I previously posted that for modern AMD GPUs like Tonga and Polaris that have a 256-bit memory interface, the minimum core/clock ratio for eth mining is ~0.56. After digging through lots of GCN architecture documents for my analysis of ZEC mining, I have determined that the fundamental limit for fully utilizing the GDDR5 memory is 0.5. This means on a Rx 480 with memory clocked at 2Ghz, the core has to be at least 1Ghz in order to fully utilize the GDDR5 memory bandwidth. The reason is that the L2 cache can transfer at most 64 bytes per core clock. GDDR5 transfers 4 bits per clock cycle in a 2 cycle burst. The GDDR memory interface is 32-bits wide, so each chip xfers 32 bytes per 2 clocks. Tonga and Polaris use 8 GGDR5 chips for 8 x 32 = 256 bits on the external memory bus.
