Sunday, November 25th 2018

AMD Doubles L3 Cache Per CCX with Zen 2 "Rome"

A SiSoft SANDRA results database entry for a 2P AMD "Rome" EPYC machine sheds light on the lower cache hierarchy. Each 64-core EPYC "Rome" processor is made up of eight 7 nm 8-core "Zen 2" CPU chiplets, which converge at a 14 nm I/O controller die, which handles memory and PCIe connectivity of the processor. The result mentions cache hierarchy, with 512 KB dedicated L2 cache per core, and "16 x 16 MB L3." Like CPU-Z, SANDRA has the ability to see L3 cache by arrangement. For the Ryzen 7 2700X, it reads the L3 cache as "2 x 8 MB L3," corresponding to the per-CCX L3 cache amount of 8 MB.

For each 64-core "Rome" processor, there are a total of 8 chiplets. With SANDRA detecting "16 x 16 MB L3" for 64-core "Rome," it becomes highly likely that each of the 8-core chiplets features two 16 MB L3 cache slices, and that its 8 cores are split into two quad-core CCX units with 16 MB L3 cache, each. This doubling in L3 cache per CCX could help the processors cushion data transfers between the chiplet and the I/O die better. This becomes particularly important since the I/O die controls memory with its monolithic 8-channel DDR4 memory controller.
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