Hey, I’m planning on doing a build with the 3950x when it becomes available. I will use this machine for software dev work most of the time, so it will be Linux on an encrypted ZFS raid, two 4k screens, proper keyboard, and of course the many many cores for good compile speed. This will also finally be a CPU with ECC support! And I plan on buying two 16GB modules.
Now, I’ve looked a bit into the whole deal with how Ryzen 3000 scales with memory speeds, and I’ve seen a topic here where someone managed to overclock their ECC RAM to 3600MHz.
What I have not seen however, is whether someone tested setting a custom Infinity Fabric clock speed/multiplier when using slower ECC RAM. In theory, one should be able run 2400MHz RAM, crank the IF clock all the way up to 1800MHz in the UEFI (or set a ratio of 1.5), right? Hell, if you squint a bit that factor doesn’t even look that unsynchronized.
Has anyone here made any experiments/benchmarks with this?
The penalty from breaking the 1:1 ratio isn’t worth it unless your scenario is such that your RAM is the faster element AND fast enough to make up for the latency penalty. Essentially, latency goes up when you go asynchronous (1:1 ratio is synchronous).
I would get the fastest ECC you can afford and overclock it within reason. I have 4x16GB DIMMs of samsung 2666 ECC Overclocked to 3333 C17. IF clock 1:1. This is with a 3900x on a Taichi x570.
Everyone’s results will be different, but with only 2 DIMMs you should be able to push a little further.
Memtest86 8.3 Pro or Linux on kernel 5.4+ will tell you if errors are occurring as you overclock. Once you start getting consistent errors at reasonable voltages, just back down the OC a notch or two.
Thanks! I just built my new workstation with two Samsung 2666MHz DIMMs. I’ve been running them at 2933MHz without issues but will try to push further and run some benchmarks when I have some time 
Just a quick confirmation: Running at 3200MHz for two weeks now without issues. When I first set tthis, I had it run Memtest for a few hours and it passed everything without any problems, too.
Did you consider this ram kit?
I’m planning a build just like yours and Im looking at this kit now.
Nope, sorry, first time I saw it. FYI, I paid about $250 (in Sweden, incl. tax) for 2x16GB.