Question: Some applications read a large data set first, then modify most or all of it. The base MSI coherence protocol will first fetch all of

Some applications read a large data set first, then modify most or all of it. The base MSI coherence protocol will first fetch all of the cache blocks in the Shared state, and then be forced to perform an invalidate operation to upgrade them to the Modified state. The additional delay has a significant impact on some workloads. An additional protocol optimization eliminates the need to upgrade blocks that are read and later written by a single processor. This optimization adds the Exclusive (E) state to the protocol, indicating that no other node has a copy of the block, but it has not yet been modified. A cache block enters the Exclusive state when a read miss is satisfied by memory and no other node has a valid copy. CPU reads and writes to that block proceed with no further bus traffic, but CPU writes cause the coherence state to transition to Modified. Exclusive differs from Modified because the node may silently replace Exclusive blocks (while Modified blocks must be written back to memory). Also, a read miss to an Exclusive block results in a transition to Shared, but does not require the node to respond with data (since memory has an up-to-date copy). Draw new protocol diagrams for a MESI protocol that adds the Exclusive state and transitions to the base MSI protocol's Modified, Shared, and Invalidate states.

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