Selective cache-to-cache lateral castouts

摘要

A data processing system includes first and second processing units and a system memory. The first processing unit has first upper and first lower level caches, and the second processing unit has second upper and lower level caches. In response to a data request, a victim cache line to be castout from the first lower level cache is selected, and the first lower level cache selects between performing a lateral castout (LCO) of the victim cache line to the second lower level cache and a castout of the victim cache line to the system memory based upon a confidence indicator associated with the victim cache line. In response to selecting an LCO, the first processing unit issues an LCO command on the interconnect fabric and removes the victim cache line from the first lower level cache, and the second lower level cache holds the victim cache line.

主权项

1. 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; andthe second lower level cache holding the victim cache line.