Frequency vs Latency

[tonsofpuppies]

I'm going to be building my first PC in a few months and I'm trying to gather as much information as I can before I begin. I'm hearing conflicting things all over the internet about the importance of frequency vs latency and I'm hoping someone here can provide me with a more definitive answer. I'm looking at the Trident Z RGB memory (32GB) and there are several options within similar price points and I'm trying to find out which one will give me the best performance.

I've heard the formula for finding the "true latency" of RAM is CL / F x 2000. For example, 15 CL / 3000 Mhz RAM would have a TL of 10 nanoseconds.

Here are my different RAM options on Newegg Canada.

14 CL / 3000 Mhz - $589.00 = 9.3 ns
14 CL / 3200 Mhz - $669.00 = 8.75 ns
16 CL / 3466 Mhz - $612.00 = 9.23 ns
16 CL / 3600 Mhz - $669.00 = 8.89 ns
18 CL / 3866 Mhz - $732.00 = 9.3 ns

So if we look at the TL, it would appear that the 3866 Mhz RAM is the worst value, as it gives the same performance as the 3000 Mhz RAM, but costs $143 more. Is this correct, or have I been mislead? Which RAM would you recommend? I was leaning towards the 14 CL / 3200 Mhz, but I'm looking for guidance and suggestions. Thank you!

 

[InvalidError]

That's only the "first word" latency. Once you have the first word on its way, you have the burst length to consider too. If you have a burst length of 8 words, then you have to add 8/MT ns to the burst completion time.

At 3000-14, you have 9.3ns first word + 8/3000M = 2.7ns = 12ns burst completion
At 3866-18, you have 9.3ns first word + 3/3866M = 2.07ns = 11.37ns burst completion

So 3866-18 is ~0.63ns faster at completing 8-word (128 bytes) bursts - a typical cache row transfer.

 

[tonsofpuppies]

That's only the "first word" latency. Once you have the first word on its way, you have the burst length to consider too. If you have a burst length of 8 words, then you have to add 8/MT ns to the burst completion time.

At 3000-14, you have 9.3ns first word + 8/3000M = 2.7ns = 12ns burst completion
At 3866-18, you have 9.3ns first word + 3/3866M = 2.07ns = 11.37ns burst completion

So 3866-18 is ~0.63ns faster at completing 8-word (128 bytes) bursts - a typical cache row transfer.

How does the RAM I was leaning towards compare to the 3866-18? Apologies, but I'm a novice when it comes to this stuff (at least the in depth aspects). Would it be...

3200-14 = 8.75 ns + 8/3200 = 2.5 ns - TOTAL = 11.25 ns

In that instance, it seems the 3200-14 is the best performer.

EDIT: The 3600-16 comes in at 11.11 ns for the same price

 

[InvalidError]

Which one is best will vary on an application-by-application basis. Some workloads are particularly sensitive to first-word latency (ex.: code compilers), others are more sensitive to bandwidth, yet more favor a balance somewhere in-between. In many cases though, the performance gains get very small once you get past DDR4-2866 or so.

Another thing to keep in mind is that not all CPUs/motherboards will manage to reliably support having two DIMMs per channel if you are going to be using these on a mainstream dual-channel platform and the likelihood of running into issues increases the higher the memory speed is. The official DDR4 spec doesn't go beyond 2400MT/s in dual-channel configuration, anything beyond that is non-standard bonus.

 

[tonsofpuppies]

Which one is best will vary on an application-by-application basis. Some workloads are particularly sensitive to first-word latency (ex.: code compilers), others are more sensitive to bandwidth, yet more favor a balance somewhere in-between. In many cases though, the performance gains get very small once you get past DDR4-2866 or so.

Another thing to keep in mind is that not all CPUs/motherboards will manage to reliably support having two DIMMs per channel if you are going to be using these on a mainstream dual-channel platform and the likelihood of running into issues increases the higher the memory speed is. The official DDR4 spec doesn't go beyond 2400MT/s in dual-channel configuration, anything beyond that is non-standard bonus.

I will be using the Z370 Aorus Gaming 7 motherboard (well, the Z390 Aorus Gaming 7 when it comes out, that's when I'll be building it), but the Z370 shouldn't be too much different. With an 8700K processor (or the 8 rumoured 8 core Coffee Lake if it becomes available later this year). My primary usage will be for gaming, both for playing and for game development (which I'll be starting university for in the Fall). Which do you think is the best choice?

EDIT: I've posted my parts list below for reference.

https://ca.pcpartpicker.com/list/8DNGP3

 

[InvalidError]

You really should consider a 2x16GB kit instead of 4x8GB kit to reduce your likelihood of getting compatibility issues at high frequencies. If you still insist on 4x8GB, at least check that the DIMMs you pick are on the motherboard's QVL (or vice-versa) as a quad-DIMM dual-channel set at the rated speed. Otherwise, you are throwing money at DIMMs which may not work in your PC - at least not at their rated speed and timings.

 

[tonsofpuppies]

You really should consider a 2x16GB kit instead of 4x8GB kit to reduce your likelihood of getting compatibility issues at high frequencies. If you still insist on 4x8GB, at least check that the DIMMs you pick are on the motherboard's QVL (or vice-versa) as a quad-DIMM dual-channel set at the rated speed. Otherwise, you are throwing money at DIMMs which may not work in your PC - at least not at their rated speed and timings.

download.gigabyte.us/FileList/Memory/mb_memory_z370-aorus-gaming-7.pdf

This is the QVL for my board.

UPDATE: This is the G.Skill page for the 3200-14 RAM, which shows the Aorus Gaming 7 listed under the Z370 boards, so I assume it's (fully) compatible?

https://www.gskill.com/en/product/f4-3200c14q-32gtzr

 

[InvalidError]

If G.Skill says this specific DIMM set has been tested with the Gaming 7, then you should be fine.

 

[tonsofpuppies]

If G.Skill says this specific DIMM set has been tested with the Gaming 7, then you should be fine.

Thank you for all of the info. I had no idea that all of this stuff even mattered. I was always under the assumption that DDR4 is DDR4 and is universally compatible. Based on my needs (playing games and game development), which set of RAM do you think would be the best performing?

I read on another message board that the CL 16 / 3600 Mhz RAM could also run at CL 14 / 3200 if desired, whilst it wouldn't work the other way around. If this is true, then obviously the 3600-16 is more versatile, but I'm not sure whether to believe it or not. I just know too little about RAM, unfortunately.

 

[InvalidError]

I was always under the assumption that DDR4 is DDR4 and is universally compatible.

The official (JEDEC) spec for DDR4 only goes up to 2666MT/s single-sided DIMM single DIMM per channel. Anything beyond that is technically overclocking and results will vary. (And with picky chips like Ryzen 1xxx, even JEDEC standard speeds can be elusive, especially prior to the AGESA 1.0.0.6 update.)

Which DIMMs will ultimately be able to get tweaked to what speeds and latencies depends on the quality of the individual dies and the DIMM PCB quality. As you wrote, 3600-16 may give you more freedom to experiment. Just as with everything when the silicon lottery is involved, mileage will vary - the DIMM manufacturer may or may not have binned (all) of their dies so tightly that consecutive models won't overlap in capabilities, albeit at the expense of some extra tweaking effort on the end-user's end.