Flushing of cache memory in a computer system

Abstract

An efficient streamlined cache coherent protocol for replacing data is provided in a multiprocessor distributed-memory computer system. In one implementation, the computer system includes a plurality of subsystems, each subsystem includes at least one processor and an associated cache and directory. The subsystems are coupled to a global interconnect via global interfaces. In one embodiment, when data is replaced from a requesting subsystem, an asynchronous flush operation is initiated. In this implementation, the flush operation includes a pair of decoupled local flush instruction and corresponding global flush instruction. By decoupling the local flush instructions from the global flush instructions, once the requesting processor in the requesting subsystem is done issuing the local flush instruction, the requesting processor does not have to wait for a corresponding response from home location associated with the data being replaced. Instead, the requesting processor is freed up quickly since there is no need to wait for an acknowledgment from the home location (home subsystem) over the global interconnect. The home subsystem responds with an appropriate ACK message. The requesting subsystem reissues a read-to-own (RTO) transaction on its local interconnect thereby retrieving and invalidating any copy(s) of the data in the requesting subsystem. A Completion message is sent to the home subsystem together with the dirty data. Subsequently, a confirmation of the completion of the flush operation can be implemented using a 'synchronization' mechanism to verify that all previously valid cache lines associated with a page have been successfully replaced with respect to their home location and the replaced cache lines are now marked 'invalid' at the home subsystem.