Thread-level Parallelism

• Look for concurrency at a granularity coarser than instructions
  • Put a chunk of consecutive instructions together and call it a thread (largely wrong!)
  • Each thread can be seen as a “dynamic” subgraph of the sequential control-flow graph: take a loop and unroll its graph
  • The edges spanning the subgraphs represent data dependence across threads (the control dependence edges are usually converted to data dependence edges through suitable transformations)
    • The goal of parallelization is to minimize such edges
    • Threads should mostly compute independently on different cores; but need to talk once in a while to get things done!

• Parallelizing sequential programs is fun, but often tedious for non-experts
  • So look for parallelism at even coarser grain
  • Run multiple independent programs simultaneously
    • Known as multi-programming
    • The biggest reason why quotidian Windows fans would buy small-scale multiprocessors and multi-core today
    • Can play games while running heavy-weight simulations and downloading movies
    • Have you seen the state of the poor machine when running anti-virus?