EPYC Genoa adds support for two DDR5 modules per channel

EPYC Genoa adds support for two DDR5 modules per channel – up to 6TB of memory per socket

At startup server processors EPYC Genoa supports the configuration of the memory subsystem with only one module per channel (1DPC – DIMM Per Channel). Support for two DIMMs per channel (2DPC) was hampered by a bug in the chip’s memory controller. AMD announced today that all EPYC Genoa processors will soon receive support for the 2DPC memory configuration – the company has already sent corresponding BIOS updates to motherboard developers.

    Image source: AMD

Image source: AMD

AMD’s new server processors offer market-leading performance and feature several advanced interfaces, including 12 DDR5 memory channels. However, EPYC Genoa was only introduced with support for DDR5 memory in a single DIMM per channel configuration, meaning that only one DDR5 module can be plugged into each of the twelve memory channels in the processor.

When introducing new processors, AMD promised to release a software update with support for 2DPC in the first quarter of 2023. With 2DPC mode, you can double the capacity of the storage subsystem and slightly increase its speed in certain scenarios and configurations. Following reports of an alleged memory subsystem failure in Genoa processors, some analysts speculated that AMD would need to resume production and upgrade existing chips.

AMD today officially announced that 2DPC support does not require a factory reset or chip modification, just a BIOS update. The company has already provided its OEM partners and board manufacturers with everything needed to create new BIOS versions. Motherboards with enough slots to support this feature are already in the design phase.

Support for 12 DDR5 channels by EPYC Genoa processors is a record for x86 processors. That’s one and a half times more than the 8 channels of the competing Intel Sapphire Rapids processor, while in the 1DPC configuration both processors support DDR5 with a nominal frequency of 4800MHz. For the 2DPC configuration, Intel specifies DDR5-4400 modules in its specification, and AMD has not yet announced the maximum DDR5 frequency for 2DPC mode.

AMD’s decision to launch EPYC Genoa without 2DPC support in the first phase makes sense as demand for 2DPC configurations is expected to be much lower. The 2DPC configuration is typically used to increase RAM capacity with little performance gain. But with 12 memory channels in a 1DPC configuration, AMD can already support up to 3TB of memory per chip using 256GB DIMMs. Support for 2DPC increases this capacity to 6 TB DDR5 per processor socket, but here the problem of lack of space to fit 12-channel memory slots in a standard two-processor server comes to the fore.

Even installing 24 DIMM slots for a 1DPC configuration already causes many problems due to lack of space. It’s hard to imagine doubling the number of slots for a 2DPC configuration – a two socket server would require 48 slots! Experts assume that most 2DPC configurations will be used in single-socket servers or dual-socket servers will not use the maximum number of channels.

    Image source: Tomshardware.com

Image source: Tomshardware.com

In fact, AMD partners are already required to use special “narrow” memory slots for Genoa motherboards to accommodate 12 slots next to the socket. AMD has confirmed that there have been multiple incidents of physical damage to motherboard DIMM slots due to narrow slots and other tricks to pack DIMMs more densely. Of course, this does not indicate a global problem with the platform, but points to the difficulties that AMD already has with “only” 12 memory slots.

Also, the problems of 2DPC mode extend beyond the physical placement of memory slots. Adding more DIMMs per channel results in slower memory speeds, and increasing the number of channels introduces further system complexity. Even additional empty slots can decrease the maximum memory speed.

DDR5 requires more complex motherboard designs with more layers and better materials than DDR4, which drives up costs. The task becomes more complicated as the rated frequency of memory modules increases, and market insiders suspect that support for 2DPC mode may end as DDR6 memory begins to be used.

About the author

Dylan Harris

Dylan Harris is fascinated by tests and reviews of computer hardware.

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