Confused: DDR4 for Skylake-X, recommended, overclocked, tested and SPD speed (?)

[tomb75]

Hello!

I'm confused trying to match RAM for my next build. Maybe somebody can help me understand details, please. (I reviewed Memory Tutorial and not found the answers).

I'm thinking about i7-7820X CPU and ASRock Fatal1ty X299 Professional Gaming i9. I need 64 GB of RAM (4 x 16GB sticks), and would like to buy G.Skill Trident Z RGB.

Facts:
1. Intel web page says: "Memory type: DDR4-2666" for i7-7820X.
2. G.Skill recommends F4-2400C15Q-64GTZR for this mainboard. Tested speed: 2400 MHz, CL15, 1.2V. SPD Speed: 2133 MHz.
3. G.Skill also recoomends F4-2666C15Q-64GVR for this mainboard. Tested speed: 2666 MHz, CL15, 1.2V. SPD Speed: 2133 MHz.

Now questions:
1. Should I choose 2666 MHz, which matches CPU "memory type"? I don't like it, it is not RGB.
2. Should I keep 2666 MHz speed, matching CPU, or 2400 MHz is also fine?
3. Can I take 2400 MHz and easily overclock it to 2666 MHz?
4. What is SPD Speed? Safest/stable speed?

I hope somebody can clarify it for me, or just point the source of the knowlegde.

Thanks in advance!

 

[Urfaust]

I would go higher with the type of build you project, but since you gonna have lots of bp in quad channel already you could maybe focus on getting tight latency with a good kit like at least 3000/3200 at 1.35 or better, those Samsung B-die at 1.4v to get really great timings with the inherently high bandwith of x299's quad channel.

No need to match you cpu speed, it doesn't match like that anyway. SPD is practically the default frequency, it's actually a little module physically embedded on your sticks carrying informations on your kit profiles (JEDEC/XMP) that will be detected when your motherboard posts. You first time boot at default...then set XMP profile of your kit or manual on your own, as long as it's supported by the motherboard you will choose, this is what you should check first, not Intel or the ram vendors, it's the motherboard QVL list that matters, eventually you can even exceed it but that's what you should base your choice on to begin with.


Your XMP profile of your SPD will be safe, as in you just select XMP in bios and it sets everything for you as defined by the advertized spec of your kit. You can still eventually go higher than the XMP profile or what you motherboard allows, for example i run 4000 Mhz DDR4 on a Maximus Hero which is officially only supporting 3733 Mhz max and at lower timing that the XMP profile.

 

[tomb75]

No need to match you cpu speed, it doesn't match like that anyway. SPD is practically the default frequency, it's actually a little module physically embedded on your sticks carrying informations on your kit profiles (JEDEC/XMP) that will be detected when your motherboard posts. You first time boot at default...then set XMP profile of your kit or manual on your own, as long as it's supported by the motherboard...

Thanks for these clarifications. I understand more now.

...the motherboard QVL list that matters, eventually you can even exceed it but that's what you should base your choice on to begin with.

Yes, you are right. I started from motherboard QVL and found some interesting kits. Then I checked if memory producers have tested already available kits with new hardware, and found out that G.Skill has many kits to choose, which I can buy here - it is also important. Samsung kits are not easily available in my geographical location. BTW: I already skipped the idea of RGB, it has to be powerful, not colorful.

Your XMP profile of your SPD will be safe, as in you just select XMP in bios and it sets everything for you as defined by the advertized spec of your kit. You can still eventually go higher than the XMP profile or what you motherboard allows, for example i run 4000 Mhz DDR4 on a Maximus Hero which is officially only supporting 3733 Mhz max and at lower timing that the XMP profile.

So, there is also a chance to overclock it a little bit above spec. Thanks for info.

I would go higher with the type of build you project, but since you gonna have lots of bp in quad channel already you could maybe focus on getting tight latency with a good kit like at least 3000/3200 at 1.35 or better...

Correct me, if I'm wrong: I dig a little bit more, and find out that:
1. Frequency at CPU side is the frequency of the memory controller. i7-7820X has 2666 MHz, meaning 2.666 billion 64-bit transfers per second multiplied by 4 channels = about 85 GB/s. So, CPU is not able to access more data than 85 GB in one second.
2. It is worth to have higher performance RAMs to avoid bottlenecks, due to various delays, here CAS is the most important factor. And generally faster RAM has higher frequency-to-CL ratio, e.g. 2400 MHz CL 15 is faster than 2666 MHz CL 18, because 2400/15=160 and 2666/18=148.11.

Can anybody confirm above points, please? (Maybe I misunderstood something)

 

[Urfaust]

Yes, you are right. I started from motherboard QVL and found some interesting kits. Then I checked if memory producers have tested already available kits with new hardware, and found out that G.Skill has many kits to choose, which I can buy here - it is also important. Samsung kits are not easily available in my geographical location. BTW: I already skipped the idea of RGB, it has to be powerful, not colorful.

The Trident are great, i had the 4266 kit until a few weeks ago, they exist in non RGB red or black (no fan of that either), now switched to HOF for some reasons. When i meant Samsung B-die, it's the modules inside, you ll get some in the Gskill and some other good kits. What's also important is at what voltage they are rated, for example at 1.35/1.4v it means it's very good modules, hand picked IC for the latter and will go even higher than their default already very fast XMP profiles. I have mine at 4000 Mhz CL16 @ 1.45v, not even trying hard yet.

So, there is also a chance to overclock it a little bit above spec. Thanks for info.

Yes definetly, if your motherboard plays nice you will be able to go higher and depending how far are you willing to go on the voltage and things, the sky is the limit with some, either by improving the frequency and/or timings, which is another form of extracting more performance on its own, again with the x299 you will have plenty of bandwidth already (almost x2 compared to dual chanel) so probably having good timings will give you the most benefits at some point, and you need good modules to get great timings as well, it's not really different in this regard than increasing frequency.

But yes higher frequency kit have "slower" latency however it's compensated by the raw speed. Basically your true latency is about the same at 1600Mhz CL8 vs 3200 CL16 in DDR4 but you have doubled your bandwidth in the meantime.
Great article on that matter : http://www.crucial.com/usa/en/memory-performance-speed-latency



All this might in the end depends on what you will use the most on this build. Some softwares like frequency some others are more latency happy. For gaming you are mostly improving your minimum framerate under cpu bound scenarios. Content creators type of software can show very significant differences with better ram as they are always only limited by the cpu.

 

[tomb75]

Thanks again for further explanations and interesting article! I really appreciate it.

I think I will try to balance performance and my budget. For now, I'm aiming at 32 GB, 3200 MHz, CL14. I do not play games too much, so it is not the target here. I need to speed up my current 5-years old PC: i7-2600K @ 4400 MHz, 8 GB RAM. I'm focused on developing/compiling C++ projects on Linux virtual machine, while creating content in the background on Windows (includes x264/x265 encoding). I'm also planning to use some ramdisk and fast M.2 drive. I hope new build speeds up everything at least twice.

Edit: I'm also interested in 10GbE LAN for near future, that's why I want this mainboard.