Invalidation vs. Update

• Two main classes of protocols:
  • Dictates what action should be taken on a store
  • Invalidation-based protocols invalidate sharers when a store miss appears
  • Update-based protocols update the sharer caches with new value on a store
  • Advantage of update-based protocols: sharers continue to hit in the cache while in invalidation-based protocols sharers will miss next time they try to access the line
  • Advantage of invalidation-based protocols: only store misses go on bus and subsequent stores to the same line are cache hits
• 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
• Overhead of initiating small updates
  • Invalidation-based protocols are much more popular
  • Some systems support both or maybe some hybrid based on dynamic sharing pattern of a cache line