to design languages that were both simpler and more powerful than their contemporaries – that these are not opposing goals. All of the participants agreed that in practice, most popular languages seem to accrete lots of cruft from small changes that seem good at the time, but contribute to an overly complex system.

Lucky or good?

Another theme that came up quite a bit was the role of luck in the speakers’ success. Many of them attributed their success to luck, leaving it there, but I liked Dahlia Malkhi’s reference to a golfer who hit a hole-in-one. He was asked if that was a result of luck or training, and replied that he was lucky, but he had to train a lot in order to get that lucky.

Beware elegance?

Several speakers (notably Butler Lampson and Joseph Sifakis) mentioned that they tend to be suspicious of clean, elegant solutions to problems, because such solutions often don’t work well in the real world. I’d never heard it put so generally, especially by leaders in the field, as that goes against a common intuition among mathy people that’s usually nurtured as part of our education. (That’s still a good thing – as Einstein famously said, it’s important to strive for simplicity, but be careful not to go to far.) In fact, Sifakis attributed the lack of serious work in rigorous system design to researchers’ preference for nice theories, even if those theories don’t match reality. (While probably a factor, this explanation seems to leave out the economic cost of such rigor as an important reason why many systems today aren’t built the way he suggests.)

System verification

In the Systems Architecture and Verification session, Vint Cerf noted that automatic verifiers don’t seem to work well for many types of systems we build and asked Sifakis and E. Allen Emerson whether there existed interactive tools that would help programmers test assertions about their systems, rather than automatically trying to verify the whole thing. Emerson pointed out that this is called semi-automatic verification, but still seemed more interested in the fully-automatic kind. Vint’s idea made me think of a sort of extensible lint, since lint is already an admittedly limited tool for checking a fixed set of assertions about a program. But despite its limits, lint is incredibly useful (at least in languages like C and JavaScript) for rooting out large classes of bugs, and it would be interesting to think about a more interactive workflow that would free the tool from having to report only things it knows are problems. (People generally won’t use an error-checking tool that reports many false positives, but they might use a tool that can evaluate static assertions about their code in a less rigid context.)

“What”-based programming

Alan Kay and others talked about the idea of “what”-based programming, rather than the “how”-based approaches we typically use today. The idea is that humans tell the computer what to do, and some engine under the hood figures out how to do it. Kay demonstrated a graphical environment based on this idea, and then wondered why we couldn’t build more complex systems (including operating systems) that way. Bill, Robert, and I tried for a while to imagine what this would look like. On the one hand, many classes of device drivers are similar enough that you could imagine modeling some pieces of them with a declarative “what”-based description, but interaction with physical devices often requires precise sequences of register reads and writes and it’s hard to imagine encoding that without essentially describing “how”. Achieving good performance may be challenging, since humans writing code for such systems today necessarily describe how to organize them to be fast. And if you could solve this for something as constrained as device drivers, how could you generalize it for the schedulers or VM system, without encoding detailed knowledge into the engine that actually translates the “what” to the “how”? You could also imagine that debugging such systems would be very difficult. Still, I found the idea compelling, because there are many cases where we do build “what”-based descriptions and the result is that it’s much easier to verify both that the description does what the human wants it to do and that the “what”-to-“how” system properly translates it (e.g., Meta-D, a Cloud Analytics module that describes a family of DTrace scripts declaratively, or even the D language itself). It would be interesting to hear from Alan Kay what he was thinking in posing this question.

Computation in biology and physics

I was especially intrigued by Leonard Adleman‘s remarks during the “Algorithmic View of the Universe” panel, in which he talked about vastly different notions of computation and how results based on the Turing model, plus the Church-Turing thesis, can inform physics and biology. He discussed protein folding in the cell as a form of computation, and what implications that has for biologists trying to understanding cellular processes. Later he wondered what implications the proven constraints of the Turing model, taken as physical laws, would have on quantum mechanics (e.g., that certain types of time travel allowed by QM must actually be impossible).

These were just a few of the bits I found most interesting, but the whole weekend was a humbling experience. Besides being able to see so many important figures in the field, it was a good opportunity to step outside the confines of day-to-day engineering, which for me tends toward a time horizon of a few years. And most of the talks provoked interesting discussions. So thanks to all the speakers, and to the ACM for putting together the event and making the video available.