1. Data Structures and Algorithms in Parallel Computing
Lecture 5

2. BSP Processors + network + synchronization Superstep
Concurrent parallel computation
Message exchanges between processors
Barrier synchronization
All processors reaching this
point wait for the rest

3. Supersteps A BSP algorithm is a sequence of supersteps
Computation superstep
Many small steps
Example: floating point operations (addition, subtraction, etc.)
Communication superstep
Communication operations each transmitting a data word
Example: transfer a real number between 2 processors
In theory we distinguish between the 2 types of supersteps
In practice we assume a single superstep

4. Some applications Pagerank Single Source Shortest Path (SSSP)
Connected Components

5. Pagerank Analysis algorithm to determine the importance of a document
Based on the number of references to it and the importance of the source documents
Named after Larry Page

6. Pagerank

7. Pagerank
Source: wikipedia

8. Solving Pagerank System of linear equations
Iterative loop till convergence

9. Pagerank in Pregel

10. Experimental results On Apache Giraph
Taken from

11. SSSP
Find shortest path between a single source vertex and every other vertex in the graph
Dijsktra’s algorithm for sequential computations

12. Sequential SSSP
Source: wikipedia

13. SSSP in Pregel

14. Experimental results
Binary trees

15. Connected components (recap)
Label 2 vertices with same label iff there is a path between the two
Sequentially it can be achieved by depth first or breadth first search

16. CC in Pregel Use graph contraction Algorithm
Each vertex starts with a label
Each vertex sends its label to all neighbors
Each vertex replaces its label with the minimum (maximum) value it receives from neighbors
Algorithm stops when convergence is achieved

17. Experimental results

18. Apache Giraph Pregel is proprietary
Giraph is an open source Pregel implementation
Runs on standard Hadoop
Computation is executed in memory
Can be a job in a pipeline (MapReduce)
Uses Zookeeper for synchronization

19. Building an application
Create a custom vertex by extending BasicVertex
Create a custom input format
Adjacency list where each line looks like
vertexID neighborID1 neighborID2 …
Extend the TextVertexInputFormat
Create a custom output format
Extend the TextVertexOutputFormat

20. What’s next? Vertex centric vs. subgraph centric Load balancing ...
Importance of partitioning and graph type
...