Monday, March 22, 2010

Intrinstic Robustness of the Price of Anarchy

Everybody loves cookies. Even us, the students in 15-589U class, love cookies. Thus, for Thursday's class, Ryan, who wishes us all to attend the talk, will bring cookies. Alas, we are 12 students in the class, and Ryan only has 11 cookies. So Ryan is willing to give us the cookies if we all reach a consensus on who gets a cookie. Splitting a cookie is unheard of, and neither is sharing a cookie. (Yes, we all love cookies that much!)

So essentially, each of us has two options: "ask for a cookie" or "not to ask for a cookie". If everybody asks for a cookie, no one gets a cookie, as there as only 11 cookies and 12 students. If 11 or fewer people ask for a cookie, each gets 1 cookie. If you don't ask for a cookie, you always get no cookies. So, how will we reach a consensus?

Well, that depends on how collegial we are. If we care about getting as many cookies as possible as a class, someone steps down, and the rest get 11 cookies. But what if each of us cares only about how many cookies (s)he gets? It is clear that no matter what other students do, each of us is always better off by asking for a cookie! Therefore, we all ask for cookies, and no one gets any! So, even though as a collective, we could gain 11 cookies, we end up getting 0, and everybody's passion for cookies is left unsatisfied.

Let's contemplate a bit about this example. Here is a game, where acting selfishly is globally worse than playing for overall good. In fact, we get a ratio of 11/0, which is infinitely worse. What about other games? Can we measure how much worse will we do by acting selfishly in a given game G? In a set G of games?

This is precisely the question that Koutsoupias and Papadimitriu raised in STOC'99. Suppose we model the game by costs rather than payoffs, so everybody wishes to minimize their costs. How much more do we (altogether) pay for playing selfishly, in comparison to playing for the overall good? In the words of Koutsoupias and Papadimitriu, who considered routing games, how much faster would the internet be if it had a manager? This is the Price of Anarchy, the ratio between the worst cost of a Nash-Equilibrium in the game, to the cost of the social optimum.

Ever since Koutsoupias and Papadimitriu raised the question, there have been numerous attempts at bounding the POA of various classes of games. The question was even extended to other models of players (such as regret-minimizing, myopic best response players etc.). In STOC 2009, Tim Roughgarden introduced the notion of smoothness in games, and showed the following:

(i) Smoothness gives an immediate (and very elegant) upper bound on the POA of a game.
(ii) The same bound applies to other models of players.
(iii) This bound gives the exact POA for the class of congestion games.

In Thursday's talk, we will survey the definition of POA and the various models of players considered in the past. We will then go on to stating in exact and proving all the above 3 results.

As a side note: the first two results are pretty straight-forward, so even people who have no background in Game Theory are welcome to attend the talk.

And just to clarify: Ryan will NOT actually bring cookies to Thursday's class.


  1. I like your cookie analogy, now I wish I was in Pittsburgh to see this lecture ;-)

  2. even i wish the same......its going to b an interesting session....