Shared-cache clusters in a system with a fully shared memory

Interest in the concept of clustered caches has been growing in recent years. The advantages of sharing data and instruction streams among two or more microprocessors are understood; however, clustering also introduces new challenges in cache and memory coherency when system design requirements indicate that two or more of these clusters are needed. This paper describes the shared L2 cache cluster design found in the S/390® G4 server. This novel cache design consists of multiple shared-cache clusters, each supporting up to three microprocessors, forming a tightly coupled symmetric multiprocessor with fully coherent caches and main memory. Because this cache provides the link between an existing S/390 system bus and the new, high-performance S/390 G4 microprocessor chips, the paper addresses the challenges unique to operating shared caches on a common system bus.