Which One is Better?

• Difficult to answer
  • Depends on program behavior and hardware cost
• When is update-based protocol good?
  • What sharing pattern? (large-scale producer/consumer)
  • Otherwise it would just waste bus bandwidth doing useless updates
• When is invalidation-protocol good?
  • Sequence of multiple writes to a cache line
  • Saves intermediate write transactions
• Also think about the overhead of initiating small updates for every write in update protocols
  • Invalidation-based protocols are much more popular
  • Some systems support both or maybe some hybrid based on dynamic sharing pattern of a cache line

MSI Protocol

• Forms the foundation of invalidation-based writeback protocols
  • Assumes only three supported cache line states: I, S, and M
  • There may be multiple processors caching a line in S state
  • There must be exactly one processor caching a line in M state and it is the owner of the line
  • If none of the caches have the line, memory must have the most up-to-date copy of the line
• Processor requests to cache: PrRd , PrWr
• Bus transactions: BusRd , BusRdX , BusUpgr , BusWB