Waiting Algorithms

• Busy wait (common in multiprocessors)
  • Waiting processes repeatedly poll a location (implemented as a load in a loop)
  • Releasing process sets the location appropriately
  • May cause network or bus transactions
• Block
  • Waiting processes are de-scheduled
  • Frees up processor cycles for doing something else
• Busy waiting is better if
  • De-scheduling and re-scheduling take longer than busy waiting
  • No other active process
  • Does not work for single processor
• Hybrid policies: busy wait for some time and then block

Implementation

• Popular trend
  • Architects offer some simple atomic primitives
  • Library writers use these primitives to implement synchronization algorithms
  • Normally hardware primitives for acquire and possibly release are provided
  • Hard to offer hardware solutions for waiting
  • Also hardwired waiting may not offer that much of flexibility