Some Definitions

• Task
  • Arbitrary piece of sequential work
  • Concurrency is only across tasks
  • Fine-grained task vs. coarse-grained task: controls granularity of parallelism (spectrum of grain: one instruction to the whole sequential program)
• Process/thread
  • Logical entity that performs a task
  • Communication and synchronization happen between threads
• Processors
  • Physical entity on which one or more processes execute

Decomposition

• Find concurrent tasks and divide the program into tasks
  • Level or grain of concurrency needs to be decided here
  • Too many tasks: may lead to too much of overhead communicating and synchronizing between tasks
  • Too few tasks: may lead to idle processors
  • Goal: Just enough tasks to keep the processors busy
• Number of tasks may vary dynamically
  • New tasks may get created as the computation proceeds: new rays in ray tracing
  • Number of available tasks at any point in time is an upper bound on the achievable speedup