Large-Scale Machine Learning with Apache Spark

MLlib: Scalable Machine Learning on Spark

Xiangrui Meng

What is MLlib?

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What is MLlib?

MLlib is a Spark subproject providing machine learning primitives:

• initial contribution from AMPLab, UC Berkeley

• shipped with Spark since version 0.8

• 35 contributors

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What is MLlib?Algorithms:!

• classification: logistic regression, linear support vector machine (SVM), naive Bayes, classification tree

• regression: generalized linear models (GLMs), regression tree

• collaborative filtering: alternating least squares (ALS)

• clustering: k-means

• decomposition: singular value decomposition (SVD), principal component analysis (PCA)

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Why MLlib?

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scikit-learn?

Algorithms:!

• classification: SVM, nearest neighbors, random forest, …

• regression: support vector regression (SVR), ridge regression, Lasso, logistic regression, …!

• clustering: k-means, spectral clustering, …

• decomposition: PCA, non-negative matrix factorization (NMF), independent component analysis (ICA), …

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Mahout?

Algorithms:!

• classification: logistic regression, naive Bayes, random forest, …

• collaborative filtering: ALS, …

• clustering: k-means, fuzzy k-means, …

• decomposition: SVD, randomized SVD, …

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Vowpal Wabbit?H2O?

R?MATLAB?

Mahout?

Weka?scikit-learn?

LIBLINEAR?

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GraphLab?

Why MLlib?

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• It is built on Apache Spark, a fast and general engine for large-scale data processing.

• Run programs up to 100x faster than Hadoop MapReduce in memory, or 10x faster on disk.

• Write applications quickly in Java, Scala, or Python.

Why MLlib?

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Spark philosophy

Make life easy and productive for data scientists:

• Well documented, expressive API’s

• Powerful domain specific libraries

• Easy integration with storage systems

• … and caching to avoid data movement

Word count (Scala)

val counts = sc.textFile("hdfs://...") .flatMap(line => line.split(" ")) .map(word => (word, 1L)) .reduceByKey(_ + _)

Word count (SparkR)lines <- textFile(sc, "hdfs://...") words <- flatMap(lines, function(line) { unlist(strsplit(line, " ")) }) wordCount <- lapply(words, function(word) { list(word, 1L) }) counts <- reduceByKey(wordCount, "+", 2L) output <- collect(counts)

Gradient descent

val points = spark.textFile(...).map(parsePoint).cache() var w = Vector.zeros(d) for (i <- 1 to numIterations) { val gradient = points.map { p => (1 / (1 + exp(-p.y * w.dot(p.x)) - 1) * p.y * p.x ).reduce(_ + _) w -= alpha * gradient }

w w � ↵ ·nX

i=1

g(w;xi, yi)

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// Load and parse the data. val data = sc.textFile("kmeans_data.txt") val parsedData = data.map(_.split(‘ ').map(_.toDouble)).cache() !// Cluster the data into two classes using KMeans. val clusters = KMeans.train(parsedData, 2, numIterations = 20) !// Compute the sum of squared errors. val cost = clusters.computeCost(parsedData) println("Sum of squared errors = " + cost)

k-means (scala)

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k-means (python)# Load and parse the data data = sc.textFile("kmeans_data.txt") parsedData = data.map(lambda line: array([float(x) for x in line.split(' ‘)])).cache() !# Build the model (cluster the data) clusters = KMeans.train(parsedData, 2, maxIterations = 10, runs = 1, initialization_mode = "kmeans||") !# Evaluate clustering by computing the sum of squared errors def error(point): center = clusters.centers[clusters.predict(point)] return sqrt(sum([x**2 for x in (point - center)])) !cost = parsedData.map(lambda point: error(point)) .reduce(lambda x, y: x + y) print("Sum of squared error = " + str(cost))

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Dimension reduction+ k-means

// compute principal components val points: RDD[Vector] = ... val mat = RowMatrix(points) val pc = mat.computePrincipalComponents(20) !// project points to a low-dimensional space val projected = mat.multiply(pc).rows !// train a k-means model on the projected data val model = KMeans.train(projected, 10)

Collaborative filtering// Load and parse the data val data = sc.textFile("mllib/data/als/test.data") val ratings = data.map(_.split(',') match { case Array(user, item, rate) => Rating(user.toInt, item.toInt, rate.toDouble) }) !// Build the recommendation model using ALS val numIterations = 20 val model = ALS.train(ratings, 1, 20, 0.01) !// Evaluate the model on rating data val usersProducts = ratings.map { case Rating(user, product, rate) => (user, product) } val predictions = model.predict(usersProducts)

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Why MLlib?• It ships with Spark as

a standard component.

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Spark community

One of the largest open source projects in big data:

• 170+ developers contributing

• 30+ companies contributing

• 400+ discussions per month on the mailing list

30-day commit activity

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MapReduceStormYarnSpark

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Out for dinner?!

• Search for a restaurant and make a reservation.

• Start navigation.

• Food looks good? Take a photo and share.

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Why smartphone?

Out for dinner?!

• Search for a restaurant and make a reservation. (Yellow Pages?)

• Start navigation. (GPS?)

• Food looks good? Take a photo and share. (Camera?)

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Why MLlib?

A special-purpose device may be better at one aspect than a general-purpose device. But the cost of context switching is high: • different languages or APIs

• different data formats

• different tuning tricks

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Spark SQL + MLlib// Data can easily be extracted from existing sources, // such as Apache Hive. val trainingTable = sql(""" SELECT e.action, u.age, u.latitude, u.longitude FROM Users u JOIN Events e ON u.userId = e.userId""") !// Since `sql` returns an RDD, the results of the above // query can be easily used in MLlib. val training = trainingTable.map { row => val features = Vectors.dense(row(1), row(2), row(3)) LabeledPoint(row(0), features) } !val model = SVMWithSGD.train(training)

Streaming + MLlib

// collect tweets using streaming !// train a k-means model val model: KMmeansModel = ... !// apply model to filter tweets val tweets = TwitterUtils.createStream(ssc, Some(authorizations(0))) val statuses = tweets.map(_.getText) val filteredTweets = statuses.filter(t => model.predict(featurize(t)) == clusterNumber) !// print tweets within this particular cluster filteredTweets.print()

GraphX + MLlib// assemble link graph val graph = Graph(pages, links) val pageRank: RDD[(Long, Double)] = graph.staticPageRank(10).vertices !// load page labels (spam or not) and content features val labelAndFeatures: RDD[(Long, (Double, Seq((Int, Double)))] = ... val training: RDD[LabeledPoint] = labelAndFeatures.join(pageRank).map { case (id, ((label, features), pageRank)) => LabeledPoint(label, Vectors.sparse(features ++ (1000, pageRank)) } !// train a spam detector using logistic regression val model = LogisticRegressionWithSGD.train(training)

Why MLlib?• Built on Spark’s lightweight yet powerful APIs.

• Spark’s open source community.

• Seamless integration with Spark’s other components.

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• Comparable to or even better than other libraries specialized in large-scale machine learning.

Why MLlib?

• Scalability

• Performance

• User-friendly APIs

• Integration with Spark and its other components

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Logistic regression

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Logistic regression - weak scaling

• Full dataset: 200K images, 160K dense features. • Similar weak scaling. • MLlib within a factor of 2 of VW’s wall-clock time.

MLbase VW Matlab0

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n=6K, d=160Kn=12.5K, d=160Kn=25K, d=160Kn=50K, d=160Kn=100K, d=160Kn=200K, d=160K

MLlib

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n=12K, d=160Kn=25K, d=160Kn=50K, d=160Kn=100K, d=160Kn=200K, d=160K

Fig. 5: Walltime for weak scaling for logistic regression.

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Fig. 6: Weak scaling for logistic regression

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1 Machine2 Machines4 Machines8 Machines16 Machines32 Machines

Fig. 7: Walltime for strong scaling for logistic regression.

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Fig. 8: Strong scaling for logistic regression

with respect to computation. In practice, we see comparablescaling results as more machines are added.

In MATLAB, we implement gradient descent instead ofSGD, as gradient descent requires roughly the same numberof numeric operations as SGD but does not require an innerloop to pass over the data. It can thus be implemented in a’vectorized’ fashion, which leads to a significantly more favor-able runtime. Moreover, while we are not adding additionalprocessing units to MATLAB as we scale the dataset size, weshow MATLAB’s performance here as a reference for traininga model on a similarly sized dataset on a single multicoremachine.

Results: In our weak scaling experiments (Figures 5 and6), we can see that our clustered system begins to outperformMATLAB at even moderate levels of data, and while MATLABruns out of memory and cannot complete the experiment onthe 200K point dataset, our system finishes in less than 10minutes. Moreover, the highly specialized VW is on average35% faster than our system, and never twice as fast. Thesetimes do not include time spent preparing data for input inputfor VW, which was significant, but expect that they’d be aone-time cost in a fully deployed environment.

From the perspective of strong scaling (Figures 7 and 8),our solution actually outperforms VW in raw time to train amodel on a fixed dataset size when using 16 and 32 machines,and exhibits stronger scaling properties, much closer to thegold standard of linear scaling for these algorithms. We areunsure whether this is due to our simpler (broadcast/gather)communication paradigm, or some other property of the sys-tem.

System Lines of CodeMLbase 32

GraphLab 383Mahout 865

MATLAB-Mex 124MATLAB 20

TABLE II: Lines of code for various implementations of ALS

B. Collaborative Filtering: Alternating Least Squares

Matrix factorization is a technique used in recommendersystems to predict user-product associations. Let M 2 Rm⇥n

be some underlying matrix and suppose that only a smallsubset, ⌦(M), of its entries are revealed. The goal of matrixfactorization is to find low-rank matrices U 2 Rm⇥k andV 2 Rn⇥k, where k ⌧ n,m, such that M ⇡ UV

T .Commonly, U and V are estimated using the following bi-convex objective:

min

U,V

X

(i,j)2⌦(M)

(Mij � U

Ti Vj)

2+ �(||U ||2F + ||V ||2F ) . (2)

Alternating least squares (ALS) is a widely used method formatrix factorization that solves (2) by alternating betweenoptimizing U with V fixed, and V with U fixed. ALS iswell-suited for parallelism, as each row of U can be solvedindependently with V fixed, and vice-versa. With V fixed, theminimization problem for each row ui is solved with the closedform solution. where u

⇤i 2 Rk is the optimal solution for the

i

th row vector of U , V⌦i is a sub-matrix of rows vj such thatj 2 ⌦i, and Mi⌦i is a sub-vector of observed entries in the

MLlib

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Logistic regression - strong scaling

• Fixed Dataset: 50K images, 160K dense features. • MLlib exhibits better scaling properties. • MLlib is faster than VW with 16 and 32 machines.

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MLlib

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Collaborative filtering

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Collaborative filtering

• Recover a ra-ng matrix from a subset of its entries. ?

?

?

?

?

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Alternating least squares (ALS)

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ALS - wall-clock time

• Dataset: scaled version of Netflix data (9X in size). • Cluster: 9 machines. • MLlib is an order of magnitude faster than Mahout. • MLlib is within factor of 2 of GraphLab.

System Wall-‐clock /me (seconds)

Matlab 15443

Mahout 4206

GraphLab 291

MLlib 481

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Implementation

Implementation of k-means

Initialization:

• random

• k-means++

• k-means||

Iterations:

• For each point, find its closest center.

• Update cluster centers.

Implementation of k-means

li = argminj

kxi � cjk22

cj =

Pi,li=j xjPi,li=j 1

Implementation of k-meansThe points are usually sparse, but the centers are most likely to be dense. Computing the distance takes O(d) time. So the time complexity is O(n d k) per iteration. We don’t take any advantage of sparsity on the running time. However, we have

kx� ck22 = kxk22 + kck22 � 2hx, ci

Computing the inner product only needs non-zero elements. So we can cache the norms of the points and of the centers, and then only need the inner products to obtain the distances. This reduce the running time to O(nnz k + d k) per iteration. !However, is it accurate?

Implementation of ALS

• broadcast everything • data parallel • fully parallel

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Broadcast everything• Master loads (small)

data file and initializes models.

• Master broadcasts data and initial models.

• At each iteration, updated models are broadcast again.

• Works OK for small data.

• Lots of communication overhead - doesn’t scale well.

• Ships with Spark ExamplesMaster

Workers

Ratings

Movie!Factors

User!Factors

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Data parallel• Workers load data

• Master broadcasts initial models

• At each iteration, updated models are broadcast again

• Much better scaling

• Works on large datasets

• Works well for smaller models. (low K)

MasterWorkers

Ratings

Movie!Factors

User!Factors

Ratings

Ratings

Ratings

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Fully parallel• Workers load data

• Models are instantiated at workers.

• At each iteration, models are shared via join between workers.

• Much better scalability.

• Works on large datasets

MasterWorkers

RatingsMovie!FactorsUser!Factors




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Implementation of ALS

• broadcast everything • data parallel • fully parallel • block-wise parallel

• Users/products are partitioned into blocks and join is based on blocks instead of individual user/product.

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New features for v1.0

• Sparse data support

• Classification and regression tree (CART)

• Tall-and-skinny SVD and PCA

• L-BFGS

• Model evaluation

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MLlib v1.1?• Model selection!

• training multiple models in parallel

• separating problem/algorithm/parameters/model

• Learning algorithms!• Latent Dirichlet allocation (LDA)

• Random Forests

• Online updates with Spark Streaming

• Optimization algorithms!• Alternating direction method of multipliers (ADMM)

• Accelerated gradient descent

And?

Contributors

Ameet Talwalkar, Andrew Tulloch, Chen Chao, Nan Zhu, DB Tsai, Evan Sparks, Frank Dai, Ginger Smith, Henry Saputra, Holden Karau, Hossein Falaki, Jey Kottalam, Cheng Lian, Marek Kolodziej, Mark Hamstra, Martin Jaggi, Martin Weindel, Matei Zaharia, Nick Pentreath, Patrick Wendell, Prashant Sharma, Reynold Xin, Reza Zadeh, Sandy Ryza, Sean Owen, Shivaram Venkataraman, Tor Myklebust, Xiangrui Meng, Xinghao Pan, Xusen Yin, Jerry Shao, Sandeep Singh, Ryan LeCompte

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Interested?• Website: http://spark.apache.org

• Tutorials: http://ampcamp.berkeley.edu

• Spark Summit: http://spark-summit.org

• Github: https://github.com/apache/spark

• Mailing lists: [email protected] [email protected]

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http://spark.apache.org

http://ampcamp.berkeley.edu

http://spark-summit.org

https://github.com/apache/spark

mailto:[email protected]

mailto:[email protected]

Large-Scale Machine Learning with Apache Spark

Engineering

160k dense

weak scaling

factors user

strong scaling

logistic regression

updated models

naive bayes

train

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