Objectives_template

	Directory overhead Directory overhead Need 6 bits to maintain the head node id NUMA-Q scales up to 64 nodes Need 2 bits for encoding three states: HOME, FRESH, GONE A system with P nodes, M bytes of memory, and cache block size of B bytes has M/B cache blocks per node 2 + log(P) bits needed for directory entry per cache block Total overhead = (M/B)(log (P) + O(1))P O(Plog(P)) Cache overhead Extended RAC tags for storing upstream and downstream pointers 2log(P) per cache block Total increased tag DRAM area is O(P*log(P)) Handling read miss Requester on missing the RAC as well as quad snoop sends a read request to home Allocates a block in RAC and marks its state PENDING CASE A: directory is HOME state Change directory state to FRESH Change head pointer to requester id Send reply to requester Requester fills cache block in RAC, forwards it to requesting processor, changes RAC block state to ONLY_FRESH CASE B: directory state is FRESH Home changes head pointer to requester id Sends reply with data read from memory and the old head node id Requester sends a request to the previous head expressing intention to become the new head Old head changes its upstream pointer to point to the requester and the RAC state to MID_VALID or TAIL_VALID; sends an acknowledgment to requester Requester changes its downstream pointer to old head and upstream pointer to home; also changes RAC line state to HEAD_FRESH Observe the strict request-reply nature of the protocol CASE C: directory state is GONE Means head node has an exclusive copy of the cache line Home replies to the requester with the head node id, but does not change the state of the directory Requester sets RAC line state to PENDING and sends a data request to the head node Old head changes RAC line state to TAIL_VALID, sets its upstream pointer to the requester, and sends data to requester Requester sets RAC line state to HEAD_DIRTY, sets its upstream pointer to home and downstream pointer to old head Note that directory remains in GONE state and memory is not updated (similar to an M to O transition) Handling races Suppose when the requester’s (say A) message reaches the old head (say B) the RAC line is in PENDING state SCI doesn’t have any pending state in directory or doesn’t use NACKs (actually uses, but small in number) B does become the new head (has to because the home has already updated the directory), but inherits the PENDING state from A Any subsequent request will come to B and will become the new pending head Ultimately the PENDING state is resolved along the chain starting from A upstream FIFO nature of the pending list guarantees fairness Also, no problem related to sizing the buffers for holding pending requests (no extra space needed