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Distributed Computing Seminar Lecture 4: Clustering an Overview and Sample MapReduce Implementation Christophe Bisciglia, Aaron Kimball, & Sierra Michels-Slettvet Summer 2007 Except as otherwise noted, the content of this presentation is 2007 Google Inc. and licensed under the Creative Commons Attribution 2.5 License. Outline Clustering Intuition Clustering Algorithms The Distance Measure Hierarchical vs. Partitional K-Means Clustering Complexity Clustering MapReducing a large data set with K-Means and Canopy Clustering Canopy Clustering What is clustering? Google News They didn't pick all 3,400,217 related articles by hand... Or Amazon.com Or Netflix... Other less glamorous things... Hospital Records Scientific Imaging Related genes, related stars, related sequences markets, product positioning Market Research Segmenting Social Network Analysis Data mining Image segmentation... The Distance Measure How the similarity of two elements in a set is determined, e.g. euclidean distance Manhattan Distance Inner Product Space Maximum Norm Or any metric you define over the space... Types of Algorithms hierarchical clustering vs. Partitional Clustering hierarchical clustering Builds or breaks up a hierarchy of clusters. Partitional Clustering Partitions set into all clusters simultaneously. Partitional Clustering Partitions set into all clusters simultaneously. K-Means Clustering Super simple Partitional Clustering Choose the number of clusters, k Choose k points to be cluster centers Then... K-Means Clustering iterate { Compute distance from all points to all kcenters Assign each point to the nearest k-center Compute the average of all points assigned to all specific k-centers Replace the k-centers with the new averages } But! The complexity is pretty high: k * n * O ( distance metric ) * num (iterations) Moreover, it can be necessary to send tons of data to each Mapper Node. Depending on your bandwidth and memory available, this could be impossible. Furthermore There are three big ways a data set can be large: There are a large number of elements in the set. Each element can have many features. There can be many clusters to discover Conclusion Clustering can be huge, even when you distribute it. Canopy Clustering Preliminary step to help parallelize computation. Clusters data into overlapping Canopies using super cheap distance metric. Efficient Accurate Canopy Clustering While there are unmarked points { pick a point which is not strongly marked call it a canopy center mark all points within some threshold of it as in it's canopy strongly mark all points within some stronger threshold } After the canopy clustering... Resume hierarchical or partitional clustering as usual. Treat objects in separate clusters as being at infinite distances. MapReduce Implementation: Problem Efficiently partition a large data set (say... movies with user ratings!) into a fixed number of clusters using Canopy Clustering, K-Means Clustering, and a euclidean distance measure. The Distance Metric The Canopy Metric ($) The K-Means Metric ($$$) Steps! Get Data into a form you can use (MR) Picking Canopy Centers (MR) Assign Data Points to Canopies (MR) Pick K-Means Cluster Centers K-Means algorithm (MR) Iterate! Data Massage This isn't interesting, but it has to be done. Selecting Canopy Centers Assigning Points to Canopies K-Means Map Elbow Criterion Choose a number of clusters s.t. adding a cluster doesn't add interesting information. Rule of thumb to determine what number of Clusters should be chosen. Initial assignment of cluster seeds has bearing on final model performance. Often required to run clustering several times to get maximal performance Conclusions Clustering is slick And it can be done super efficiently And in lots of different ways Tomorrow! Graph algorithms.

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