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Algorithmic Game Theoryand Internet Computing
Vijay V. Vazirani
New Market Models
and Algorithms
Markets
Stock Markets
Internet
Revolution in definition of markets
Revolution in definition of markets
New markets defined byGoogle AmazonYahoo!Ebay
Revolution in definition of markets
Massive computational power available
for running these markets in a
centralized or distributed manner
Revolution in definition of markets
Massive computational power available
for running these markets in a
centralized or distributed manner
Important to find good models and
algorithms for these markets
Theory of Algorithms
Powerful tools and techniques
developed over last 4 decades.
Theory of Algorithms
Powerful tools and techniques
developed over last 4 decades.
Recent study of markets has contributed
handsomely to this theory as well!
Adwords Market
Created by search engine companiesGoogleYahoo!MSN
Multi-billion dollar market
Totally revolutionized advertising, especially
by small companies.
New algorithmic and game-theoretic questions
Monika Henzinger, 2004: Find an on-line
algorithm that maximizes Google’s revenue.
The Adwords Problem:
N advertisers; Daily Budgets B1, B2, …, BN Each advertiser provides bids for keywords he is interested
in.
Search Engine
The Adwords Problem:
N advertisers; Daily Budgets B1, B2, …, BN Each advertiser provides bids for keywords he is interested
in.
Search Enginequeries (online)
The Adwords Problem:
N advertisers; Daily Budgets B1, B2, …, BN Each advertiser provides bids for keywords he is interested
in.
Search EngineSelect one Ad
Advertiser pays his bid
queries (online)
The Adwords Problem:
N advertisers; Daily Budgets B1, B2, …, BN Each advertiser provides bids for keywords he is interested
in.
Search EngineSelect one Ad
Advertiser pays his bid
queries (online)
Maximize total revenue
Online competitive analysis - compare with best offline allocation
The Adwords Problem:
N advertisers; Daily Budgets B1, B2, …, BN Each advertiser provides bids for keywords he is interested
in.
Search EngineSelect one Ad
Advertiser pays his bid
queries (online)
Maximize total revenue
Example – Assign to highest bidder: only ½ the offline revenue
Example:
$1 $0.99
$1 $0
Book
CD
Bidder1 Bidder 2
B1 = B2 = $100
Queries: 100 Books then 100 CDs
Bidder 1 Bidder 2
Algorithm Greedy
LOST
Revenue100$
Example:
$1 $0.99
$1 $0
Book
CD
Bidder1 Bidder 2
B1 = B2 = $100
Queries: 100 Books then 100 CDs
Bidder 1 Bidder 2
Optimal Allocation
Revenue199$
Generalizes online bipartite matching
Each daily budget is $1, and
each bid is $0/1.
Online bipartite matching
advertisers queries
Online bipartite matching
advertisers queries
Online bipartite matching
advertisers queries
Online bipartite matching
advertisers queries
Online bipartite matching
advertisers queries
Online bipartite matching
advertisers queries
Online bipartite matching
advertisers queries
Online bipartite matching
Karp, Vazirani & Vazirani, 1990:
1-1/e factor randomized algorithm.
Online bipartite matching
Karp, Vazirani & Vazirani, 1990:
1-1/e factor randomized algorithm. Optimal!
Online bipartite matching
Karp, Vazirani & Vazirani, 1990:
1-1/e factor randomized algorithm. Optimal!
Kalyanasundaram & Pruhs, 1996:
1-1/e factor algorithm for b-matching:
Daily budgets $b, bids $0/1, b>>1
Adwords Problem
Mehta, Saberi, Vazirani & Vazirani, 2005:
1-1/e algorithm, assuming budgets>>bids.
Adwords Problem
Mehta, Saberi, Vazirani & Vazirani, 2005:
1-1/e algorithm, assuming budgets>>bids.
Optimal!
New Algorithmic Technique
Idea: Use both bid and
fraction of left-over budget
New Algorithmic Technique
Idea: Use both bid and
fraction of left-over budget
Correct tradeoff given by
tradeoff-revealing family of LP’s
Historically, the study of markets
has been of central importance,
especially in the West
A Capitalistic Economy
depends crucially on pricing mechanisms,
with very little intervention, to ensure:
Stability Efficiency Fairness
Do markets even have inherentlystable operating points?
General Equilibrium TheoryOccupied center stage in Mathematical
Economics for over a century
Do markets even have inherentlystable operating points?
Leon Walras, 1874
Pioneered general
equilibrium theory
Supply-demand curves
Irving Fisher, 1891
Fundamental
market model
Fisher’s Model, 1891
milkcheese
winebread
¢¢
$$$$$$$$$$$$$$$$$$
$$
$$$$$$$$
People want to maximize happiness – assume
linear utilities.Find prices s.t. market clears
Fisher’s Model n buyers, with specified money, m(i) for buyer i k goods (unit amount of each good) Linear utilities: is utility derived by i
on obtaining one unit of j Total utility of i,
i ij ijj
U u xiju
]1,0[
x
xuuij
ijj iji
Fisher’s Model n buyers, with specified money, m(i) k goods (each unit amount, w.l.o.g.) Linear utilities: is utility derived by i
on obtaining one unit of j Total utility of i,
Find prices s.t. market clears, i.e.,
all goods sold, all money spent.
i ij ijj
U u xiju
xuu ijj iji
Arrow-Debreu Theorem, 1954
Celebrated theorem in Mathematical Economics
Established existence of market equilibrium under very general conditions using a deep theorem from topology - Kakutani fixed point theorem.
Kenneth Arrow
Nobel Prize, 1972
Gerard Debreu
Nobel Prize, 1983
Arrow-Debreu Theorem, 1954
.
Highly non-constructive
Adam Smith
The Wealth of Nations
2 volumes, 1776.
‘invisible hand’ of the market
What is needed today?
An inherently algorithmic theory of
market equilibrium
New models that capture new markets
Beginnings of such a theory, within
Algorithmic Game Theory
Started with combinatorial algorithms
for traditional market models
New market models emerging
Combinatorial Algorithm for Fisher’s Model
Devanur, Papadimitriou, Saberi & V., 2002
Using primal-dual schema
Primal-Dual Schema
Highly successful algorithm design
technique from exact and
approximation algorithms
Exact Algorithms for Cornerstone Problems in P:
Matching (general graph) Network flow Shortest paths Minimum spanning tree Minimum branching
Approximation Algorithms
set cover facility location
Steiner tree k-median
Steiner network multicut
k-MST feedback vertex set
scheduling . . .
No LP’s known for capturing equilibrium allocations for Fisher’s model
Eisenberg-Gale convex program, 1959
DPSV: Extended primal-dual schema to
solving nonlinear convex programs
2s
1s
2t
1t
A combinatorial market
2s
1s
2t
1t
A combinatorial market
)(ec
2s
1s
2t
1t
A combinatorial market
)1(m
)2(m
)(ec
A combinatorial market
Given: Network G = (V,E) (directed or undirected)Capacities on edges c(e)Agents: source-sink pairs
with money m(1), … m(k)
Find: equilibrium flows and edge prices
1 1( , ),...( , )k ks t s t
Flows and edge prices
f(i): flow of agent i p(e): price/unit flow of edge e
Satisfying: p(e)>0 only if e is saturated flows go on cheapest paths money of each agent is fully spent
Equilibrium
Kelly’s resource allocation model, 1997
Mathematical framework for understanding
TCP congestion control
Highly successful theory
TCP Congestion Control
f(i): source rate prob. of packet loss (in TCP Reno)
queueing delay (in TCP Vegas) p(e):
TCP Congestion Control
f(i): source rate prob. of packet loss (in TCP Reno)
queueing delay (in TCP Vegas)
Kelly: Equilibrium flows are proportionally fair:
only way of adding 5% flow to someone’s
dollar is to decrease 5% flow from
someone else’s dollar.
p(e):
primal process: packet rates at sources
dual process: packet drop at links
AIMD + RED converges to equilibrium
in the limit
TCP Congestion Control
Kelly & V., 2002: Kelly’s model is a
generalization of Fisher’s model.
Find combinatorial polynomial time
algorithms!
Jain & V., 2005:
Strongly polynomial combinatorial algorithm
for single-source multiple-sink market
Single-source multiple-sink market
Given: Network G = (V,E), s: sourceCapacities on edges c(e)Agents: sinks
with money m(1), … m(k)
Find: equilibrium flows and edge prices
1,..., kt t
Flows and edge prices
f(i): flow of agent i p(e): price/unit flow of edge e
Satisfying: p(e)>0 only if e is saturated flows go on cheapest paths money of each agent is fully spent
Equilibrium
s
t1
t2
2
2
110$
10$
s
t1
t2
2
2
1 10$
10$
$5
$5
s
t1
t2
2
2
1 10$
10$
120$
s
t1
t2
2
2
1 120$
10$
$10
$40
$30
Jain & V., 2005:
Strongly polynomial combinatorial algorithm
for single-source multiple-sink market
Ascending price auctionBuyers: sinks (fixed budgets, maximize flow)Sellers: edges (maximize price)
Auction of k identical goods
p = 0; while there are >k buyers:
raise p; end; sell to remaining k buyers at price p;
s
t1
t2
t3
t4
Find equilibrium prices and flows
s
t1
t2
t3
t4
Find equilibrium prices and flows
m(1)
m(2)
m(3)m(4)
cap(e)
s
t1
t2
t3
t4
min-cut separating from all the sinkss
60
s
t1
t2
t3
t4
p
60
s
t1
t2
t3
t4
p
60
Throughout the algorithm:
s itc(i): cost of cheapest path from to
sink demands flow ( )
( )( )
m if i
c iit
s
t1
t2
t3
t4
p
: ( )i c i p
60
sink demands flow ( )
( )m i
f ip
it
Auction of edges in cut
p = 0; while the cut is over-saturated:
raise p; end; assign price p to all edges in the cut;
s
t1
t2 t3
t4
pp 0
0(2)c p
60 50 (2) 10f
s
t1
t2 t3
t4
p0
p
0(2)c p
60 50 0(1) (3) (4)c c c p p
s
t1
t2 t3
t4
p0
p1
60 50 20
0(2)c p
0 1(1) (3)c c p p
(1) (3) 30f f
s
t1
t2 t3
t4
p0
p1
p
60 50 20
s
t1
t2 t3
t4
p0
p1 p
2
60 50 200 1 2(4)c p p p
(4) 20f
s
t1
t2 t3
t4
p0
p1 p
2 nested cuts
60 50 20
Flow and prices will:
Saturate all red cutsUse up sinks’ moneySend flow on cheapest paths
s
t1
t2
t3
t4
Implementation
s
t1
t2
t3
t4
t
s
t1
t2
t3
t4
t
Capacity of edge =tt i
( )( )
( )
m if i
c i
s
t1
t2
t3
t4
t
min s-t cut
60
s
t1
t2
t3
t4
t
p
60
s
t1
t2
t3
t4
t
p
60
s
t1
t2
t3
t4
t
p tt i
Capacity of edge =
( )( )
m if i
p
: ( )i c i p
s
t1
t2 t3
t4
t
pp 0
0(2)c p
60 50
f(2)=10
s
t1
t2 t3
t4
t
p0
p
60 50
s
t1
t2 t3
t4
t
p0
p1
0(2)c p
0 1(1) (3) (4)c c c p p
60 50 20
s
t1
t2 t3
t4
t
p0
p1
p
s
t1
t2 t3
t4
t
p0
p1 p
2
0 1 2(4)c p p p
Eisenberg-Gale Program, 1959
max ( ) log
. .
:
: 1
: 0
ii
i ij ijj
iji
ij
m i u
s t
i u
j
ij
u xx
x
Lagrangian variables: prices of goods
Using KKT conditions:
optimal primal and dual solutions
are in equilibrium
Convex Program for Kelly’s Model
max ( ) log ( )
. .
: ( )
: ( ) ( )
, : 0
i
pip
pi
m i f i
s t
i f i f
e flow e c e
i p f
JV Algorithm
primal-dual alg. for nonlinear convex program
“primal” variables: flows
“dual” variables: prices of edges
algorithm: primal & dual improvements
Allocations Prices
Rational!!
Irrational for 2 sources & 3 sinks
s1 t1
1
s2
t2
1
t21 2
$1 $1
$1
Irrational for 2 sources & 3 sinks
s1 t1
1
s2
t2
1
t2
31
3
3
Equilibrium prices
Max-flow min-cut theorem!
Other resource allocation markets
2 source-sink pairs (directed/undirected) Branchings rooted at sources (agents)
Branching market (for broadcasting)
s1 s2
)1(m3s
)2(m)(ec
(3)m
Branching market (for broadcasting)
s1 s2
)1(m3s
)2(m)(ec
(3)m
Branching market (for broadcasting)
s1 s2
)1(m3s
)2(m)(ec
(3)m
Branching market (for broadcasting)
s1 s2
)1(m3s
)2(m)(ec
(3)m
Branching market (for broadcasting)
Given: Network G = (V, E), directed edge capacities sources, money of each source
Find: edge prices and a packing
of branchings rooted at sources s.t. p(e) > 0 => e is saturated each branching is cheapest possible money of each source fully used.
S V
Eisenberg-Gale-type program for branching market
max ( ) log ii Sm i b
s.t. packing of branchings
Other resource allocation markets
2 source-sink pairs (directed/undirected) Branchings rooted at sources (agents) Spanning trees Network coding
Eisenberg-Gale-Type Convex Program
max ( ) log iim i u
s.t. packing constraints
Eisenberg-Gale Market
A market whose equilibrium is captured
as an optimal solution to an
Eisenberg-Gale-type program
Theorem: Strongly polynomial algs for
following markets :2 source-sink pairs, undirected (Hu, 1963)spanning tree (Nash-William & Tutte, 1961)2 sources branching (Edmonds, 1967 + JV, 2005)
3 sources branching: irrational
Theorem: Strongly polynomial algs for
following markets :2 source-sink pairs, undirected (Hu, 1963)spanning tree (Nash-William & Tutte, 1961)2 sources branching (Edmonds, 1967 + JV, 2005)
3 sources branching: irrational
Open: (no max-min theorems):2 source-sink pairs, directed2 sources, network coding
EG[2]: Eisenberg-Gale markets with 2 agents
Theorem: EG[2] markets are rational.
Chakrabarty, Devanur & V., 2006:
EG[2]: Eisenberg-Gale markets with 2 agents
Theorem: EG[2] markets are rational.
Combinatorial EG[2] markets: polytope
of feasible utilities can be described via
combinatorial LP.
Theorem: Strongly poly alg for Comb EG[2].
Chakrabarty, Devanur & V., 2006:
EG
Rational
Comb EG[2]
SUA
EG[2]
3-source branching
Fisher
2 s-s undir
2 s-s dir
Single-source
Efficiency of Markets
‘‘price of capitalism’’ Agents:
different abilities to control prices idiosyncratic ways of utilizing resources
Q: Overall output of market when forced
to operate at equilibrium?
Efficiency
( )( ) min
max ( )I
equilibrium utility Ieff M
utility I
Efficiency
Rich classification!
( )( ) min
max ( )I
equilibrium utility Ieff M
utility I
1/(2 1)k
Market EfficiencySingle-source 1
3-source branching
k source-sink undirected
2 source-sink directed arbitrarily
small
1/ 2
. . 1/( 1)l b k
Other properties:
Fairness (max-min + min-max fair) Competition monotonicity
Open issues
Strongly poly algs for approximatingnonlinear convex programsequilibria
Insights into congestion control protocols?