Implementations

• Must invalidate all cached copies before allowing a store to proceed
  • Need to know where the cached copies are
  • Solution1: Never mind! Just tell everyone that you are going to do a store
    • Leads to broadcast snoopy protocols
    • Popular with small-scale bus-based CMPs and SMPs
    • AMD Opteron implements it on a distributed network (the Hammer protocol)
• Solution2: Keep track of the sharers and invalidate them when needed
  • Where and how is this information stored?
  • Leads to directory-based scalable protocols
• Directory-based protocols
  • Maintain one directory entry per memory block
  • Each directory entry contains a sharer bitvector and state bits
  • Concept of home node in distributed shared memory multiprocessors
  • Concept of sparse directory for on-chip coherence in CMPs