1 Bulk Synchronous Parallel Computing Trevor Schaub Jim Sellers This presentation was prepared for Professor Stefan Dobrev in partial fulfillment of the requirements for the course CSI 4140

2 Outline Model description Why it is interesting/important Relationship to real life Some basic techniques, simple sample program to demonstrate how the approach to algorithm design differs from PRAM and MPP

3 Why do we need change? Diverse parallel computers and models Shared mem. vs. message passing, no clear winner Use network locality vs. don’t Conclusion: parallel code is often computer specific. Need unifying model

4 Why BSP? Provides consistent, general framework to develop scaleable software Similar to OO – a way of thinking and writing programs (paradigm) Not system specific Proceed in “supersteps” Decouples communication and synchronization The deadlock problem is minimized

5 What is BSP? BSP is a process of dividing a task into steps called supersteps Each process performs the task by doing the supersteps in order The fundamental concept is the superstep itself, and it’s structure

6 What’s a superstep? 1. Processes perform as much as possible using local data 2. Communication and synchronization 3. Move on to next superstep superstep …

7 Dissection of a superstep Local computation Global Communication Synchronization Threads/Processes TIMETIME

8 Model description What are the benefits of a superstep? Easier debugging Can look at supersteps in isolation Removes problem of deadlock Separate computation from communication Allows reasoning of the correctness of the code nearly as easy as sequential code. Easier shift for programmers used to sequential programming

9 Relationship to real life Is BSP in use today? Yes, sometimes without even realizing it. When should you use BSP? Becomes more useful with more communication Benefits vary with the problem

10 Basic techniques Identify the sections that can be that need communication or synchronization Modify program so that sync. happens after communication Separate ones are grouped where possible Before each communication group there is a global barrier The code between the end of each communication group becomes a superstep

11 Example: See code examplecode example

12 References COP5937.html ting.html s12.pdf ode2.html

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