Memory consistency #

• Definition of consistency: allowed behavior of loads and stores to different addresses in parallel system

Memory operation ordering #

• Program defines sequence of loads and stores
• Four types of orderings:
  • Wx -> Ry: write to X must commit before subsequent read from Y
  • Rx -> Ry: read from X must commit before subsequent read from Y
  • Rx -> Wy: read from X must commit before subsequent write to Y
  • Wx -> Wy: write to X must commit before subsequent write to Y

Sequential Consistency #

• All ops executed in some sequential orders
  • As if manipulation if single shared memory
• Each thread’s operations happen in program order
  • Maintains all four memory operation orderings

Relaxed consistency #

• Allow certain orderings to be violated
• Allows us to hide memory latency: overlap memory access operations with other operations when they are independent
  • Mem. access in cache coherent system may require more work than just reading bits from memory