Handling a write miss

• Request opcode could be upgrade or read exclusive
  • State busy: send NACK
  • State unowned: if ReadX send cache block, change state to M, mark owner in vector; if upgrade what do you do?
  • State shared: send reply (upgrade ack or ReadX reply) with number of sharers, send invalidations to sharers, change state to M, mark owner in vector; sharers send invalidation acknowledgments to requester directly
    • What if outgoing request network queue fills up before all invalidations are sent?
  • State M: same as read miss except directory remains in PDEX state until completion message (no data) is received from owner; directory remains in M state, only the owner changes; how do you handle upgrades here?

Serializing requests

• The tricky situation is collection of invalidation acknowledgments
  • Note from previous slides that even before all acknowledgments are collected at the requester, the directory at home goes to M state with the new owner marked
  • A subsequent request will get forwarded to the new owner (at this point directory goes to PSH or PDEX state)
  • The owner is responsible for serializing the new request with the previous write
    • The write is not complete until all invalidation acknowledgments are collected
    • OTT (aka CRB) of the owner is equipped to block any incoming request until all the acknowledgments and the reply from home are collected (early interventions)
  • Note that there is no completion message back to home

Handling writebacks

• Valid directory states: M or busy; cannot be S or I
• State M
  • Just clear directory entry, write block to memory
  • Need to send writeback acknowledgment to the evicting processor (explanation coming up)
• State busy
  • How can this happen? (Late intervention race)
  • Can NACK writeback? What support needed for this?
  • Better solution: writeback forwarding
  • Any special consideration at the evicting node?
    • Drop intervention (how?)
  • How does the directory state change in this case?