[SOLVED] Which memory speed for Ryzen 7 2700X?

NovaTronMC

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Last year I upgraded from an i7 4820k to a R7 2700X, due to the increased christmas demand, memory kits were a bit limited for my choice, and had to go with a Corsair Vengeance RGB PRO 16GB kit, it runs at 3000MHz with a 15 CAS Latency. From then, I began listening about 3600MHz kits being much better for Ryzen CPUs. I'm planning on getting 32GBs of ram, so I wanted to know if I should get another 3000MHz 16gb kit or sell my current and get a 32GB kit with 3600MHz at 18 CL.
Thanks for your time!
 

Karadjgne

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3200MHz.

The Ryzen 2000 series uses Infinity fabric, same as all the other Ryzens. That's the speed of communication between the cores and linked to the data rate (DR) of the ram in a 1:1 ratio. Upto a point. In the 2000 series cpus, that point is 3466MHz, after which IF changes to a 2:1 ratio, effectively cutting transmission speeds in half.

So figure 3200MHz DDR has a data rate of 1600MHz. 3600MHz will have a DR of 1800MHz, but when applied to IF at a 2:1 ratio, you get effective speeds of just 900MHz on the fclock.

3200MHz ram is considerably stronger performance gain than 3600MHz (1600 vs 900).

It wasn't until the 3000 series that that cap was changed to 3733MHz, making 3600MHz the sweet spot for performance, but still leaves the 2000 series at 3200MHz as the best value per performance.

Even 3000MHz (1500 fclock) is stronger than 3600MHz (900 fclock).
 

beers

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What board do you have? I used a 2700x on this CHVI but it had issues before some of the latest AGESA patches for 3200 CL14.

Higher RAM freq would benefit the infinity fabric speed although the IMC on those CPUs isn't the best, but it also depends on your MB and the traces back to the CPU
 

NovaTronMC

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What board do you have? I used a 2700x on this CHVI but it had issues before some of the latest AGESA patches for 3200 CL14.

Higher RAM freq would benefit the infinity fabric speed although the IMC on those CPUs isn't the best, but it also depends on your MB and the traces back to the CPU
I have the Asus X470-F Strix, I really like how the RGB Pro heatsink looks, as it matches my setup, however, I can't seem to find speeds over 3000MHz which have a lower latency than 15 (3200MHz go for 16CL and 3600MHz for 18CL).
 
You are not likely going to be able to run a Ryzen 2700X with RAM running at DDR4-3600 speed. Only the 3rd-gen Ryzens can typically handle that. 2nd-gen Ryzen processors usually top out around DDR4-3200 or so. The difference between 3200 and 3000 will generally be quite minor.

Adding RAM from other kits does have the potentially to cause stability issues though. So selling the existing kit and replacing it with a 2x16GB kit of DDR4-3200 might be a reasonable option.

If you are just using this system for gaming or most other tasks though, you probably don't actually need 32GB at this time. It could be useful for the future, but for now, 16GB is plenty for gaming and most other common desktop tasks, and you are unlikely to see much performance benefit from adding more.
 
As for the reason CAS latency tends to appear higher with faster RAM, it's because at higher speeds, each clock cycle is shorter, so it takes more of those clock cycles to equal a given length of time. That number is referring to the number of cycles, not an absolute measurement of time.

DDR4-3000 CL15 has a true CAS latency of 10ns. DDR4-3200 CL16 also has a true CAS latency of 10ns. As does DDR4-3600 CL18. You'll notice that the CL number divided into the frequency equals the same result for all of them, as they are all referring to the same length of time. They will all take the same amount of time to begin accessing a location in memory, though the faster memory can then transfer data faster after that point.
 

NovaTronMC

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As for the reason CAS latency tends to appear higher with faster RAM, it's because at higher speeds, each clock cycle is shorter, so it takes more of those clock cycles to equal a given length of time. That number is referring to the number of cycles, not an absolute measurement of time.

DDR4-3000 CL15 has a true CAS latency of 10ns. DDR4-3200 CL16 also has a true CAS latency of 10ns. As does DDR4-3600 CL18. You'll notice that the CL number divided into the frequency equals the same result for all of them, as they are all referring to the same length of time. They will all take the same amount of time to begin accessing a location in memory, though the faster memory can then transfer data faster after that point.
Alright, I get the difference now, however I'm still a bit in doubt. Should I keep my 3000MHz kit or sell it and get a 3600MHz kit?
 

Karadjgne

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3200MHz.

The Ryzen 2000 series uses Infinity fabric, same as all the other Ryzens. That's the speed of communication between the cores and linked to the data rate (DR) of the ram in a 1:1 ratio. Upto a point. In the 2000 series cpus, that point is 3466MHz, after which IF changes to a 2:1 ratio, effectively cutting transmission speeds in half.

So figure 3200MHz DDR has a data rate of 1600MHz. 3600MHz will have a DR of 1800MHz, but when applied to IF at a 2:1 ratio, you get effective speeds of just 900MHz on the fclock.

3200MHz ram is considerably stronger performance gain than 3600MHz (1600 vs 900).

It wasn't until the 3000 series that that cap was changed to 3733MHz, making 3600MHz the sweet spot for performance, but still leaves the 2000 series at 3200MHz as the best value per performance.

Even 3000MHz (1500 fclock) is stronger than 3600MHz (900 fclock).
 

bfollett

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I'm not sure if you'll read this post after having already picked "Best Solution", but it looks like from reading your original post that you are considering adding a second set of 16 GB Dram to get to 32GB. That approach may not work. 2 different sets of memory even of the same brand/model number has no guarantee of working together. Memory sticks are fickle. Only 4 stick kits that have been tested together by the manufacturer are guaranteed to work. If you add a second set of memory to your existing set it may or may not work. Just luck of the draw.
 

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