What I heard is that 6000 was the sweet spot for AMD atm
I realized I am **** near illiterate on DDR5 settings and AM5, so I did some googling.
I found
this test at Techpowerup to be the most helpful.
So, definitions:
FCLK: The Clock speed of the Fabric that connects the chiplets on the package.
UCLK: The Clock speed of the Uncore or I/O die which notably includes the memory controller.
MCLK: The RAM clocks in Mhz (will be half of the MT/s figure because DDR)
Apparently previously (on AM4) AMD used to recommend running FCLK : UCLK : MCLK at 1:1:1, but with AM5, they now recommend just letting FCLK float at auto, and running UCLK and MCLK at 1:1.
UCLK is typically stable up to 3000Mhz but can get difficult to get stable above that. (which probably explains the DDR5-6000 recommendation) but there are gains to be had if you can get your UCLK stable above 3000.
According to that test, the following configurations seem to trade blows depending on workload/title:
| DDR MT/s | MCLK | UCLK | FCLK |
| DDR5-6400 | 3200 Mhz | 3200 Mhz (1:1) | 2100 Mhz (Auto) |
| DDR5-7200 | 3600 Mhz | 1800 Mhz (1:2) | 2100 Mhz (Auto) |
| DDR5-8000 | 4000 Mhz | 2000 Mhz (1:2) | 2100 Mhz (Auto) |
They did this test on a Ryzen 9-9950x
I clicked through like 20 pages of results, only to find that there was no clear winner between these three. They almost seemingly randomly traded blows near the top, though I think the DDR5-6400 one was closer to the top on the type of things I do more, but that may just be in my head.
So the question is what to do if you are buying into the platform and don't know if you will win the I/O Die silicon lottery and be able to hit a 3200 UCLK. I guess one could buy DDR5-6400 ram, and just clock it down to 6000 if the uncore can't be coaxed to do, but then you won't get help from EXPO (Apparently AMD's new alternative to XMP?) to hit the subtimings and have to go in there and tinker yourself.
Or do you just assume you'll never hit a UCLK of 3200, and just buy DDR5-6000 RAM and be happy with EXPO working and not having the headache of tinkering with all the memory subtimings....
Or one could just go for DDR5-8000, use a 1:2 divider between the MCLK and UCLK and thus a clean 2000Mhz on the UCLK, but may not fully take advantage of your floating auto setting on the FCLK if it goes above 2000 (which it did for the reviewer).
I hear many people say that DDR5-8000 loses some performance due tot he more relaxed timings, but I don't think that is the case. You can get DDR5-8000 with a CAS latency of 36 clock cycles, but since the clock is so much faster at 8000, that is equivalnet to 6400/8000*36=28.8 at DDR5-6400, which is actually lower than the lowest CAS latency DDR5-6400 sticks I can find anywhere. (lowest I can find is CAS=30).
I think any performance loss here is more likely due to a lack of full utilization of the FLCK,
if it automatically goes to 2100, which it did for the reviewers system.
So maybe, just maybe, we should get some cheap garbage ram, test where the FCLK lands at auto, and then buy the lowest CAS latency RAM we can find at FCLK * 4 ?
(DDR5-8000 if FCLK=2000
or DDR5-8400 if FCLK=2100)?
That said, the lowest CAS latency I can find at DDR5-8400 is 40. That's equivalent to 30.47 at 6400, so a slight timing penalty there...
Who the hell knows.
There do seem to be several options for DDR5-8200 with a CL of 38. That lands you at a DDR5-64000 equivalency of 29.6. Maybe that splits the difference between all of them? I mean, that presumes you can get the RAM stable that fast to begin with. And it probably gets pretty hot if you run it there...
Timings seem to scale with clock speed up to ~DDR5-8200 judging by what I ca find for sale, above that, things look like they start to fall apart rather quickly on the timings side, so the DDR5-8400 idea may not be the best, at least not until better RAM comes out.
I mean, really, between any of these configurations the difference is +/- 1% or so in most tests, so it really doesn't amount to much anyway, unless one uses something slower than the three options above.
