Using Write Mask to Support Hybrid Write-Back and Write-Through Cache Policy on Many-Core Architectures

Abstract

Write-back cache policy can greatly save bandwidth consumption for write operations.It′s particularly beneficial in many-core architecture.Normally CMP uses write-invalid or write-update cache protocol like directory based MESI which is hardly scalable and too complex.Alternatively the authors implemented scope consistency(and lock-based cache coherence protocol) on chip,add write-mask for each cacheline of L1 Dcache to record the written byte′s location and solve the false sharing problem.To further optimize the write-mask storage overhead,two methods are proposed.First the authors set store instructions with 1/2/3 bytes write-through property and let every 4-byte data has 1 bit write-mask.This method can compress the chip area of write-mask to 27.9% of origin byte-grain design.Secondly they design write mask buffer whose entry counts 12.5% of total number of Dcache blocks and compress the area overhead to 17.7% of origin without performance lost.On Godson-T 64-core platform which uses scope consistency,they use write-mask to implement hybrid WB/WT cache policy(in the scope range with possible data race we implement write-through,but out of the scope range without data race they choose write-back).Three splash2 programs and two biological programs are evaluate.The results show that performance improvement is above 24% compared to completely write-through and no performance lost under the two storage optimizations.