Originally Posted by Offler
High RAM frequency also benefits to Ryzens, not sure about current 3000s. Combo of my Gskills Xflare and Threadripper is the most stable CPU+RAM combo I ever had, in both oc and non-oc scenarios. I agree with most of the specs here, one thing might help to RAM OC - if its 8 layered board.
Zen 2 has some interesting behavior when it comes to the Infinity Fabric clock (FCLK) and memory clock (MCLK). As a reminder for those who forgot or didn't know, DDR (double data rate) buses transfer data on both the rising and falling edge of the clock instead of just the rising or just the falling edge, hence why a MCLK of 1800MHz translates to an equivalent DDR clock of 3600MHz. I'm going to neglect talking about UCLK as well to simplify things but the general idea is still the same (just with one less frequency to get confused about).
On Zen and Zen+, the FCLK was strictly tied to MCLK (no decoupling) and the FCLK couldn't really go that high to begin with, hence why Ryzen 1000 and 2000 had a lot of difficulty with higher speed RAM kits. In the end, the higher your MCLK (and therefore FCLK), the better performance you'd get.
Zen 2 on the other hand decouples the FCLK from the MCLK so that the FCLK, which usually tops out around 1800-1900MHz on the current Ryzen 3000 IMCs, doesn't hold MCLK back. By default, once you're at 1900MHz FCLK it will swap to a 2:1 MCLK:FCLK ratio. In 1:1, an MCLK of 1900MHz (DDR# 3800MHz) would result in a FCLK of 1900MHz. In 2:1, a 1900MHz MCLK would result in a meager FCLK of 950MHz so there's actually a performance penalty until you reach some insane MCLK speeds (we're talking like 7000MHz+ RAM).
There's the option to specify the MCLK:FCLK ratio such that they aren't perfect multiples of each other so you don't have that giant drop in performance from going directly to 2:1 from 1:1 but that on its own still has some penalties in performance due to desync between the two.
Hence on Zen 2, you really only want RAM between 3600-3800MHz and 3800MHz is if your IMC can even handle it. You actually get worse performance on those 3933MHz+ kits unless you want to go through the arduous process of messing with FCLK:MCLK ratios. It's not worth the hassle so you're better off just sticking to the range where MCLK maxes out FCLK while still keeping the 1:1 ratio. Besides, timings get looser far faster than frequency can keep up once your frequencies are in the 4000MHz+ range so at that point your memory performance will get very bad very quickly.
It's worth trying to get FCLK as high as you can since that will affect die-to-die or CCX to CCX communication speeds. It's not very expensive to do so either since 32GB (2x16 or 4x8) Micron Rev.E kits are only $140-$150 anyways and those kits are known to clock very easily up to the mid to high 4000MHz range. You just spend the 10 minutes using the Ryzen DRAM calculator and copying in the timings that provides into BIOS. Literally just try 1:1 w/ your memory at 3800MHz and then go to 3733 then 3600 if your IMC can't handle the higher speeds without needing to switch to 2:1. It's free performance at no cost.
Number of layers doesn't matter that much. All that matters is the layout/topology. As long as the board isn't doing something truly horrendous like stacking components on top of the memory traces, it'll be fine. There's horrible 8-layer motherboards and very good 4 or 6-layer motherboards. The IMC is likely to be the limiting factor anyways. We're not clocking the RAM to insane speeds or hoping for insanely tight timings.