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My keynote at the large-scale recommender systems workshop at Recsys 2013.

Aapo KyröläFollow

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- Large-scale Recommender Systems on Just a PC LSRS 2013 keynote (RecSys ’13 Hong Kong) Aapo Kyrölä Ph.D. candidate @ CMU http://www.cs.cmu.edu/~akyrola Twitter: @kyrpov Big Data – small machine
- My Background • Academic: 5th year Ph.D. @ Carnegie Mellon. Advisors: Guy Blelloch, Carlos Guestrin (UW) 2009 2012 + Shotgun : Parallel L1-regularized regression solver (ICML 2011). + Internships at MSR Asia (2011) and Twitter (2012) • Startup Entrepreneur Habbo : founded 2000
- Outline of this talk 1. Why single-computer computing? 2. Introduction to graph computation and GraphChi 3. Recommender systems with GraphChi 4. Future directions & Conclusion
- Large-Scale Recommender Systems on Just a PC Why on a single machine? Can’t we just use the Cloud?
- Why use a cluster? Two reasons: 1. One computer cannot handle my problem in a reasonable time. 1. I need to solve the problem very fast.
- Why use a cluster? Two reasons: 1. One computer cannot handle my problem in a reasonable time. Our work expands the space of feasible (graph) problems on one machine: - Our experiments use the same graphs, or bigger, than previous papers on distributed graph computation. (+ we can do Twitter graph on a laptop) - Most data not that “big”. 1. I need to solve the problem very fast. Our work raises the bar on required performance for a “complicated” system.
- Benefits of single machine systems Assuming it can handle your big problems… 1. Programmer productivity – Global state – Can use “real data” for development 2. Inexpensive to install, administer, less power. 3. Scalability.
- Efficient Scaling Distributed Graph System Task 7 Task 6 Task 5 Task 4 Task 3 Single-computer system (capable of big tasks) Task 2 Task 1 Task 2 Task 3 Task 4 Task 5 Task 6 Task 1 6 machines (Significantly) less than 2x throughput with 2x machines T11 T10 T9 T8 T7 T6 T5 T4 T3 T2 T1 Task 1 Exactly 2x 2 Task Task 3 throughput with 2x Task 4 machines 5 Task Task 6 Task 10 Task 11 Task 12 12 machines Time T Time T
- GRAPH COMPUTATION AND GRAPHCHI
- Why graphs for recommender systems? • Graph = matrix: edge(u,v) = M[u,v] – Note: always sparse graphs • Intuitive, human-understandable representation – Easy to visualize and explain. • Unifies collaborative filtering (typically matrix based) with recommendation in social networks. – Random walk algorithms. • Local view vertex-centric computation
- Vertex-Centric Computational Model • Graph G = (V, E) – directed edges: e = (source, destination) – each edge and vertex associated with a value (user-defined type) – vertex and edge values can be modified • (structure modification also supported) A B Data Data Data Data Data Data Data Data Data Data GraphChi – Aapo Kyrola 12
- Vertex-centric Programming • “Think like a vertex” • Popularized by the Pregel and GraphLab projects Data Data Data Data Data { // modify neighborhood } Data Data Data Data Data MyFunc(vertex)
- What is GraphChi Both in OSDI’12!
- The Main Challenge of Disk-based Graph Computation: Random Access << 5-10 M random edges / sec to achieve “reasonable performance” 100s reads/writes per sec ~ 100K reads / sec (commodity) ~ 1M reads / sec (high-end arrays)
- Details: Kyrola, Blelloch, Guestrin: “Large-scale graph computation on just a PC” (OSDI 2012) Parallel Sliding Windows or Only P large reads for each interval (sub-graph). P2 reads on one full pass.
- GraphChi Program Execution For T iterations: For p=1 to P For v in interval(p) updateFunction(v) For T iterations: For v=1 to V updateFunction(v) “Asynchronous”: updates immediately visible (vs. bulk-synchronous).
- Performance GraphChi can compute on the full Twitter follow-graph with just a standard laptop. ~ as fast as a very large Hadoop cluster! (size of the graph Fall 2013, > 20B edges [Gupta et al 2013])
- GraphChi is Open Source • C++ and Java-versions in GitHub: http://github.com/graphchi – Java-version has a Hadoop/Pig wrapper. • If you really really want to use Hadoop.
- RECSYS MODEL TRAINING WITH GRAPHCHI
- Overview of Recommender Systems for GraphChi • Collaborative Filtering toolkit (next slide) • Link prediction in large networks – Random-walk based approaches (Twitter) – Talk on Wednesday.
- GraphChi’s Collaborative Filtering Toolkit • Developed by Danny Bickson (CMU / GraphLab Inc) • Includes: – – – – – – – – Alternative Least Squares (ALS) Sparse-ALS SVD++ LibFM (factorization machines) GenSGD Item-similarity based methods PMF CliMF (contributed by Mark Levy) – …. See Danny’s blog for more information: http://bickson.blogspot.com /2012/12/collaborativefiltering-with-graphchi.html Note: In the C++ -version. Java-version in development by a CMU team.
- TWO EXAMPLES: ALS AND ITEM-BASED CF
- Example: Alternative Least Squares Matrix Factorization (ALS) • Task: Predict ratings for items (movies) by users. • Model: – Latent factor model (see next slide) Reference: Y. Zhou, D. Wilkinson, R. Schreiber, R. Pan: “Large-Scale Parallel Collaborative Filtering for the Netflix Prize” (2008)
- ALS: Product – Item bipartite graph 0.4 2.3 -1.8 2.9 1.2 4 Women on the Verge of a Nervous Breakdown 2.3 2.5 3.9 0.02 0.04 2.1 3.141 3 The Celebration 8.7 -3.2 2.8 0.9 0.2 2.9 0.04 City of God 4.1 2 Wild Strawberries 5 User’s rating of a movie modeled as a dot-product: <factor(user), factor(movie)> La Dolce Vita
- ALS: GraphChi implementation • Update function handles one vertex a time (user or movie) • For each user: – Estimate latent(user): minimize least squares of dot-product predicted ratings • GraphChi executes the update function for each vertex (in parallel), and loads edges (ratings) from disk – Latent factors in memory: need O(V) memory. – If factors don’t fit in memory, can replicate to edges. and thus store on disk Scales to very large problems!
- ALS: Performance Matrix Factorization (Alternative Least Squares) Netflix (99M edges), D=20 GraphChi (Mac Mini) GraphLab v1 (8 cores) 0 2 4 6 8 10 12 Minutes Remark: Netflix is not a big problem, but GraphChi will scale at most linearly with input size (ALS is CPU bounded, so should be sub-linear in #ratings).
- Example: Item Based-CF • Task: compute a similarity score [e,g. Jaccard] for each movie-pair that has at least one viewer in common. – Similarity(X, Y) ~ # common viewers – Output top K similar items for each item to a file. – … or: create edge between X, Y containing the similarity. • Problem: enumerating all pairs takes too much time.
- Women on the Verge of a Nervous Breakdown 3 Solution: Enumerate all The Celebration triangles of the graph. New problem: how to City of God enumerate triangles if the graph does not fit in RAM? Wild Strawberries La Dolce Vita
- Enumerating Triangles (Item-CF) • Triangles with edge (u, v) = intersection(neighbors(u), neighbors(v)) • Iterative memory efficient solution (next slide)
- Algorithm: • Let pivots be a subset of the vertices; • Load all neighbor-lists (adjacency lists) of pivots into RAM • Use now GraphChi to load all vertices from disk, one by one, and compare their adjacency lists to the pivots’ adjacency lists (similar to merge). • Repeat with a new subset of pivots. PIVOTS
- Triangle Counting Performance Triangle Counting twitter-2010 (1.5B edges) GraphChi (Mac Mini) Hadoop (1636 machines) 0 50 100 150 200 250 Minutes 300 350 400 450
- FUTURE DIRECTIONS & FINAL REMARKS
- Single-Machine Computing in Production? • GraphChi supports incremental computation with dynamic graphs: – Can keep on running indefinitely, adding new edges to the graph Constantly fresh model. – However, requires engineering – not included in the toolkit. • Compare to a cluster-based system (such as Hadoop) that needs to compute from scratch.
- Unified Recsys Platform for GraphChi? • Working with masters students at CMU. • Goal: ability to easily compare different algorithms, parameters – Unified input, output. – General programmable API (not just file-based) – Evaluation process: Several evaluation metrics; Cross-validation, held-out data… – Run many algorithm instances in parallel, on same graph. – Java. • Scalable from the get-go.
- DataDescriptor data deﬁnition column1 : categorical column2: real column3: key column4: categorical Input data Algorithm X: Input Algorithm Input Descriptor map(input: DataDescriptor) GraphChi Preprocessor aux data GraphChi Input
- aux data Disk GraphChi Input Algorithm X Training Program Held-out data (test data) Algorithm Y Training Program Algorithm X Predictor training metrics test quality metrics Algorithm Z Training Program
- Recent developments: Disk-based Graph Computation • Recently two disk-based graph computation systems published: – TurboGraph (KDD’13) – X-Stream (SOSP’13 in October) • Significantly better performance than GraphChi on many problems – Avoid preprocessing (“sharding”) – But GraphChi can do some computation that XStream cannot (triangle counting and related); TurboGraph requires SSD – Hot research area!
- Do you need GraphChi – or any system? • Heck, for many algorithms, you can just mmap() over your (binary) adjacency list / sparse matrix, and write a for-loop. – See Lin, Chau, Kang Leveraging Memory Mapping for Fast and Scalable Graph Computation on a PC (Big Data ’13) • Obviously good to have a common API – And some algos need more advanced solutions (like GraphChi, XStream, TurboGraph) Beware of the hype!
- Conclusion • Very large recommender algorithms can now be run on just your PC or laptop. – Additional performance from multi-core parallelism. – Great for productivity – scale by replicating. • In general, good single machine scalability requires care with data structures, memory management natural with C/C++, with Java (etc.) need low-level byte massaging. – Frameworks like GraphChi hide the low-level. • More work needed to ‘’productize’’ current work.
- Thank you! Aapo Kyrölä Ph.D. candidate @ CMU – soon to graduate! (Currently visiting U.W) http://www.cs.cmu.edu/~akyrola Twitter: @kyrpov

- This talk has two main goals: 1) to little bit challenge how we think about scalability: in this case, show how just a single machine, a Mac Mini, can solve very big problems – that people often use something like Hadoop for; 2) to talk about GraphChi, which is my research system and show how to implement rec sys for that.
- HOW MANY KNOW GRAPHLAB? So because of my industry experience, on working with very large systems, I always focus on very practical solutions. And it is because of this experience of working with distributed systems, that I really understand the benefits of avoiding it!
- Let me ask it otherway round. Why would you want to use a cluster?Most people do not have multi-tera or petabyte datasets.
- Let me ask it otherway round. Why would you want to use a cluster?
- This is a made-up example to illustrate a point. Relate to netflix off-line.Here we have chosen T to be the time the single machine system, such as GraphChi, solves the one task. Let’s assume the cluster system needs 6 machines to solve the problem, and does it about 7 times faster than GraphChi. Then in Time T it solves 7 tasks while GraphChi solves 6 tasks with the same cluster.Now if we double the size of the cluster, to twelve machines: cluster systems never have linear speedup, so let’s assume the performance increases by say 50%. Of course this is just fake numbers, but similar behavior happens at some cut-off point anyway. Now GraphChi will solve exactly twice the number of tasks in time T.
- We are not only ones thinking this way…Add MSR paper?
- Let’s now discuss what is the computational setting of this work. Let’s first introduce the basic computational model.
- Note about edge-centric?
- So as a recap, GraphChi is a disk-based GraphLab. While GraphLab2 is incredibly powerful on big clusters, or in the cloud, you can use GraphChi to solve as big problems on just a Mac Mini. Of course, GraphLab can solve the problems way faster – but I believe GraphChi provides performance that is more then enough for many. Spin-off of GraphLab projectDisk based GraphLabOSDI’12
- I will now briefly demonstrate why disk-based graph computation was not a trivial problem. Perhaps we can assume it wasn’t, because no such system as stated in the goals clearly existed. But it makes sense to analyze why solving the problem required a small innovation, worthy of an OSDI publication. The main problem has been stated on the slide: random access, i.e when you need to read many times from many different locations on disk, is slow. This is especially true with hard drives: seek times are several milliseconds. On SSD, random access is much faster, but still far a far cry from the performance of RAM. Let’s now study this a bit.
- So how does GraphChi work? I don’t have time to go to details now. It is based on an algorithm we invented called Parallel Sliding Windows. In this model you split the graph in to P shards, and the graph is processed in P parts. For each part you load one shard completely in to memory, and load continuous chunks of data from the other shards. All in all, you need very small number of random accesses, which are the bottleneck of disk based computing. GraphChi is good on both SSD and hard drive!
- Another, perhaps a bit surprising motivation comes from thinking about scalability in large scale.The industry wants to compute many tasks on the same graph. For example, to compute personalizedRecommendations, same task is computed for people in different countries, different interests groups, etc.Currently: you need a cluster just to compute one single task. To compute tasks faster, you grow the cluster.But this work allows a different way. Since one machine can handle one big task, you can dedicate one taskPer machine.Why does this make sense? * Clusters are complex, and expensive to scale. * while in this new model, it is very simple as nodes do not talk to each other, and you can double the throughput by doubling the machinesThere are other motivations as well, such as reducing costs and energy. But let’s move on.
- Single machine systems are easy to programBut currently need specialized solutions while if you use Hadoop etc., you can use same framework for wide variety of problems

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