Question Only half of the installed RAM is detected though all 4 banks are reported ?

[ReluctantTinkerer]

I own a Gigabyte ax370 Gaming K5 motherboard with a Ryzen 7 2700x CPU. I recently populated its 4 RAM banks with 4 x 16GB Corsair Vengeance Pro DDR4 2933 Mhz RAM. The BIOS (which I updated to the latest 51h version) reports 64GB RAM at 2667 Mhz. However, my OS (Arch Linux) only sees 32GB.

From https://en.wikichip.org/wiki/amd/ryzen_7/2700x I understood that for all four banks to "work", the top speed would be 1866. Is that true? If so, why doesn't SMP automatically do this? I have also tried to find if I can manually set the speed in the BIOS but the values seem to be greyed out. Is there a way to do this?

 

[Roland Of Gilead]

Hey there,

It's the limitations of the chipsets and CPU for that Gen. Typically, the IMC (Internal memory controller) can run 2 DIMM at 2933, but when 4 are populated it may drop down to 2133mhz. But if the only other jedec timing on your DIMMs are 1866, then that's what it is.

Your mobo may support 3200mhz, but the IMC isn't able for it.

 

[ReluctantTinkerer]

Hey there,

It's the limitations of the chipsets and CPU for that Gen. Typically, the IMC (Internal memory controller) can run 2 DIMM at 2933, but when 4 are populated it may drop down to 2133mhz. But if the only other jedec timing on your DIMMs are 1866, then that's what it is.

Your mobo may support 3200mhz, but the IMC isn't able for it.

Fair enough I suppose. Do you know how to tell the IMC to throttle down to 1866? It seems to automatically select 2667.

 

[Roland Of Gilead]

If you open up CPU-z, you can get the Jedec timings that are resident on the DIMMS. Open the speed tab. You will see options that are available. Some default to 1866 (older), and others default to 2400/2666mhz. Often overclockable ram (xmp) only have two options.

 

[ReluctantTinkerer]

I'm on Linux, and CPU-X has no such functionality. Isn't there a way to do this in the BIOS menu?

 

[ReluctantTinkerer]

In the BIOS menu, there is an "auto" value of the System Memory Multiplier. I can't seem to change that. Is it locked by something?

 

[ReluctantTinkerer]

Anyway, I resolved it by having BIOS reset itself. Now it works.

I selected the XMP Profile 1, saved the configuration, turned off the computer, re-seated all four DIMMs, and now I have 62,7 GiB at, according to lshw, 2666 Mhz. When I rebooted and entered the BIOS setup screen, it told me it had reset and showed 65536MB at 2684,74 Mhz.