Synchronization Types

• Mutual exclusion
  • Synchronize entry into critical sections
  • Normally done with locks
• Point-to-point synchronization
  • Tell a set of processors (normally set cardinality is one) that they can proceed
  • Normally done with flags
• Global synchronization
  • Bring every processor to sync
  • Wait at a point until everyone is there
  • Normally done with barriers

Synchronization

• Normally a two-part process: acquire and release; acquire can be broken into two parts: intent and wait
  • Intent: express intent to synchronize (i.e. contend for  the lock, arrive at a barrier)
  • Wait: wait for your turn to synchronization (i.e. wait until you get the lock)
  • Release: proceed past synchronization and enable other contenders to synchronize
  • Waiting algorithms do not depend on the type of synchronization

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