How to interpret System Memory Specification?

prueba2306 · March 26, 2019

I'm trying to assemble my first computer. I don't need a powerful GPU (I practically don't play games) BUT I need a good amount of cores/multi threading (multiple programs opened). I realize that Ryzen 5 2400G is the best option for me (cheap and powerful enough for at least 4/5 years).

Looking for information about what I need to acquire for the best possible performance I realize the that there is a "System Memory Specification: DDR4-2933 dual-channel" (is the same for most Ryzen 3/5/7)

But in this page https://en.wikichip.org/wiki/amd/ryzen_5/2400g#Memory_controller, in the memory configuration part, it changes depending on the motherboard (dual channel) and the numbers of ram slots used:

Single Rank

2 of 2 DDR4-2933
2 of 4 DDR4-2933
4 of 4 DDR4-2133

Double Rank

2 of 2 DDR4-2667
2 of 4 DDR4-2400
4 of 4 DDR4-1866

So here are the questions:

1) What does "DDR4-2933 dual-channel" really means? That if I buy DDR4-3000 or faster I would not be using them in their full potential? Because i read in some forums that buying DDR4-3200 improves the speed of the system. Is that true or is a "placebo effect"?

2)I know that the CAS latency is important too, and the best way to get "the real" speed of the RAM is dividing the speed/CAS. I found out a cheap ram that runs 2400/10=240 against a 2933/14=209.5 or even a 3000/14=214.3. According to the specification, are you telling me that it won't be able to "squeeze" the 3000 one but it will be able with the 2400 one? How does that makes sense?

3)According to the previous table, does that mean the best speed of the ram would be 2133 if i use all the slots? Then doesn't make sense to buy anything faster than 2133. I'm not gonna use all the slots right now but i plan to expand ram in the future (16 GB to begin with and later 32GB or maybe even 64GB)

I really would appreciate your help trying to understand this stuff.

Mira Yurizaki · March 26, 2019

57 minutes ago, prueba2306 said:

1) What does "DDR4-2933 dual-channel" really means? That if I buy DDR4-3000 or faster I would not be using them in their full potential? Because i read in some forums that buying DDR4-3200 improves the speed of the system. Is that true or is a "placebo effect"?

Dual channel memory is when you take two memory modules and use them together in a way that combines of both. RAM normally uses a 64-bit wide data bus. Dual channeling increases that to 128-bits. It may be faster for extended data transfers but it won't help much for many smaller ones since latency is more of a factor there.

RAM speed does help with Ryzen's performance, but it depends on the task and it seems to generally not matter:

https://www.anandtech.com/show/11857/memory-scaling-on-ryzen-7-with-team-groups-night-hawk-rgb/5

https://www.anandtech.com/show/12621/memory-scaling-zen-vega-apu-2200g-2400g-ryzen/2

Quote

2)I know that the CAS latency is important too, and the best way to get "the real" speed of the RAM is dividing the speed/CAS. I found out a cheap ram that runs 2400/10=240 against a 2933/14=209.5 or even a 3000/14=214.3. According to the specification, are you telling me that it won't be able to "squeeze" the 3000 one but it will be able with the 2400 one? How does that makes sense?

The division should be the other way around as latency is given in clock cycles.

10/2400 MHz = ~4.1667ns
14/2933 MHz = ~4.7733ns
14/3000 MHz = ~4.6667ns

In human time scales, this largely doesn't matter.

Quote

3)According to the previous table, does that mean the best speed of the ram would be 2133 if i use all the slots? Then doesn't make sense to buy anything faster than 2133. I'm not gonna use all the slots right now but i plan to expand ram in the future (16 GB to begin with and later 32GB or maybe even 64GB)

I believe the table is only if you're using a completely stock configuration, independent of what the motherboard actually supports. The motherboard determines the RAM speeds you can run in your system. For example they claim on a 1700X (https://en.wikichip.org/wiki/amd/ryzen_7/1700x) the highest supported speed is DDR4-2666. But you can definitively say that's not the case with people here on the forums with a 1700X using faster RAM. And I myself have a 2700X which is more or less a die shrink of the 1700X, and I'm running DDR4-3200 RAM just fine.