Early return indication for read exclusive requests in shared memory architecture
View Patent ↗An early return indication is used to notify a first communications interface, prior to a response being received from any of a plurality of sources coupled to a second communications interface, that the return data can be used by the first communications interface when it is received thereby from a source of the return data if the source has an exclusive copy of the return data. By doing so, the first communications interface can often prepare for forwarding the return data over its associated communication link such that the data can be forwarded with little or no latency once the data is retrieved from its source, and may be able to initiate the return of data over the communication link prior to all responses being received from the other sources. The early return indication may also serves as an early coherency indication in that the first communications interface is no longer required to wait for updating of a coherency directory to complete prior to forwarding the return data over the communication link.
1. A method of processing a request for data in a data processing system of the type including a first communications interface configured to be coupled to at least one requester over a first communications link and a second communications interface configured to be coupled to a plurality of sources over a second communications link, the method comprising, in response to a request by the requester:
forwarding the request to the plurality of sources over the second communications interface;
tracking responses to the request from each of the plurality of sources; and
prior to receiving a response from any of the plurality of sources, indicating to the first communications interface that return data received from one of the sources in response to the request can be used by the first communications interface if the source has an exclusive copy of the return data.
2. The method of claim 1 , further comprising:
accessing coherency information stored in a coherency directory to identify the source of return data for the request; and
updating the coherency directory based upon the request;
wherein indicating to the first communications interface that the return data can be used by the first communications interface is performed prior to updating the coherency directory.
3. The method of claim 1 , wherein the first communications interface comprises a processor bus interface configured to be coupled to at least one processor over a processor bus, wherein the request is generated by the processor and received by the processor bus interface, wherein the first and second communications interfaces and the control logic are disposed in a first node in a multi-node data processing system, and wherein the second communications interface comprises an internodal interface configured to coupled the first node to a second node.
4. The method of claim 1 , further comprising, in the first communications interface, preparing for communication of the return data over the communications link in response to the indication, and prior to the return data being available to the communications interface, by arbitrating for the communications link to line up the return data.
5. The method of claim 1 , wherein the indication to the communications interface comprises a conditional early coherence indicator.
6. The method of claim 1 , further comprising, in the first communications interface, after the indication is made that return data received from one of the sources in response to the request can be used by the first communications interface if the source has an exclusive copy of the return data, determining whether the return data received from the source indicates that the source has an exclusive copy of the return data.
7. The method of claim 1 , further comprising, in the first communications interface, initiating communication of the return data over the first communications link when received thereby from the source of the return data over the second communications interface prior to responses being received from all of the plurality of sources if a determination is made that the source has an exclusive copy of the return data.
8. The method of claim 7 , further comprising waiting until responses have been received from all sources if a determination is made that the source does not have an exclusive copy of the return data.