Question on DDR5 memory speed vs CPU capability

Fwiw, there is this list from AMD about memory compatibility for their CPUs. Select '7000 Series or higher' from the Tier dropdown.
Not sure how up to date this list is.

I just said you cannot blindly buy RAM and expect to work at the speed you want. You have to do some research based on your motherboard and CPU. And yes, CPU speed is an important factor to consider, either you like it or not.

Note: I did not experiment, I read as much info for my computer purchases and parts to make sure it would work as intended for the performance I expected...

Problem is that using all memory controller channels tends to overload IMC (Internal Memory Controller) located in the CPU specially when running high frequency and compounded by larger memory capacity. Ryzen has another memory controller in separate I/O chip. IMC is not directly connected to cores o not affected with core frequency. Zen4 and now Zen5 IMC is decoupled form I/O chip so it's not necessary to run RAM at same frequency as those two but when ratio is not 1:1:1 (all at same base frequency) some RAM performance could be lost.
That (over)load possibility is with practically all consumer CPUs but server CPUs have much beefier IMC with more channels to accommodate more channels for more memory although those also tend to run at lower frequency for greater reliability.
Edit.
It goes without saying that perfectly matched memory like in factory matched kits has greater chance of success.

So reviews were rights, I got 5600 and it can run only at 4800, despite the fact that motherboard supports 7200 and CPU should support at least 5200 in a dual channel, while AMD suggests 6000 as the sweet spot. My only hope is that new BIOS will fix it eventually.

TairikuOkami said:

So reviews were rights, I got 5600 and it can run only at 4800, despite the fact that motherboard supports 7200 and CPU should support at least 5200 in a dual channel, while AMD suggests 6000 as the sweet spot. My only hope is that new BIOS will fix it eventually.

(snip)

Click to expand...

You can't select an XMP (extreme memory profile) in the BIOS settings?


(It's a little strange that the manual refers to XMP. That's an Intel term.)

Don't know what problem you have but mine is running above it's specifications

bobkn said:

You can't select an XMP (extreme memory profile) in the BIOS settings?

https://download.asrock.com/Manual/Software/AMD%20B650/Software_BIOS%20Setup%20Guide_English.pdf

(It's a little strange that the manual refers to XMP. That's an Intel term.)

Click to expand...

It's not strange, AMD can work with inte XMP or AMD's EXPO.

CountMike said:

It's not strange, AMD can work with inte XMP or AMD's EXPO.

Click to expand...

I didn't mean to imply that an AMD board couldn't use XMP.

On my ASUS x570 board, the BIOS setting was called DOCP. I figured it was some sort of NIH (not invented here) thing, but that's probably wrong.

Whatever they called it, I used the feature.

(Edited to correct a missing negative.)

bobkn said:

You can't select an XMP (extreme memory profile) in the BIOS settings?

Click to expand...

Yes, XMP or EXPO, but it will result in black screen and it also permanently breaks the ability to restart PC.

bobkn said:

I didn't mean to imply that an AMD board could use XMP.

On my ASUS x570 board, the BIOS setting was called DOCP. I figured it was some sort of NIH (not invented here) thing, but that's probably wrong.

Whatever they called it, I used the feature.

Click to expand...

DOCP is what Asus call XMP or EXPO settings, so it's just Asus's way to circumvent royalties to Intel- MSI calls it e-XMP for instance.

As said before, you cannot always force an incompatible RAM profile. You must match your CPU capabilities. Today RAM is a little more flexible, but some generations back you should match the CPU FSB with the RAM FSB. Buying the wrong type you either had to use RAM in less than nominal speed or it wouldn't work at all. In both cases you just wasted money on a faster RAM your system didn't support. I think this is your case there, Either select a slower speed compatible to your CPU or replace the RAM with a matching one.

spapakons said:

As said before, you cannot always force an incompatible RAM profile. You must match your CPU capabilities. Today RAM is a little more flexible, but some generations back you should match the CPU FSB with the RAM FSB. Buying the wrong type you either had to use RAM in less than nominal speed or it wouldn't work at all. In both cases you just wasted money on a faster RAM your system didn't support. I think this is your case there, Either select a slower speed compatible to your CPU or replace the RAM with a matching one.

Click to expand...

There is another dimension with Ryzen CPUS. They have another chip for I/O which also regulates RAM, so for memory path it goes like this
IMC (internal Memory Controller) inside main chip where cores are > FCLK which is in I/O chip > RAM itself. Ideally all should run at RAM base frequency which at end has to be multiplied by 2 as it's DDR (Double Data Rate) so all is in ratio of 1:1:1
Lets say for Zen3.
IMC=1600MHz > FCLK=1600MHz which gives 3200MTS/MHz ram frequency (remember that's DDR).
With Zen4 (7xxx, 8xxx and now new Zen5 (9xxx) that ratio is decoupled and can be 1:2:1 or different.
Lets say Zen4or 5:
IMC 3000 MHz> FCLK 2000 > RAM is 3000MHz * 2=6000MHz. So all those values should be in harmony for best operation, stability and performance.
My RAM has those values in it's XMP but I was able to set it manually for end result of 6200MHz Cl30 (instead of 32) FCLK 2100 which pushed IMC to 3100MHz.
BIOS did recognize memory's XMP (6000MHz, CL32) I changed only FCLK, memory frequency and CL to 30. Results of allvalues can be seen here: