US Patent 7774551 Hierarchical cache coherence directory structure

A method for maintaining cache coherence comprises coordinating operations among a plurality of processors distributed among a plurality of nodes coupled by an interconnect fabric and managing cache coherence in a plurality of memory directories respectively associated with the processor plurality in combination with a node controller directory cache associated with a node controller coupled between the processor plurality and the interconnect fabric. The method further comprises maintaining memory coherence directory information comprising identifying processors within a node in a first portion of bits of a memory directory entry coupled to an associated processor in the node and identifying subsets of processors external to the node in the system in a second portion of bits.