Handling write miss

• CASE A: requester is in HEAD_DIRTY state already
  • Directory must be in GONE state
  • Only need to invalidate sharers
  • Requester sends an invalidation to the next sharer
  • A sharer upon receiving an invalidation sends a roll-out request to its next sharer (unless TAIL); the receiving node sets its upstream pointer properly and sends a roll-out acknowledgment
  • Eventually roll-out request is acknowledged, the sharer invalidates its RAC line and sends a reply back to head with the id of the next sharer
  • Head moves on to purge the sharer with received id
  • During the entire process requester’s RAC line remains in PENDING state
  • Note that home is not at all involved here
• CASE B: requester is in ONLY_DIRTY state
  • No transaction needed
• CASE C: requester is in HEAD_FRESH state
  • Send state change request to home (FRESH to GONE)
  • Once acknowledgment from home is received list purging can be started
  • What if the home is in a state different from FRESH with a different head node?
    • The only case in SCI when a NACK is generated
    • The requester on receiving the NACK changes its state to PENDING and initiates a new write request to home for transitioning to ONLY_DIRTY
• CASE D: requester in MID_FRESH or TAIL_FRESH state
  • First it must roll out from the list and attach itself to the head in HEAD_FRESH state (recall that only the head node can write)
  • This roll-out may require acknowledgments from upstream and downstream neighbors (if MID) or just the upstream neighbor (if TAIL)
  • Follow CASE C
• CASE E: requester not a sharer
  • First get the block in HEAD_DIRTY state
  • Follow CASE A