[SOLVED] What is CAS latency? How much does it affect Zen 2?

xxscienceboyxx

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Nov 30, 2018
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Hello,
I’m thinking of getting a 3900x, and I bought some corsair LPX 3600 MHz ram for it. I’m not sure what the CAS latency for it is. What exactly is CAS latency though?
Is there much of a difference between RAM of the same MHz but different CAS latency?
Also would it be better to get 3200 MHz ram with a lower latency?
 
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QwerkyPengwen

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I got 3600 MHz ram with C18. Does that mean it won't be able to run at 3600 MHz? If so, would 3200 MHz ram with C16 latency be faster for the 3900x?
I bought the 3600 mhz C18 ram for $66 with a black friday deal. Should I return it and buy 3200 MHz C16 RAM for ($5-10) more (costs more because there aren't any deals now)?
as I said, the timings and speed are connected.
3200MHz with CL16 is the same as 3600MHz CL18 (in regards to the CAS Latency specifically) but the 3600MHz speed in and of itself is faster if you can run it at that speed or close to it.

But getting RAM to run at speeds higher than 3200MHz with Ryzen can be a chore sometimes depending what CPU you have, and what motherboard you have and whether or not the RAM is on the QVL or not.

But also as stated, you get diminishing returns on anything higher than 3200MHz.
Overall performance of the CPU and the system and games becomes less exponential the higher above 3200MHz you go.

So while overall performance will be better, it won't be a ton better.

If you are worried about it, and have the option to get your money back and instead get the 3200MHz RAM, then do that.
But make sure it's not some cheapo RAM and that it will run at it's rated speed on your system.

Overall I would say since you bought it, just stick with it.
If you can get it to run at it's speed or close to it either with XMP/DOCP profiles, or by manually dialing in the speed, timings, and voltage, then great, but if not, then you can still at least run it at 3200MHz with tighter timings, or dial in a clock speed in between the two speeds at custom timings, or just have it on hand for when you upgrade CPU and mobo in the future and can get it to run at it's rated speed then (assuming you can't get it to run that speed on your current hardware)
 
CAS Latency (CL) stands for Column Address Strobe. It's the number of clock cycles that pass from when an instruction is given for a particular column and the moment the data is available. In general the lower the CAS latency the better.

The 3900x processor supports up to 3200MHz officially however 3600MHz is possible with more selective Timings and a low CL14. There is no guarantee you will attain the rated frequency of your OC RAM which depends on the strength of the IMC.
 

QwerkyPengwen

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The higher the speed, the higher the latency/timings. It's all connected.
The latency can matter in only certain situations, but it won't make a huge difference and makes no difference in standard everyday things.

If you can run the RAM at 3600mhz then great, but when you go above 3200mhz you get less performance gains for extra money when it comes to buying the RAM.
But since you already have it then use it.
You can find the timings on the sticker that is in the RAM stick
 

xxscienceboyxx

Commendable
Nov 30, 2018
176
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1,695
3
The 3900x processor supports up to 3200MHz officially however 3600MHz is possible with more selective Timings and a low CL14. There is no guarantee you will attain the rated frequency of your OC RAM which depends on the strength of the IMC.
I got 3600 MHz ram with C18. Does that mean it won't be able to run at 3600 MHz? If so, would 3200 MHz ram with C16 latency be faster for the 3900x?
I bought the 3600 mhz C18 ram for $66 with a black friday deal. Should I return it and buy 3200 MHz C16 RAM for ($5-10) more (costs more because there aren't any deals now)?
 
Last edited:

QwerkyPengwen

Splendid
Ambassador
I got 3600 MHz ram with C18. Does that mean it won't be able to run at 3600 MHz? If so, would 3200 MHz ram with C16 latency be faster for the 3900x?
I bought the 3600 mhz C18 ram for $66 with a black friday deal. Should I return it and buy 3200 MHz C16 RAM for ($5-10) more (costs more because there aren't any deals now)?
as I said, the timings and speed are connected.
3200MHz with CL16 is the same as 3600MHz CL18 (in regards to the CAS Latency specifically) but the 3600MHz speed in and of itself is faster if you can run it at that speed or close to it.

But getting RAM to run at speeds higher than 3200MHz with Ryzen can be a chore sometimes depending what CPU you have, and what motherboard you have and whether or not the RAM is on the QVL or not.

But also as stated, you get diminishing returns on anything higher than 3200MHz.
Overall performance of the CPU and the system and games becomes less exponential the higher above 3200MHz you go.

So while overall performance will be better, it won't be a ton better.

If you are worried about it, and have the option to get your money back and instead get the 3200MHz RAM, then do that.
But make sure it's not some cheapo RAM and that it will run at it's rated speed on your system.

Overall I would say since you bought it, just stick with it.
If you can get it to run at it's speed or close to it either with XMP/DOCP profiles, or by manually dialing in the speed, timings, and voltage, then great, but if not, then you can still at least run it at 3200MHz with tighter timings, or dial in a clock speed in between the two speeds at custom timings, or just have it on hand for when you upgrade CPU and mobo in the future and can get it to run at it's rated speed then (assuming you can't get it to run that speed on your current hardware)
 
I got 3600 MHz ram with C18. Does that mean it won't be able to run at 3600 MHz? If so, would 3200 MHz ram with C16 latency be faster for the 3900x?
I bought the 3600 mhz C18 ram for $66 with a black friday deal. Should I return it and buy 3200 MHz C16 RAM for ($5-10) more (costs more because there aren't any deals now)?
No it does not mean they will not work at 3600MHz however being OC RAM there is no guarantee and profiles such as XMP will not work. OC RAM without a profile requires manual Bios input to determine correct CL Timings and Voltage.

You would be better of stability wise with a kit at 3200MHz C16 which would be about the same performance as 3600MHz at CL18.
 

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