1. science
2. computer science

Large-Scale Matrix Factorization
with Distributed Stochastic Gradient Descent
KDD 2011
Rainer Gemulla, Peter J. Haas, Erik Nijkamp and Yannis Sismanis
Presenter: Jiawen Yao
Dept. CSE, UT Arlington
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Outline
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Matrix Factorization
Stochastic Gradient Descent
Distributed SGD
Experiments
Summary
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Matrix completion
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Matrix Factorization
• As Web 2.0 and enterprise-cloud applications proliferate, data
mining becomes more important
• Real application
• Set of users
• Set of items (movies, books, products, …)
• Feedback (ratings, purchase, tags,…)
• Predict additional items a user may like
• Assumption: Similar feedback
Similar taste
• Matrix Factorization
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Matrix Factorization
• Example - Netflix competition
• 500k users, 20k movies, 100M movie ratings, 3M
question marks
Avatar
The Matrix
Up
Alice
?
4
2
Bob
3
2
?
Charlie
5
?
3
• The goal is to predict missing entries (denoted by ?)
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Matrix Factorization
• A general machine learning problem
• Recommender systems, text indexing, face recognition,…
• Training data
𝑉: 𝑚 × 𝑛 input matrix (e.g., rating matrix)
Z: training set of indexes in 𝑉 (e.g., subset of known ratings)
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Output
find a approximation 𝑉 ≈ 𝑊𝐻 which have the smallest loss
𝑎𝑟𝑔𝑚𝑖𝑛𝑊,𝐻 𝐿(𝑉, 𝑊, 𝐻)
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The loss function
• Loss function
– Nonzero squared loss
𝐿𝑁𝑍𝑆𝐿
𝐿𝑁𝑍𝑆𝐿 =
𝑖,𝑗:𝑉𝑖𝑗 ≠0
𝑉𝑖𝑗 − 𝑊𝐻
2
𝑖𝑗
• A sum of local losses over the entries in 𝑉𝑖𝑗
𝐿=
(𝑖,𝑗)∈𝑍
𝑙 𝑉𝑖𝑗 , 𝑊𝑖∗ , 𝐻∗𝑗
• Focus on the class of nonzero decompositions
𝑍=
𝑖, 𝑗 : 𝑉𝑖𝑗 ≠ 0
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The loss function
• Find best model
𝑎𝑟𝑔𝑚𝑖𝑛𝑊,𝐻
(𝑖,𝑗)∈𝑍
𝐿𝑖𝑗 (𝑊𝑖∗ , 𝐻∗𝑗 )
– 𝑊𝑖∗ row i of matrix W
– 𝐻∗𝑗 column j of matrix H
– To avoid trivialities, we assume there is at
least one training point in every row and in every
column.
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Prior Work
• Specialized algorithms
– Designed for a small class of loss functions
– GKL loss
• Generic algorithms
– Handle all differentiable loss functions that decompose into summation form
– Distributed gradient descent (DGD), Partitioned SGD (PSGD)
– The proposed
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Successful applications
• Movie recommendation
>12M users, >20k movies, 2.4B ratings
36GB data, 9.2GB model
• Website recommendation
51M users, 15M URLs, 1.2B clicks
17.8GB data, 161GB metadata, 49GB model
• News personalization
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Stochastic Gradient Descent
Find minimum 𝜃 ∗ of function L
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Stochastic Gradient Descent
Find minimum 𝜃 ∗ of function L
Pick a starting point 𝜃0
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Stochastic Gradient Descent
Find minimum 𝜃 ∗ of function L
Pick a starting point 𝜃0
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Stochastic Gradient Descent
Find minimum 𝜃 ∗ of function L
Pick a starting point 𝜃0
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Stochastic Gradient Descent
Find minimum 𝜃 ∗ of function L
Pick a starting point 𝜃0
Approximate gradient 𝐿′ (𝜃0 )
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Stochastic Gradient Descent
Find minimum 𝜃 ∗ of function L
Pick a starting point 𝜃0
Approximate gradient 𝐿′ (𝜃0 )
Stochastic difference equation
𝜃𝑛+1 = 𝜃𝑛 − 𝜖𝑛 𝐿′ (𝜃𝑛 )
Under certain conditions, asymptotically
approximates (continuous) gradient
descent
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Stochastic Gradient Descent for Matrix Factorization
Set 𝜃 = (𝑊, 𝐻) and use
𝐿(𝜃) =
𝐿′ (𝜃) =
(𝑖,𝑗)∈𝑍
(𝑖,𝑗)∈𝑍
𝐿𝑖𝑗 𝑊𝑖∗ , 𝐻∗𝑗
𝐿′𝑖𝑗 𝑊𝑖∗ , 𝐻∗𝑗
𝐿′ (𝜃, 𝑧) = 𝑁𝐿′𝑧 (𝑊𝑖𝑧∗ , 𝐻∗𝑗𝑧 )
Where N = 𝑍 and training point z is chosen randomly from the training set.
𝑍=
𝑖, 𝑗 : 𝑉𝑖𝑗 ≠ 0
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Stochastic Gradient Descent for Matrix Factorization
SGD for Matrix Factorization
Input: A training set Z, initial values W0 and H0
1. Pick a random entry 𝑧 ∈ 𝑍
2. Compute approximate gradient 𝐿′ (𝜃, 𝑧)
3. Update parameters
𝜃𝑛+1 = 𝜃𝑛 − 𝜖𝑛 𝐿′ (𝜃𝑛 , 𝑧)
4. Repeat N times
In practice, an additional projection is used
𝜃𝑛+1 = Π𝐻 [𝜃𝑛 − 𝜖𝑛 𝐿′ 𝜃𝑛 , 𝑧 ]
to keep the iterative in a given constraint set 𝐻 = {𝜃: 𝜃 ≥ 0}.
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Stochastic Gradient Descent for Matrix Factorization
Why stochastic is good ?
• Easy obtain
• May help in escaping local minima
• Exploit repetition with the data
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Distributed SGD (DSGD)
SGD steps depend on each other
𝜃𝑛+1 = 𝜃𝑛 − 𝜖𝑛 𝐿′ (𝜃𝑛 , 𝑧)
How to distribute?
•



Parameter mixing (ISGD)
Map: Run independent instances of SGD on subsets of the data
Reduce: Average results once at the end
Does not converge to correct solution
• Iterative Parameter mixing (PSGD)
 Map: Run independent instances of SGD on subsets of the data (for
some time)
 Reduce: Average results after each pass over the data
 Converges slowly
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Distributed SGD (DSGD)
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Stratified SGD
Proposed Stratified SGD to obtain an efficient DSGD for matrix factorization
𝐿 𝜃 = 𝜔1 𝐿1 𝜃 + 𝜔2 𝐿2 𝜃 + ⋯ + 𝜔𝑞 𝐿𝑞 𝜃
The sum of loss function 𝐿𝑠 , and 𝑠 is a stratum.
A stratum is a part or partition of dataset
SSGD runs standard SGD on a single stratum at a time, but switches
strata in a way that guarantees correctness
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Stratified SGD algorithm
Suppose a stratum sequence {𝛾𝑛 }, each 𝛾𝑛 takes values in {1, … , q}
𝜃𝑛+1 = Π𝐻 [𝜃𝑛 − 𝜖𝑛 𝐿′𝛾𝑛 𝜃𝑛 ]
Appropriate sufficient conditions
for the convergence of SSGD can
be from stochastic approximation
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Distribute SSGD
SGD steps depend on each other
𝜃𝑛+1 = Π𝐻 [𝜃𝑛 − 𝜖𝑛 𝐿′𝛾𝑛 𝜃𝑛 ]
An SGD step on example 𝑧 ∈ 𝑍
1. Reads W𝑖𝑧∗ , H∗𝑗𝑧
2. Performs gradient computation 𝐿′𝑖𝑗 𝑊𝑖𝑧∗ , 𝐻∗𝑗𝑧
3. Updates W𝑖𝑧∗ and H∗𝑗𝑧
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Problem Structure
SGD steps depend on each other
𝜃𝑛+1 = Π𝐻 [𝜃𝑛 − 𝜖𝑛 𝐿′𝛾𝑛 𝜃𝑛 ]
An SGD step on example 𝑧 ∈ 𝑍
1. Reads W𝑖𝑧∗ , H∗𝑗𝑧
2. Performs gradient computation 𝐿′𝑖𝑗 𝑊𝑖𝑧∗ , 𝐻∗𝑗𝑧
3. Updates W𝑖𝑧∗ and H∗𝑗𝑧
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Problem Structure
SGD steps depend on each other
𝜃𝑛+1 = Π𝐻 [𝜃𝑛 − 𝜖𝑛 𝐿′𝛾𝑛 𝜃𝑛 ]
An SGD step on example 𝑧 ∈ 𝑍
1. Reads W𝑖𝑧∗ , H∗𝑗𝑧
2. Performs gradient computation 𝐿′𝑖𝑗 𝑊𝑖𝑧∗ , 𝐻∗𝑗𝑧
3. Updates W𝑖𝑧∗ and H∗𝑗𝑧
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Problem Structure
SGD steps depend on each other
𝜃𝑛+1 = Π𝐻 [𝜃𝑛 − 𝜖𝑛 𝐿′𝛾𝑛 𝜃𝑛 ]
An SGD step on example 𝑧 ∈ 𝑍
1. Reads W𝑖𝑧∗ , H∗𝑗𝑧
2. Performs gradient computation 𝐿′𝑖𝑗 𝑊𝑖𝑧∗ , 𝐻∗𝑗𝑧
3. Updates W𝑖𝑧∗ and H∗𝑗𝑧
Not all steps are dependent
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Interchangeability
Definition 1. Two elements 𝑧1 , 𝑧2 ∈ 𝑍 are interchangeable if they share
neither row nor column
When𝑧𝑛 , 𝑧𝑛+1 are interchangeable, the SGD steps
𝜃𝑛+1 = 𝜃𝑛 − 𝜖𝐿′ 𝜃𝑛 , 𝑧𝑛
𝜃𝑛+2 = 𝜃𝑛 − 𝜖𝐿′ 𝜃𝑛 , 𝑧𝑛 − 𝜖𝐿′ (𝜃𝑛+1 , 𝑧𝑛+1 )
= 𝜃𝑛 − 𝜖𝐿′ 𝜃𝑛 , 𝑧𝑛 − 𝜖𝐿′ (𝜃𝑛 , 𝑧𝑛+1 )
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A simple case
Denote by 𝑍 𝑏 the set of training points in block 𝒁𝑏 . Suppose we run T
steps of SSGD on Z, starting from some initial point 𝜃0 = (𝑊0 , 𝐻0 ) and
using a fixed step size 𝜖. Describe an instance of the SGD process by a
training sequence 𝜔 = 𝑧0 , 𝑧1 , … , 𝑧𝑇−1 of T training points.
𝑇−1
𝜃𝑛+1 𝜔 = 𝜃𝑛 𝜔 + 𝜖
𝑌𝑛 (𝜔)
𝑛=0
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A simple case
𝑇−1
𝜃𝑛+1 𝜔 = 𝜃𝑛 𝜔 + 𝜖
𝑌𝑛 (𝜔)
𝑛=0
Consider the subsequence σ𝑏 (𝜔) means training points from block 𝒁𝑏 ;
the length 𝑇𝑏 𝜔 = |σ𝑏 𝜔 |.
The following theorem suggests we can run SGD on each block
independently and then sum up the results.
Theorem 3 Using the definitions above
𝑑 𝑇𝑏 𝜔 −1
𝜃𝑇 𝜔 = 𝜃0 + 𝜖
𝑌𝑘 (σ𝑏 (𝜔))
𝑏=1
𝑘=0
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A simple case
If we divide into d independent map tasks Γ1 , … Γ𝑑 . Task Γ𝑏 is responsible
for subsequence σ𝑏 (𝜔) : It takes 𝑍 𝑏 , 𝑊 𝑏 and 𝐻 𝑏 as input, performs the
block-local updates σ𝑏 (𝜔)
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The General Case
Theorem 3 can also be applied into a general case.
“d-monomial”
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The General Case
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Exploitation
Block and distribute the input matrix 𝑉
High-level approach (Map only)
1. Pick a “diagonal”
2. Run SGD on the diagonal (in parallel)
3. Merge the results
4. Move on to next “diagonal”
Steps 1-3 form a cycle
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Exploitation
Block and distribute the input matrix 𝑉
High-level approach (Map only)
1. Pick a “diagonal”
2. Run SGD on the diagonal (in parallel)
3. Merge the results
4. Move on to next “diagonal”
Steps 1-3 form a cycle
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Exploitation
Block and distribute the input matrix 𝑉
High-level approach (Map only)
1. Pick a “diagonal”
2. Run SGD on the diagonal (in parallel)
3. Merge the results
4. Move on to next “diagonal”
Steps 1-3 form a cycle
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Exploitation
Block and distribute the input matrix 𝑉
High-level approach (Map only)
1. Pick a “diagonal”
2. Run SGD on the diagonal (in parallel)
3. Merge the results
4. Move on to next “diagonal”
Steps 1-3 form a cycle
Step 2:
Simulate sequential SGD
1. Interchangeable blocks
2. Throw dice of how many iterations
per block
3. Throw dice of which step sizes
per block
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Exploitation
Block and distribute the input matrix 𝑉
High-level approach (Map only)
1. Pick a “diagonal”
2. Run SGD on the diagonal (in parallel)
3. Merge the results
4. Move on to next “diagonal”
Steps 1-3 form a cycle
Step 2:
Simulate sequential SGD
1. Interchangeable blocks
2. Throw dice of how many iterations
per block
3. Throw dice of which step sizes
per block
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Exploitation
Block and distribute the input matrix 𝑉
High-level approach (Map only)
1. Pick a “diagonal”
2. Run SGD on the diagonal (in parallel)
3. Merge the results
4. Move on to next “diagonal”
Steps 1-3 form a cycle
Step 2:
Simulate sequential SGD
1. Interchangeable blocks
2. Throw dice of how many iterations
per block
3. Throw dice of which step sizes
per block
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Experiments
• Compare with PSGD, DGD, ALS methods
• Data
Netflix Competition
100M ratings from 480k customers on 18k movies
Synthetic dataset
10M rows, 1M columns, 1B nonzero entries
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Experiments
Test on two well-known loss functions
𝐿𝑁𝑍𝑆𝐿 =
𝑖,𝑗:𝑉𝑖𝑗 ≠0
𝑉𝑖𝑗 − 𝑊𝐻
𝐿𝐿2 = 𝐿𝑁𝑍𝑆𝐿 + 𝜆 𝑊
2
𝐹
2
𝑖𝑗
+ 𝐻
2
𝐹
It works well on a variety kind of loss functions, results for other loss
functions can be found in their technique report.
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Experiments
The proposed DSGD converges faster and achieves better results than others
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Experiments
1. The processing time remains constant as the size increase
2. To very large datasets on larger clusters, the overall running time increases by
a modest 30%
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Summary
• Matrix factorization
• Widely applicable via customized loss functions
• Large instances (millions * millions with billions of
entries)
• Distributed Stochastic Gradient Descent
• Simple and versatile
• Achieves
• Fully distributed data/model
• Fully distributed processing
• Same or better loss
• Faster
• Good scalability
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Thank you!
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