A Look into Intel Xeon 6's Memory Subsystem

Summary

Intel’s server dominance has been shaken by high core count competition from the likes of AMD and Arm. Xeon 6, Intel’s latest server platform, aims to address this with a more scalable chiplet strategy. Chiplets are now arranged side by side, with IO chiplets on either side of the chip. It’s reminiscent of the arrangement Huawei used for Kunpeng 920, but apparently without the uniform chiplet height restrictions of Huawei’s design. Intel also scales up to three compute dies, as opposed to a maximum of two on Kunpeng 920.Compared to Emerald Rapids and Granite Rapids, Xeon 6 uses a more aggressive chiplet strategy. Lower speed IO and accelerators get moved to separate IO dies. Compute dies contain just cores and DRAM controllers, and use the advanced Intel 3 process. The largest Xeon 6 SKUs incorporate three compute dies and two IO dies, scaling up to 128 cores per socket. It’s a huge core count increase from the two prior generations.AWS has Xeon 6 instances generally available with their r8i virtual machine type, providing an opportunity to check out Intel’s latest chiplet design from a software performance perspective. This will be a short look. Renting a large cloud instance for more detailed testing is prohibitively expensive.AWS’s r8i instance uses the Xeon 6 6985P-C. This SKU is not listed on Intel’s site or other documentation, so a brief overview of the chip is in order. The Xeon 6 6985P-C has 96 Redwood Cove cores that clock up to 3.9 GHz, and each have 2 MB of L2 cache. Redwood Cove is a tweaked version of Intel’s Golden Cove/Raptor Cove, and has previously featured on Intel’s Meteor Lake client platform. Redwood Cove brings a larger 64 KB L1 instruction cache among other improvements discussed in another article. Unlike their Meteor Lake counterparts, Xeon 6’s Redwood Cove cores enjoy AVX-512 support with 2x 512-bit FMA units, and 2x512-bit load + 1x512-bit store throughput to the L1 data cache. AMX support is present as well, providing specialized matrix ...