Snoop filter for cache coherency in a data processing system

A method of data transfer in a data processing system having a shared data resource and a network comprising a plurality of nodes, the shared data resource accessible by a plurality of request nodes of the network via a home node of the network, where a home node serves as a home for data associated with a set of data addresses where the plurality of nodes access the shared data resource via the home node, the method comprising:

• receiving, by the home node, a write-back request from a first request node of the plurality of request nodes, to store write-back data to a system address in the shared data resource;
• responsive to the write-back request, the home node:
• determining a state of the write-back data;
• when the state of the write-back data is in a ‘clean’ or ‘unique’ state or when, upon receipt of the write-back request, no data associated with the system address is stored in a cache of the home node, storing the write-back data as ‘clean’ data in the cache of the home node;
• when write-back data is already stored in the cache of the home node, dropping the write-back data without storing it in the cache of the home node;
• and
• updating a presence vector in a snoop filter of the home node;
• receiving, by the home node, a read request for data stored at the system address in the shared data resource, where the read request is sent to the home node by the first request node of the plurality of request nodes;
• and
• responsive to the read request, the home node, when the requested data is not stored in the cache of the home node but is stored in a local cache of the second request node:
• retrieving the requested data from the second request node;
• storing the cache of the home node with the returned requested data, marking the returned requested data in the cache as ‘dirty’ to indicate that the returned requested data has not been written back to the shared data resource in response to the read request and updating the snoop filter of the home node to indicate that the first request node has a copy of the requested data;
• and
• forwarding the returned requested data to the first request node.

A data processing system comprising:

• a home node configured to monitor access, by a plurality of processors of the data processing system, to a set of addresses in a shared data resource;
• and
• a snoop filter of the home node, the snoop filter comprising a snoop filter cache coupled to the home node,
• where the home node comprises a system cache and a snoop filter,
• where each processor of the plurality of processors has a local cache and a cache controller, and
• where, when data requested by a first processor of the plurality of processors is retrieved by the home node from a second processor of the plurality of processors and forwarded to the first processor, the requested data retrieved from the second processor is also stored in the system cache and marked as being in a ‘dirty’ state to indicate that the requested data has not been written back to the shared data resource in response to the data request;
• and
• where the snoop filter cache does not distinguish between a local cache that stores the block of data in a ‘SharedDirty’ state and local caches that store the block of data in a ‘SharedClean’ state.