A Shared-Forwarding State Based Multiple-Tier Cache Coherency Protocol

In CC-NUMA architecture system, in order to reduce the overhead of cache coherency maintenance, large scale CC-NUMA system usually employs multi-tier cache coherency domain method, so as to effectively alleviate the contradictions between system scalability and coherency maintenance overhead. The traditional MESI, MESIF, MOESI protocols mainly aim at the single tier coherency domain, and do not take into account the characteristic that query business accounts for dominant in the large database applications, therefore there are many problems such as high frequency of cross domain operations, low execution efficiency and so on when the protocols are used in multi-tier cache coherency domain. To address the above problems, this paper presents a multi-tier cache coherency protocol called MESI-SF based on the shared-forwarding state. The protocol creates a shared-forwarding state called Share-F, and thus there exists remote data copy with readable forwarding state in multiple coherency domains at the same time. In this way, within the same domain data copy with shared-forwarding state can directly response to read requests, and thus the protocol can effectively reduce the number of cross domain operations and enhance the system performance. Experimental simulation with SPLASH-2 test suites shows that, for the two tiers cache coherency domain system, compared with the MESI protocol, MESI-SF can reduce 23.0% visits that cross the clumps, and the average instruction execution cycle is reduced by 7.5%; compared with the MESIF protocol, MESI-SF can reduce 12.2% visits that cross the clumps, and the average cycles per instruction (CPI) is reduced by 5.95%.