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Regret Minimization and the Price of Total Anarchy Paper by A. Blum, M. Hajiaghayi, K. Ligett, A.Roth Presented by Michael Wunder

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Nash Anarchy vs. Total Anarchy In a multiagent setting, want to find the ratio between the socially optimal value and the “selfish” agent outcome Traditionally, assumed to be Nash, where no agent has incentive to change Can also find the price of total anarchy, when selfish agents act repeatedly to minimize regret over previous actions

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Why Regret Minimization? Finding Nash equilibria can be computationally difficult Not clear that agents would converge to it, or remain in one if there are several Regret minimization is realistic because there are efficient algorithms that minimize regret, it is locally computed, and players improve by lowering regret

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Results comparing prices Shows how PoTA compares with PoA Four classes of games Hotelling Games Valid Games Atomic Linear Congestion Games Parallel Link Congestion Games

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Preliminaries (maximization) A i : set of pure strategies for player i S i : set of mixed strategies for player i (distributions over A i ) Social Utility Function: Individual utility function: Strategy set if player i changes from s i to s’ i : ’

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Preliminaries (cont.) Regret of Player i given action sets A: The difference between action taken and best available action over all timesteps Price of Total Anarchy: Ratio of social value of best strategies to the “regret minimizers” Socially Optimal Value:

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Hotelling Games Problem: k sellers must set up a vendor stand on a graph to sell to n tourists, who buy from first seller along a path Strategy set A i = V S1 S2 T1

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Hotelling Games cont. Social welfare at time t: To maximize fairness (and maximize the lowest player), split all vertices equally OPT = n/k Si T1

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Hotelling Games cont. Claim: Price of anarchy = (2k-2)/k Proof: Consider alternate set: Some player h achieves: If player i plays same strategy as h, the expected payoff is: Therefore, Price of Anarchy

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Hotelling with Total Anarchy The price of total anarchy is also (2k-2)/k Proof from symmetry: Let O t i be the set of plays at time t by players other than i Δ i t->u be the difference between expected payoff from choosing from O t i at time step u, and n/(2k-2) For all i, for all 1 u + Δ i u->t >=0 Imagine a (2k-2) player game where there is a time t and a time u player for each original player but i If player i replaces a random player, α i = n/(2k-2)

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Hotelling Total Anarchy Proof If player i replaces a time t player, and all other time t players are removed, player i’s payoff only improves The expected payoff of player i from picking an action o t i uniformly at random from O t i and playing over all T rounds:

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Generalized Hotelling Games The above proof does not use specifics of the game as described In general, PoTA is (2k-2)/k even in the presence of arbitrarily many Byzantine players making arbitrary decisions Regret-minimizing players may not converge to a Nash equilibrium, and play can cycle forever

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Valid Games, Price of Anarchy Valid games are a broad class of games that includes a market sharing game, the facility location problem, and others. Example: Cable television market sharing Game is bipartite graph G = ((V,U),E). Each v in V is a player, each u in U is a market Markets have value and cost Players have budget Players may enter adjacent markets, and receive value of market divided by players in market

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Valid Games Definition For a set function f, define the derivative of f at X in V in direction D in V-X to be f’D(X)=f(X U D)-f(X) A game is valid if: For X in A, γ i’(X)>= γ i’(A) for all i in V – A (submodularity) ( Vickrey )

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Valid Games Price of Anarchy Vetta shows that for any Nash equilibrium strategy S, if γ is non- decreasing, γ(S) >= OPT/2 PoTA matches PoA While PoA does not hold with the addition of Byzantine players, PoTA does

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Total Anarchy w/Byzantines Show by contradiction:

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Total Anarchy w/Byzantines So there is a regret minimizing player i which violates the regret minimizing condition.

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Atomic Congestion Games An atomic congestion game is a minimization game consisting of k players and a set of facilities V (a i over V i ) Each facility e has a latency function f e (l e ) Each player i has weight w i (unweighted w i = 1) Player i experiences cost: load on facility l e

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Atomic Congestion Games Consider two types of social utility function: linear and makespan in parallel link networks Linear Edge Costs: Social utility:

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Congestion Games PoA Price of Anarchy with unweighted players, sum social utility function, and linear cost functions is 2.5 (Christodoulou et al. 2005) Claim: Price of Total Anarchy is the same: “By assuming regret minimization, each player’s time average cost is no better than the cost of best action in hindsight. That is, no better than optimal strategy.”

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Congestion Games: PoTA Proof: for all i: Summing over all players: After math:

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Congestion Games: PoTA For atomic congestion games with unweighted players, sum social function, and polynomial latency functions of degree d, PoTA <= d d 1-o(1)

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Parallel Link Congestion Game n identical links, k weighted players Each player pays sum of weights of jobs on link chosen Social cost is total weight of worst loaded link (makespan):

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2 Parallel Links: PoTA For 2 links, Price of Total Anarchy matches Price of Anarchy = 3/2, but only in expectation

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n Parallel Links: PoTA With n parallel links, PoTA is not the same as PoA PoTA with makespan utility and n links is Ω(n ½ ), versus O(log n/ log log n) for PoA Proof: with n links and n players, OPT = 1 We can construct a situation with negative regret but with maximum latency = Ω(n ½ )

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n Parallel Links: PoTA Divide the players into groups of size n ½ /2 and rotate each group to take link 1 The rest distribute evenly on the remaining links Each player has average latency 5/4 – ½ (n -½ ) If a player plays a fixed link, the average latency is 2 – ½ (n -½ ) Therefore, players have negative regret but maximum latency = Ω(n ½ )

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Conclusion Thank you!

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