RAM Clock Speed VS Cas Latency

[Darth Scoob]

At a given first word latency, say 10ns, would higher speed memory or lower cas latency memory perform better? Between several kits of ram that are equivalent except one is 3200MT/S CL16, another is 3600MT/S CL18, and a third is 4000MT/S CL20, which would perform best? They would all have a first word latency of 10ns, but would there be a verifiable trend in speed for certain loads like gaming or rendering?

I haven't been able to find any information about this online. If you know of a resource with this data please let me know.

[CasoF13]

[RONOTHAN##]

There are so many other factors to this than just CL and frequency that affect performance, so there isn't a definitive "4000 cl20" will be the best.

 

As a general rule, given a certain latency in ns, more frequency is more better, but it does matter how those clocks are being utilized. If you're comparing 3200 16-18-18-36 to 4000 CL20-40-40-60, the 3200MHz kit will perform better because of how loose tRCD is. Plus secondary and tertiary timings are important as well, arguably more important than the primaries. 

 

That's not even considering different memory controller settings. If this is an AMD CPU, performance is best when the FCLK is the same as the MCLK, and since Ryzen 5000 series CPUs generally can't go above 3800MT/s in 1:1 mode, the 3600 CL18 kit will work best if you just enable XMP (if you use the 4000 Cl20 kit and just lower the frequency to 3800MHz that would work as well and will still perform the best with lower overall latency because the link between the memory controller and the cores is faster). On Intel platforms 11th gen and beyond you need to worry about memory gear. 3200MHz CL16 will universally run in Gear 1 where the memory controller has the lowest latency. 3600MHz, however, is iffy whether it'll run in Gear 1 without adjusting VCCSA which if you have a locked SKU isn't possible. 4000 Gear 1 is possible with adjusting VCCSA, but on the locked SKUs it's not and you'll be forced to run in Gear 2 where the performance will be worse than 3200MHz CL16. 

[TrigrH]

7 minutes ago, Darth Scoob said:

At a given first word latency, say 10ns, would higher speed memory or lower cas latency memory perform better? Between several kits of ram that are equivalent except one is 3200MT/S CL16, another is 3600MT/S CL18, and a third is 4000MT/S CL20, which would perform best? They would all have a first word latency of 10ns, but would there be a verifiable trend in speed for certain loads like gaming or rendering?

I haven't been able to find any information about this online. If you know of a resource with this data please let me know.

1. First world latency is only calculated off the first number. The other primary timings are very important also. Let alone tuning the rest of the timing set manually.

2. XMP is just a profile that loads certain timings. Typically the better this profile is, the better "bin" of ram you have.

3. The best memory speed is often capped by the platform you are using. Here is some examples:

If you are gaming on 12th gen intel with ddr4 you might want to stick to a lower speed which keeps you in gear 1. But if you have ddr5 you might want to go as high as you can afford.

On AM4 Ryzen? Stick to ~3800mhz and tune timings from there. This gives better performance for that platform.

4. Faster kits might perform better in some workloads and worse in others due to the latency.

 

TLDR: You want the highest speed that is optimal for your platform AND workload, then tune from there.