Lenovo Has a CXL Memory Monster with 128x 128GB DDR5 DIMMs

At OCP Summit 2024, we saw one more CXL monster, the Lenovo ThinkSystem SR860 V3. This system is listed on Lenovo’s website as having “Up to 16TB of TruDDR5 memory in 64x slots.” At the OCP Summit 2024, we saw this beast and how Lenovo is doing it with the help of CXL and Astera Labs Leo.

The idea of having a four-socket server is not new. A challenge, however, is that with a 8 channel Intel Xeon, you are limited by the number of DDR5 DIMM slots. Each 8-channel Xeon supports 2 DIMMs Per Channel (2DPC) for 16 DIMMs per socket. Four sockets gives us 64 DDR5 SODIMMs. Filling those with 128GB DDR5 RDIMMs only gets one to a paltry 8TB of memory. For some, a large consumer SSD-sized memory capacity is simply not enough.

With this option installed, the top part of the chassis becomes a giant memory forest. The four CPUs sit on the motherboard at the bottom each with 16 DDR5 DIMM slots. Above that, is this CXL memory forest with another 64 DIMM slots.

The 64 DIMMs are connected directly to the Xeon CPUs, and then 64 additional DIMM slots are connected via CXL memory expansion. That gives us 128 DIMM slots. With 128GB RDIMMs, that is 16TB of memory capacity.

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