Managing cache coherency in a data processing apparatus

A method of managing cache coherency in a data processing apparatus, the data processing apparatus comprising a plurality of processing units performing data processing operations requiring access to data in shared memory, each processing unit when access to data is required issuing an access request specifying an address in memory associated with that data, each processing unit having a cache associated therewith storing a subset of said data for access by that processing unit, said cache comprising a plurality of data arrays and a corresponding plurality of TAG arrays, each data array including a plurality of cache lines storing data values, and the corresponding TAG array having a plurality of TAG entries, where each TAG entry stores a TAG value that is associated with the data values in a corresponding cache line, the cache being arranged as a plurality of segments, where each segment comprises a plurality of said cache lines, each cache being responsive to at least a subset of the access requests issued by its associated processing unit to perform a lookup procedure involving access to at least one TAG array of the cache, each cache having indication circuitry associated therewith containing segment filtering data, the indication circuitry being separate from the TAG arrays of the cache and being responsive to an address portion of the address to reference the segment filtering data to provide, for each of at least a subset of the segments, an indication as to whether the data associated with that address is either definitely not stored in that segment or is potentially stored within that segment, the indications produced by the indication circuitry being used by the cache to affect the lookup procedure, the method comprising the steps of:

• providing cache coherency circuitry employing a cache coherency protocol configured to ensure data accessed by each processing unit is up-to-date;
• providing snoop indication circuitry associated with the cache coherency circuitry;
• deriving the content of the snoop indication circuitry from the segment filtering data of each said indication circuitry;
• for certain access requests, initiating a coherency operation during which the snoop indication circuitry is referenced to determine whether any of said caches require subjecting to a snoop operation;
• and
• for each cache for which it is determined a snoop operation should be performed, issuing a notification to that cache identifying the snoop operation to be performed,
• wherein each said notification identifies which segments of the cache are to be subjected to the snoop operation.

A data processing apparatus, comprising:

• a plurality of processing means for performing data processing operations requiring access to data in shared memory means, each processing means for issuing, when access to data is required, an access request specifying an address in memory associated with that data;
• each processing means having a cache means associated therewith for storing a subset of said data for access by that processing means, said cache means comprising a plurality of data array means and a corresponding plurality of TAG array means, each data array means including a plurality of cache lines for storing data values, and the corresponding TAG array means having a plurality of TAG entries, where each TAG entry stores a TAG value that is associated with the data values in a corresponding cache line, the cache means being arranged as a plurality of segments, where each segment comprises a plurality of said cache lines, each cache means for performing a lookup procedure in response to at least a subset of the access requests issued by its associated processing means, the lookup procedure involving access to at least one TAG array means of the cache means;
• each cache means having indication means associated therewith containing segment filtering data, the indication means being separate from the TAG arrays of the cache and for referencing, in response to an address portion of the address, the segment filtering data to provide, for each of at least a subset of the segments, an indication as to whether the data associated with that address is either definitely not stored in that segment or is potentially stored within that segment, the indications produced by the indication means being used by the cache means to affect which TAG arrays of the cache are accessed during the lookup procedure;
• and
• cache coherency means for employing a cache coherency protocol to ensure that said data accessed by each processing means is up-to-date, the cache coherency means having snoop indication means associated therewith whose content is derived from the segment filtering data of each said indication means, for certain access requests the cache coherency means initiating a coherency operation during which the snoop indication means is referenced to determine whether any of said cache means require subjecting to a snoop operation, for each cache means for which it is determined a snoop operation should be performed, the cache coherency means issuing a notification to that cache means identifying the snoop operation to be performed,
• wherein each said notification issued by the cache coherency means identifies which segments of the cache are to be subjected to the snoop operation.