As additional DIMM modules are added to a memory channel, the controller reduces the frequency due to the increased loading. So even when you populate a system with DDR4-2667 modules, the system may run at a lower DDR4 frequency.
The loading and maximum frequency for different configurations may vary from system to system. Below are the current guidelines for most systems using the EPYC processor.
This generally happens on systems designed with two DIMM sockets for each channel, having a total of 16 DIMM sockets per CPU.
For designs with two DIMM sockets per channel (16 DIMM sockets per CPU):
DIMM population/type - Maximum DDR4 rate
1 x Single Rank RDIMM - 2667
1 x Dual Rank RDIMM - 2400
2 x Single Rank RDIMM - 2133
2 x Dual Rank RDIMM - 2133
1 x 3DS RDIMM 1S - 2667
1 x 3DS RDIMM 2S - 2400
2 x 3DS RDIMM - 1866
1 x LRDIMM - 2667
2 x LRDIMM - 2133
