Prolog: Why bother?

• As an architect why should you be concerned with parallel programming?
  • Understanding program behavior is very important in developing high-performance computers
  • An architect designs machines that will be used by the software programmers: so need to understand the needs of a program
  • Helps in making design trade-offs and cost/performance analysis i.e. what hardware feature is worth supporting and what is not
  • Normally an architect needs to have a fairly good knowledge in compilers and operating systems

Agenda

• Parallel application case studies
• Steps in writing a parallel program
• Example

Ocean current simulation

• Regular structure, scientific computing, important for weather forecast
• Want to simulate the eddy current along the walls of ocean basin over a period of time
  • Discretize the 3-D basin into 2-D horizontal grids
  • Discretize each 2-D grid into points
  • One time step involves solving the equation of motion for each grid point
  • Enough concurrency within and across grids
  • After each time step synchronize the processors

Galaxy simulation

• Simulate the interaction of many stars evolving over time
• Want to compute force between every pair of stars for each time step
  • Essentially O(n²) computations (massive parallelism)
• Hierarchical methods take advantage of square law
  • If a group of stars is far enough it is possible to approximate the group entirely by a single star at the center of mass
  • Essentially four subparts in each step: divide the galaxy into zones until further division does not improve accuracy, compute center of mass for each zone, compute force,  update star position based on force
• Lot of concurrency across stars