1. 11
Recommendations for
Building Machine Learning Software
Justin Basilico
Page Algorithms Engineering May 19, 2016
@JustinBasilico

2. 22
Introduction

3. 3
Change of focus
2006 2016

4. 4
Netflix Scale
 > 81M members
 > 190 countries
 > 1000 device types
 > 3B hours/month
 > 36% of peak US
downstream traffic

5. 5
Goal
Help members find content to watch and enjoy
to maximize member satisfaction and retention

6. 6
Everything is a Recommendation
Rows
Ranking
Over 80% of what
people watch
comes from our
recommendations
Recommendations
are driven by
Machine Learning

7. 7
Machine Learning Approach
Problem
Data
AlgorithmModel
Metrics

8. 8
Models & Algorithms
 Regression (linear, logistic, elastic net)
 SVD and other Matrix Factorizations
 Factorization Machines
 Restricted Boltzmann Machines
 Deep Neural Networks
 Markov Models and Graph Algorithms
 Clustering
 Latent Dirichlet Allocation
 Gradient Boosted Decision
Trees/Random Forests
 Gaussian Processes
 …

9. 9
Design Considerations
Recommendations
• Personal
• Accurate
• Diverse
• Novel
• Fresh
Software
• Scalable
• Responsive
• Resilient
• Efficient
• Flexible

10. 10
Software Stack
http://techblog.netflix.com

11. 1111
Recommendations

12. 12
Be flexible about where and when
computation happens
Recommendation 1

13. 13
System Architecture
 Offline: Process data
 Batch learning
 Nearline: Process events
 Model evaluation
 Online learning
 Asynchronous
 Online: Process requests
 Real-time
Netflix.Hermes
Netflix.Manhattan
Nearline
Computation
Models
Online
Data Service
Offline Data
Model
training
Online
Computation
Event Distribution
User Event
Queue
Algorithm
Service
UI Client
Member
Query results
Recommendations
NEARLINE
Machine
Learning
Algorithm
Machine
Learning
Algorithm
Offline
Computation Machine
Learning
Algorithm
Play, Rate,
Browse...
OFFLINE
ONLINE
More details on Netflix Techblog

14. 14
Where to place components?
 Example: Matrix Factorization
 Offline:
 Collect sample of play data
 Run batch learning algorithm like
SGD to produce factorization
 Publish video factors
 Nearline:
 Solve user factors
 Compute user-video dot products
 Store scores in cache
 Online:
 Presentation-context filtering
 Serve recommendations
Netflix.Hermes
Netflix.Manhattan
Nearline
Computation
Models
Online
Data Service
Offline Data
Model
training
Online
Computation
Event Distribution
User Event
Queue
Algorithm
Service
UI Client
Member
Query results
Recommendations
NEARLINE
Machine
Learning
Algorithm
Machine
Learning
Algorithm
Offline
Computation Machine
Learning
Algorithm
Play, Rate,
Browse...
OFFLINE
ONLINE
V
sij=uivj Aui=b
sij
X≈UVt
X
sij>t

15. 15
Design application software for
experimentation
Recommendation 2

16. 16
Example development process
Idea Data
Offline
Modeling
(R, Python,
MATLAB, …)
Iterate
Implement in
production
system (Java,
C++, …)
Data
discrepancies
Missing post-
processing
logic
Performance
issues
Actual
output
Experimentation environment
Production environment
(A/B test) Code
discrepancies
Final
model

17. 17
Solution: Share and lean towards production
 Developing machine learning is iterative
 Need a short pipeline to rapidly try ideas
 Want to see output of complete system
 So make the application easy to experiment with
 Share components between online, nearline, and offline
 Use the real code whenever possible
 Have well-defined interfaces and formats to allow you to go
off-the-beaten-path

18. 18
Shared Engine
Avoid dual implementations
Experiment
code
Production
code
ProductionExperiment • Models
• Features
• Algorithms
• …

19. 19
Make algorithms extensible and modular
Recommendation 3

20. 20
Make algorithms and models extensible and modular
 Algorithms often need to be tailored for a
specific application
 Treating an algorithm as a black box is
limiting
 Better to make algorithms extensible and
modular to allow for customization
 Separate models and algorithms
 Many algorithms can learn the same model
(i.e. linear binary classifier)
 Many algorithms can be trained on the same
types of data
 Support composing algorithms
Data
Parameters
Data
Model
Parameters
Model
Algorithm
Vs.

21. 21
Provide building blocks
 Don’t start from scratch
 Linear algebra: Vectors, Matrices, …
 Statistics: Distributions, tests, …
 Models, features, metrics, ensembles, …
 Loss, distance, kernel, … functions
 Optimization, inference, …
 Layers, activation functions, …
 Initializers, stopping criteria, …
 …
 Domain-specific components
Build abstractions on
familiar concepts
Make the software put
them together

22. 22
Example: Tailoring Random Forests
Using Cognitive Foundry: http://github.com/algorithmfoundry/Foundry
Use a custom
tree split
Customize to
run it for an
hour
Report a
custom metric
each iteration
Inspect the
ensemble

23. 23
Describe your input and output
transformations with your model
Recommendation 4

24. 24
Application
Putting learning in an application
Feature
Encoding
Output
Decoding
?
Machine
Learned Model
Rd ⟶ Rk
Application or model code?

25. 25
Example: Simple ranking system
 High-level API: List<Video> rank(User u, List<Video> videos)
 Example model description file:
{
“type”: “ScoringRanker”,
“scorer”: {
“type”: “FeatureScorer”,
“features”: [
{“type”: “Popularity”, “days”: 10},
{“type”: “PredictedRating”}
],
“function”: {
“type”: “Linear”,
“bias”: -0.5,
“weights”: {
“popularity”: 0.2,
“predictedRating”: 1.2,
“predictedRating*popularity”:
3.5
}
}
}
Ranker
Scorer
Features
Linear function
Feature transformations

26. 26
Maximize out a single machine before
distributing your algorithms
Recommendation 5

27. 27
Problem: Your great new algorithm doesn’t scale
 Want to run your algorithm on larger data
 Temptation to go distributed
 Spark/Hadoop/etc seem to make it easy
 But building distributed versions of non-trivial ML algorithms is hard
 Often means changing the algorithm or making lots of approximations
 So try to squeeze as much out of a single machine first
 Have a lot more communication bandwidth via memory than network
 You will be surprised how far one machine can go
 Example: Amazon announced today an X1 instance type with 2TB
memory and 128 virtual CPUs

28. 28
How?
 Profile your code and think about memory
cache layout
 Small changes can have a big impact
 Example: Transposing a matrix can drop
computation from 100ms to 3ms
 Go multicore
 Algorithms like HogWild for SGD-type optimization
can make this very easy
 Use specialized resources like GPU (or TPU?)
 Only go distributed once you’ve optimized on
these dimensions (often you won’t need to)

29. 29
Example: Training Neural Networks
 Level 1: Machines in different
AWS regions
 Level 2: Machines in same AWS
region
 Simple: Grid search
 Better: Bayesian optimization using
Gaussian Processes
 Mesos, Spark, etc. for coordination
 Level 3: Highly optimized, parallel
CUDA code on GPUs

30. 30
Don’t just rely on metrics for testing
Recommendation 6

31. 31
Machine Learning and Testing
 Temptation: Use validation metrics to test software
 When things work and metrics go up this seems great
 When metrics don’t improve was it the
 code
 data
 metric
 idea
 …?

32. 32
Reality of Testing
 Machine learning code involves intricate math and logic
 Rounding issues, corner cases, …
 Is that a + or -? (The math or paper could be wrong.)
 Solution: Unit test
 Testing of metric code is especially important
 Test the whole system: Just unit testing is not enough
 At a minimum, compare output for unexpected changes across
versions

33. 3333
Conclusions

34. 34
Two ways to solve computational problems
Know
solution
Write code
Compile
code
Test code Deploy code
Know
relevant
data
Develop
algorithmic
approach
Train model
on data using
algorithm
Validate
model with
metrics
Deploy
model
Software Development
Machine Learning
(steps may involve Software Development)

35. 35
Take-aways for building machine learning software
 Building machine learning is an iterative process
 Make experimentation easy
 Take a holistic view of application where you are placing
learning
 Design your algorithms to be modular
 Optimize how your code runs on a single machine before
going distributed
 Testing can be hard but is worthwhile

36. 36
Thank You Justin Basilico
jbasilico@netflix.com
@JustinBasilico
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