Coherence and Replacement Protocol of DICE - A Bus-Based COMA Multiprocessor

As microprocessors become faster and demand more bandwidth, the already limited scalability of a shared bus decreases even further. DICE, a shared-bus multiprocessor, utilizes cache only memory architecture (COMA) to effectively decrease the speed gap between modern high-performance microprocessors and the bus. DICE tries to optimize COMA for a shared-bus medium, in particular to reduce the detrimental effects of cache coherence and the "last memory block" problem on replacement. In this paper, we present the coherence and replacement protocol of the DICE multiprocessor and its design trade-offs. We describe a four-state write-invalidate coherence protocol in detail. Replacement, which poses a unique overhead problem of COMA, requires that a victim block with ownership be relocated to a remote node in order not to discard the last cached memory block. We show that the relocation process can be efficiently implemented by using a temporary storage called relocation buffer and a priority-based selection algorithm. We present performance results that show a drastic reduction in global bus traffic compared to a traditional shared-bus multiprocessor architecture.