Help finding the fastest memory this mobo will take

3080guy · 11 Sep 2021 at 22:51

I have a 3080 and want to sell my current RAM and put birthday money towards buying 32GB of the fastest ram my motherboard will take, which is 4133mhz ram. I have this inexpensive mobo
https://www.msi.com/Motherboard/B450-A-PRO-MAX/Specification

Im very happy to put money towards 4133mhz ram, I realise the sweet spot in price/performance is for slower ram.

What Id like help with though please, is finding the best priced 4133mhz ram I can buy online for 32gb of decent quality 4000 or preferably 4133 ram.

Thanks for the help.

Dg834man · 12 Sep 2021 at 02:47

Highly unlikely even a 5000 chip will run the infinity fabric that high

3080guy · 12 Sep 2021 at 11:45

Dg834man said:
Highly unlikely even a 5000 chip will run the infinity fabric that high

I figured the latency or available bandwidth somewhere would be sped up fractionally, improving fps a tad

Minstadave · 12 Sep 2021 at 12:17

3080guy said:
I figured the latency or available bandwidth somewhere would be sped up fractionally, improving fps a tad

The best performance is at the highest speed your memory can run whilst 1:1 with your infinity fabric which is typically 3600mhz or 3800mhz depending on your CPU. You can run higher speeds but the latency penalty of going of sync with your Infinity Fabric is too great.

What CPU have you got?

3080guy · 12 Sep 2021 at 12:39

I'll have to read your previous comments a few more times and consult google to understand that properly. Thanks.

I have a Ryzen 9 3900 PRO
https://www.amd.com/en/products/cpu/amd-ryzen-9-pro-3900
12 Core, 3.1Ghz boost 4.3Ghz

Minstadave · 12 Sep 2021 at 12:56

3080guy said:
I'll have to read your previous comments a few more times and consult google to understand that properly. Thanks.

I have a Ryzen 9 3900 PRO
https://www.amd.com/en/products/cpu/amd-ryzen-9-pro-3900
12 Core, 3.1Ghz boost 4.3Ghz

Basically the fastest a 3900 with it's IF (Infinity Fabric) and RAM in sync is about 3800mhz RAM/1900mhz IF on a good chip. This is the best case scenario for most CPUs.

The IF is the speed of the main interconnect on the CPU, ideally it runs at the same speed as the RAM to keep latency as low as possible.

As DDR RAM is dual data rate the speed people refer to is usually doubled confusingly (so with the IF and RAM in sync 1800mhz IF=3600mhz RAM).

The limiting factor for performance is the fastest the IF can run on your CPU. Usually they top out between 1800 and 1900mhz with a rare few going above that even with the voltages tweaked.

You can choose to unlink your IF and RAM speeds, so could run 1800mhz IF and over 4000mhz RAM but the latency penalty means you lose performance even at really high memory speeds. It's just a waste of time to run them out of sync.

Unless your really into tweaking you want to aim for 3600mhz RAM at CAS16 as a decent sweet spot or if you've got money to burn you could get some lower latency modules and run CAS14.

3080guy · 12 Sep 2021 at 13:11

Fantastic, thanks for your really helpful and informative post!