Towards Scaling Fully Personalized PageRank

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Towards Scaling Fully Personalized PageRank

• Dániel Fogaras, Balázs Rácz

Computer and Automation Research Institute of the
Hungarian Academy of Sciences
Budapest University of Technology and Economics
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Problem formulation

• PageRank(Brin,Page,98)
• PV PageRank vector, r uniform distribution vector
• Overall quality measure of Web pages
• Pre-computation evaluate PV by power iteration
• Query order results by PV
• Personalized PageRank(Brin,Page,98)
• r preference vector of a user, query dependent
• PPV(r)PV personalized quality measure of Web
  pages
• Pre-computation r is not known. What to compute?
• Query power-iteration. 5 hours/query!!!

Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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Preliminaries

• Linearity
• Full personalization
• Pre-compute PPV(ri) for all pages
• V2 disk, V(VE) time, where V 109, E 1010,
  ???
• Topic-Sensitive PageRank (Haveliwala 01)
• Linearity
• Pre-compute PPV(ri) for a topical basis r1,,rk,
  k20
• Query user submits a topic by
• Query engine combines PPV(ri) vectors
• Scaling Personalized Web Search (Jeh, Widom, 03)
• Decomposition, linearity
• Pre-compute PPV(ri) for unit vectors r1,,rk,
  corresponding to k10.000 pages
• Query personalization over the 10.000 pages

Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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Towards full personalization

• Our algorithm
• Monte Carlo simulation, not power iteration
• Pre-compute approximate PPV(ri) for all unit
  vectors r1,,rk, knumber of pages
• Scalability quasi linear pre-computation
  sub-linear query
• Main points of this presentation
• Outline of the algorithm
• Pre-computation external-memory, distributed
• Query used to increase precision
• Error of approximation tends to zero
  exponentially
• Exact vs. approximated PPV -- space lower bounds

Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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Outline of the Algorithm

• Theorem (Jeh, Widom 03, F 03)
• Random walk starts from page u
• Uniform step with probability 1-c, stops with c
• PPV(u,v)Pr the walk stops at page v
• Monte Carlo algorithm
• Pre-computation
• From u simulate N independent random walks
• Database of fingerprints ending vertices of the
  walks from all vertices
• Query
• PPV(u,v) ( walks u?v ) / N

Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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External memory pre-computation

• Goal N independent random walks from each vertex
• Input webgraph V 109, E 1010
• VE gt memory
• Accessing the edges
• Edge scan --- stream access
• Edges sorted by source vertices

Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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External memory pre-computation (2)

• Goal N independent random walks from each vertex
• Simulate all walks together

Iteration 1 blink 1 edge scan Sort path
ends Merge with the sorted graph Each walk stops
with prob. c E( walks ) (1-c)kNV after k
iterations
Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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Distributed indexing

• M machines with fast local network connections
• memory lt VE M(memory)

Parallelize for NV walks Parts of the graph in
RAM Remote transfers batched
M3
Heuristic partition one site to one
machine Machine1 www.cnn.com/, Machine2
www.yahoo.com/ Uniform load balance ? ordinary
PR distributed equally
Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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Query, increasing precision

• Database of NV fingerprints (path endings)
• Query PPV(u) empirical distribution
• from N samples
• Theorem (Jeh, Widom, 03)
• O(u) denotes out-neighbors of u
• Query PPV(u) empirical distribution
• from NO(u) samples
• Number of fingerprints for a query
• F N(db accesses/query)

Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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Error of approximation

• Exact PPV(u,v)
• Approximate by F fingerprints PPV(u,v)
• Theorem
• If PPV(u,v) gt PPV(u, holds, then
• Pr PPV(u,v) lt PPV(u,w) lt exp( - 0.3Nd2 )
• Idea of the proof
• N( PPV(u,v) - PPV(u,w) ) (u?v) - (u?w)
• sum of F iid. random variables with values
  -1,0,1
• Bernsteins inequality
• Error of approximation ? 0 exponentially with
• F (db size/vertex)(db accesses/query) ? 8

Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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Exact versus approximate

• Model of computation
• Input G graph with V vertices
• Pre-compute a database of size D
• Query respond by accessing only the db.
• Exact
• Query u,v,w
• Decide if PPV(u,v) gt PPV(u,w) holds
• Approximate for fixed e and d
• Query u,v,w
• Decide if PPV(u,v) gt PPV(u,w) holds with error
  probability e when PPV(u,v) - PPV(u,w) gt d

Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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Lower bounds for the db size

• For the webgraph V 109
• Theorem 1
• For the Exact problem D ?(V2) sized db is
  required in worst case
• Theorem 2
• For the Approximate problem D ?(V)
• Is it possible to improve the 2nd lower bound?
• Our algorithm uses a D O(V logV) sized db

Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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Idea of the lower bound proofs

• One-way communication complexity
• Bit-vector probing (BVP)
• Theorem B m for any protocol
• Reduction from Exact-PPV to BVP

Alice has a bit vector Input x (x1, x2, , xm
)
Bob has a number Input 1 k m Xk ?
Communication B bits
Alice has x (x1, x2, , xm ) G graph with V
vertices, where V2 m Pre-compute an Exact PPV
database of size D
Bob has 1 k m u, v, w vertices PPV(u,v) ?
PPV(u,w) Xk ?
Communication Exact PPV db, D bits
Thus D B m V2
Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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Summary

• Fully personalized PR
• Monte-Carlo method, not power iteration
• Pre-computation
• External-memory, distributed
• Query
• Increase precision by (db accesses/query)
• Error of approximation
• Tends to zero exponentially
• Space lower bounds
• Quadratic for Exact PPR
• Linear for Approximate PPR

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Thank you!
Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz
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Misc

• NPPV(u,v) (u?v) Binom(N,PPV(u,v))
• Claim (by Chernoffs bound)
• Pr PPV(u,v) gt (1d) PPV(u,v) lt
• exp(-NPPV(u,v)d2/4)
• If for a protocol Prright answer (1?) / 2
  then B ? m
• PV PageRank vector, c constant, M normalized
  adjacency matrix,

Towards Scaling Fully Personalized
PageRank Dániel Fogaras, Balázs Rácz