AMD EPYC ram choice

[Caennanu]

Gday all,

 

Since i can't really find any good sites with the information i need, i turn to you all once again.

I'm about to purchase an EPYC CPU to replace my Ryzen 1700 for unraid (don't have enough pci-e lanes for what i want to do, and the choice is irrelevant for this topic).

 

Now i have several options when it comes to memory. I can buy 8 sticks of memory of 8GB each at 2133mhz, or 4 sticks of 16GB each at 3200mhz. The price comes down to about the same and i don't forsee needing more than 64GB of memory in the future).

Since EPYC supports 8 channel memory, i would think 8x8 would be better as you get more bandwidth to play with, but 4x16 would give me higher throughput per stick.

My usecase is not so much raw compute power, but more or less hosting a few vm's with gpu passthrough and hosting a selection of headless game servers (not VM's to game on, but a server like factorio or minecraft that clients can connect to).

 

So, the question is. Which is better at equal GB amounts?

4x 3200mhz (quad channel) or 8x 2133mhz (octa channel)?

[MagmaiKH]

I'm looking for information about CAS/CL for EPYC 7003 series of processors and this post comes up so other people are probably seeing it when they search.
I have read the AMD EPYC RAM guidelines and they are clear that you must populate all eight RAM channels to achieve maximum RAM bandwidth. 8x L/RDIMMs is the only recommend configuration and they are supposed to be clocked at 2,933 MHz.
There is a range of other RAM configurations that are supported; if you only use 4x DIMMs (connected to the appropriate channels) then you can clock them at 3,200 MHz.

I keep finding CL/CAS 22 ECC RDIMMs @ 3,200 MHz but I want faster than that.

It might be motherboard-specific if they allow them to clock down to 2,133. In theory it could work but maybe it doesn't for EPYCs; it's not a supported configuration.
Even if it did work I do not think that is a good idea because I presume the RAM clock speed affects the rate the EPYC cores are clocked at (similar to Ryzens).

To make a comparison between 3,200 and 2,933 clock RAM we need to know what CL/CAS clock-cycle latencies are supported by the RAM.
Unfortunately RAM performance is not determined by clock-rate but by clock-rate attenuated by the strobing latencies.
It is very much possible for slower clocked RAM to be perform better than higher-clocked RAM if the latencies of the slower clocked RAM are more favorable.

See the attached speadsheet for transfer rate calculations. You can enter the clock and CL/CAS and compare how fast the RAM is.
This speadsheet is not complete. It only accounts for CAS/CL latency. There are additional ones you would need to account for to get more accurate results but this is better than guessing.
Explanation of additional RAM timing: https://www.techpowerup.com/articles/overclocking/AMD/memory/131

CAS 22 @ 3,200 MHz is comparable to CAS 13 @ 2,133 MHz
CAS 22 @ 3,200 MHz is comparable to CAS 19 @ 2,933 MHz
However AFAICT those CAS latencies are not available at the lower clocks. It's 21 and 15 so the CAS of 22 @ 3,200 is the lowest latency server ECC DIMM RAM that's out there. This also requires you to run the EPYC server with only 4x DIMMs. If you go to 8x DIMMs for max bandwidth you incur a 5% latency penalty because it drops to 2,933 MHz though I believe this doubles the RAM bandwidth so it's worth it.

From what I remember for Ryzens you want to get 3,600 MHz RAM with the lowest CAS you can to maximize their performance.
(As new series and faster Ryzens come out that MHz may need to go up.)
However for 7003 EPYCs the supported RAM clock rates are much more limited to only 3,200 and 2,933.

 

RAM Latency.xlsx