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MOESI Protocol
- State transitions pertaining to O state
- I to O: Not possible
- E to O or S to O: Not possible
- M to O: On a BusRd /Flush (but no memory writeback )
- O to I: On CacheEvict / BusWB or { BusRdX,BusUpgr }/Flush ]
- O to S: Not possible
- O to E: Not possible
- O to M: On PrWr / BusUpgr
- At most one cache can have a line in O state at any point in time
- Two main design choices for MOESI
- Consider the example: P0 reads x, P0 writes x, P1 reads x, P2 reads x, P3 reads x, …
- When P1 launches BusRd , P0 sources the line and now the protocol has two options: 1. The line in P0 goes to O and the line in P1 is filled in state S; 2. The line in P0 goes to S and the line in P1 is filled in state O i.e. P1 inherits ownership from P0
- For distributed shared memory, the second choice is better
- According to the second choice, when P2 generates a BusRd request, P1 sources the line and transitions from O to S; P2 becomes the new owner
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