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I got an HP Z440 workstation, and bought 8 32GB 2133MHz memory modules for it for a total 256GB (two of those: https://www.ebay.com/itm/234996243550). Also as requested got a memory cooling contraption, and updated BIOS to the latest version.

The system is supposed to run at RAM 2133MHz speed, and has Xeon E5-2690 V4 which supports this memory speed.

When I install 2 or 4 modules, it indeed does, but after I add two more or go to all 8, the RAM speed drops to 1866MHz.

Any ideas what could be wrong?

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  • It would help if we had more information, such as specs of all the ram modules
    – Rohit Gupta
    Commented Jun 12, 2023 at 4:01
  • All are 8 modules are Samsung 32GB DDR4 PC4-2133P 2Rx4 ECC REG DIMM M393A4K40BB0-CPB
    – Alex Z
    Commented Jun 12, 2023 at 5:21
  • Did you enable the correct memory profile in the bios ? Maybe the memory doesnt pair well and the best it can do is 1866Mhz, it unfortunately does happen. Although at speeds this low you'd expect it to hit 2133Mhz.
    – Silbee
    Commented Jun 12, 2023 at 8:11
  • Do you know where are memory profile settings in HP bios? I seem to check all pages and can't find anything remotely related.
    – Alex Z
    Commented Jun 13, 2023 at 5:17
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    Quoting the Intel specs: “Maximum supported memory speed may be lower when populating multiple DIMMs per channel on products that support multiple memory channels.”
    – Daniel B
    Commented Jun 14, 2023 at 15:39

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Maximum memory bandwith for the Xeon E5-2690 V4 according to Intel is 76.8 GB/s, and there are 4 Memory channels available.

According to Wikipedia, for DDR4 DIMMS, Peak transfer rate for 2133MHz is 17066.67 MB/s, while for 1866MHz it is 14933.33 MB/s

Now, if we consider 1886MHz, then 8 (DIMMS) * 14933 MB/s = 119 GB/s, while 6 (DIMMS) x 14933 MB/s = 89 GB/s. So even considering some overhead, the available memory bandwith is already maxed out, and it would not make any sense clocking the memory higher.

Until you have a maximum of 4 DIMMS, there is 1 DIMM per Memory channel, and you can get more speed. On 1866MHz, 4 (DIMMS) * 14933 MB/s = 59 GB/s, the memory bandwith would not be maxed out, and not even on 2133MHz it is maxed out (4*17066MB/s = 68 GB/s), that's why you could go up to 2400MHz modules.

Unfortunately Intel does not spell out anywhere the exact implications as detailed here above, it is information that is not at all easy to find. As was noted in a comment, Intel's very generic statement (when clicking on the ? next to Memory Types) is:

“Maximum supported memory speed may be lower when populating multiple DIMMs per channel on products that support multiple memory channels.”

There is some more information available in this thread, and this article about a later generation of Xeons. Furthermore the Intel documentation page on v3 / v4 might be interesting for a deep dive.

Moreover, for a visual guide about the problem of more than 1 DIMM per memory channel, the graphics in this article about 4th Gen Intel Xeon Scalable are interesting to consider.

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  • @AlexZ if you can mark this answer as accepted, the community bot won't continue to revive your question for the next 10 years or so. Thank you.
    – 1NN
    Commented Jun 17, 2023 at 18:07

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