Selective cache-to-cache lateral castouts

A method of data processing in a data processing system including a plurality of processing units including first and second processing units and a system memory coupled by an interconnect fabric, wherein the first processing unit has a first processor core and associated first upper and first lower level caches and the second processing unit has a second processor core and associated second upper and lower level caches, said method comprising:

• in a cache directory of the first lower level cache, maintaining a chronology vector indicating a chronological access order of a plurality of cache lines in a congruence class and maintaining a plurality of confidence indicators each associated with a respective one of the plurality of cache lines in the congruence class;
• for each of the plurality of cache lines in the congruence class, setting the respective associated one of the plurality of confidence indicators to indicate a lateral castout (LCO) in response to the first processor core accessing the associated cache line while the associated cache line has a position in the chronological access order at least as recent as a predetermined position;
• in response to a data request:
• selecting, from among the plurality of cache lines in the congruence class of the first lower level cache, a victim cache line to be castout from the congruence class;
• the first lower level cache deciding, based upon the confidence indicator associated with the victim cache line, between performing a lateral castout (LCO) of the victim cache line to another lower level cache and a castout of the victim cache line to the system memory;
• in response to the first lower level cache deciding to perform the LCO:
• the first processing unit issuing a lateral castout (LCO) command on the interconnect fabric and removing the victim cache line from the first lower level cache;
• and
• the second lower level cache holding the victim cache line.