Machinations


Day 1 PODC ’09
August 11, 2009, 4:11 pm
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Editors Note: This is the first day of reports from PODC ’09 by my  student Amitabh Trehan.  Principles of Distributed Computing (PODC) is one of the premiere conferences focusing on theory of distributed computing.

An interesting talk in the morning was the presentation of the best student paper  Max Registers, Counters and Monotone Circuits (Aspnes, Attiya and Censor) presented by Keren Censor. The final  message of  the talk was `Lower bounds do not always have the final say ‘ (quoted from the talk). The paper deals with implementation of concurrent data structures in shared memory systems, where n processes communicate by reading and writing to shared multi-reader multi-writer registers. A lower bound given by Jayanti et al shows that these operations take Omega(n) space and Omega(n) time steps in the worst case, for many common data structures. On careful analysis of the lower bound proof, the authors realised that they could develop sub-linear algorithms for many useful applications where the worst case would not occur. These are for algorithms where the number of operations will be bounded e.g. for applications which have a limited life-time or several instances of the data structure can be changed. They go on to show how some of these structures can be constructed with some nice use of recursion and my favorite data structures – trees!  This reminded me of a discussion I had the previous evening with Victor Luchangco about Nancy Lynch. He contended that one trait contributing to the success of her work (and of her students) was the rigorous questioning of every assumption and definition of interest they came across. A good lesson!

Robert Van Renesse‘s keynote talk `Refining the way to Consensus‘ set up the stage for later talks on consensus.  His talk described several refinements to basic consensus algorithms from simple to non-blocking consensus, to election consensus, to voters consensus, and finally to recommender consensus. He stressed the importance of  exposing undergraduates to the consensus problem, because of the fact that this is a problem they are likely to see again the real world. What drew chuckles was a cartoon showing the ‘ideal’ programmer, a faceless human stuck to the computer screen and keyboard coding to strictly meet the specifications, and an ‘intuitive’ programmer, a happy guy with his back to the computer  smiling having no idea what the specifications he was implementing were but knowing when he was done with his coding job.  Editor Note: Look for more information on the consensus problem in a blog post in the next week or two.

There were two  excellent `game theoretic’ talks in the afternoon. Georgios Piliouras presented his paper with Robert Kleinberg and Eva Tardos `Load Balancing Without Regret in the Bulletin Board Model‘. They consider load balancing games in the bulletin board model (players can find out delay on all machines, but have no information on what their delay would have been if they had selected another machine) and show solutions using regret-minimization which are exponentially better than the correlated equilibrium. No-regret algorithms are an example of alternative (to the weaknesses of Nash equilibria) solution concepts based on average outcome of self-adapting agents who react to each other’s strategies in repeated play of the game. Martin Hoefer presented his paper with Ackermann, Berenbrink and Fischer `Concurrent Imitation Dynamics in Congestion Games‘ which discusses the dynamics emerging when agents sample and possibly  imitating other agent’s strategies when to do so will improve their utility.  Their main result is to show that this imitation strategy leads to rapid convergence (logarithmic in number of players) to approximate equilibria for congestion games.  An approximate equilibrium is one where only a small fraction of the players have latency much better or worse than the average latency.

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