# DISC-Finder: A distributed algorithm for identifying galaxy clusters - PowerPoint PPT Presentation

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DISC-Finder: A distributed algorithm for identifying galaxy clusters. Two galaxies are “friends” if they are close to each other. We analyze an undirected graph, where galaxies are vertices and their “friendships” are edges. We need to identify its connected components.

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DISC-Finder: A distributed algorithm for identifying galaxy clusters

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### DISC-Finder:A distributed algorithmfor identifying galaxy clusters

• Two galaxies are “friends” if they are close to each other

• We analyze an undirected graph, where galaxies are vertices and their “friendships” are edges

• We need to identify its connected components

### Friends-of-Friends (FoF) technique:

Identification of galaxy clusters

Sequential algorithms

• Exact: O((n∙ log n)1.5)

• Approximate: O(n)

• Divide the space into “slightly overlapping” cubes

• Randomly select a subset of galaxies

• Apply the kd-tree construction to build a balanced partition for the subset

• Use it for the full set of galaxies

• Distributed computation: Apply a sequential FoF algorithm to find the clusters within each cube

• Use any sequential FoF

• Allocate different cores to cubes

• Identify cross-cube edges and merge the respective clusters

• Apply the union-find algorithm to thegalaxies in the cube overlaps

REDUCER

MAPPER

MAPPER

MAPPER

REDUCER

REDUCER

### Distributed procedure

localclusters

galaxysets

apply localsequential FoF

divide the spaceinto cubes

• Scalable: We can apply it to massive datasets and use all available cores

• Black-box use of a sequential FoF:We can utilize any FoF algorithm

• Hadoop friendly: We have mapped all main operations into the Hadoop framework, which has resulted in very compact code (800 lines)

14,800 mlngalaxies

500 mlngalaxies

### Scalability

Time

(min)

240

SCALES TO LARGE DATA SETS

EFFECTIVELY USES AVAILBLE CORES

60

1000 mlngalaxies

15

4

8

16

32

64

256

128

Number of cores