DDR4 compatibility ...

[Konrad]

Desktop DDR4 is currently supported by only one platform, of course: Intel X99 Wellsburg chipset with a Haswell-E processor. Most X99 mobos have 8 physical DIMM slots, some have 4, 12, even 16. Most can address 64GB, some only 32GB and others up to 128GB or more. There basically are no low-grade cheap X99 mobos, they're all excellent, they're all robustly overengineered (and overpriced) for enthusiasts.

My question is: can I combine two identical 4x8GB(DS) DDR4-3000 15-15-15-35 1.35V kits (G.Skill F4-3000C15Q-32GRK x 2)? And yes, 64GB is excessive, but I often run virtual machines and processes which actively leverage more cores/threads and RAM (plus I typically allocate a RAM Disk), and my 32GB does bottleneck. Indeed, addressing too much RAM can actually slow performance down if all you do is gaming. But I do far more than just gaming and I want the fastest 64GB I can get.

I realize that many people combine mismatched kits to save money, and many select incorrect parts based on price rather than being carefully about compatibilities and timings. My focus is more on performance than price - factory-matched 8x8GB DDR4 kits are available, but the fastest of these (that I could find) is DDR4-2800 15-16-16-35. My understanding is that factory memory profiles (especially among the dozens of tertiary timings) can vary between "identically matched" kits, and - worse! - that timings are already restrictive and rated in isolation from other onboard memory.

And I realize that 3000MHz is a fairly extreme DRAM frequency (the highest available is 3333MHz, perhaps 3400MHz, but at even higher cost and only at lesser 4x4GB capacities). And I may be forced to balance underspec XMP speeds against processor speeds, especially if I push an aggressive overclock.

But can matched sets of DDR4 be installed with a not-too-unreasonable expectation of it just working (like DDR3 usually does)? Or can all 64GB be made to work through some tweaking of timings, even if they need to be loosened up or throttled down a bit? Or is the entire idea a simply incompatible fail just asking for headaches? (I have an Asus R5E mobo and i7-5960X, if that matters.)

I could find no (believable) online data from people attempting to do this. I'm kinda looking at you enterprise pro/IT guys for a better answer (or at least for a better-informed guess). DDR4 RDIMMs have already been around for a while, I notice, and can't be all that different (in terms of basic technical limits) than their DDR4 UDIMM counterparts.

[x88x]

While I have not worked with DDR4 specifically, everything I know of the patterns followed for the last decade+ says that there is no reason I can imagine why the two kits together would not "just work".

I would guess that the motherboards with a lower stated maximum RAM capacity are stated as such simply because the manufacturer did not test it with any higher capacities. In the absence of a bespoke RAM controller, as long as you have the same CPU, chipset, and DIMMs, they should behave (roughly) the same.

As for what the performance difference will be with two kits vs one kit, I would suggest contacting the RAM manufacturer directly. If you feel that the potential difference in those tertiary timings would impact your use-case, you are in a small enough percentage of the userbase that your questions are likely not going to be answered short of talking directly with their engineers.

[Airbozo]

I see no issues running 2 of those kits.

On the server space, we even mix and match manufacturers, and so long as they are confined to one part number per channel, I have had no issues. I've done this in a system running 1.5TB. In fact, I've used 8gb dimms from one manufacturer in one channel and 16gb dimms from a different manufacturer in another. I have been told, but have not tried, that you can even mix and match manufacturers in each channel so long as you make sure the chips on the dimms are the same, and the capacity is the same.

[Konrad]

Well, your advice is far more optimistic than I expected. I didn't expect it to be any sort of real problem either, I was surprised when I first discovered it could even be a problem (here). To be fair, the bulk of my information comes from overclocking/enthusiast sites, where all too many people have money to burn and not enough mental energy to learn what they really need to know. While I am indeed burning a wee bit too much money on a single system I am getting value in return, and I'm unwilling to risk system stability (along with an important work tool) to push a few more MHz I don't absolutely need.

I hope my 64GB DDR4-3000 works out lol. (Too bad them heatspreaders are so damned ugly.)

[Airbozo]

As I always tell people when buying memory; make sure you can return it and run stress tests on them to make sure the system is stable.

The stress tests uncovered one unscrupulous vendors habit of relabeling memory thinking no one would notice... Now we run similar stress tests on all our systems before shipping.

BTW: I kind of like those heat spreaders. My case is tucked into my desk so it wouldn't matter too much any way.

[Konrad]

I'm a no-window kinda guy, visual bling on my components isn't a major issue (so long as they aren't too disgustingly fugly, like the awesomely-efficient-yet-utterly-hideous Noctua NH-D15 cooler).

But if I had a choice, I would much prefer memory heatspreaders which look more like these. Or even no heatspreaders at all. No accounting for tastes, not even for my own.

I can indeed return my 4x8GB DDR4 kit if I choose. Losing a $127.50 restocking fee plus $122.43 for priority-shipping-to-USA - which is annoying, given that I could choose no-rush $5.99 shipping to me when I placed the order, curse you newegg.ca!

I don't personally consider a system "stable" unless it can run 100%+ stress tests (like multiple Prime95 instances or whatever) for a full 24 hours - uninterrupted by reboots, crashes, freezes, or thermal shutdown. I love my overclocks - who doesn't? - but no amount of performance is meaningful to me if I can't depend on it running reliably. Not just some gamer who thinks "stable" means being able to run a game for a few hours at a stretch between BSoDs, not some LN2-sputtering volt-modding lunatic who thinks "stable" means a finicky beyond-the-threshold system doesn't crash just barely long enough to screenshot a lucky record-breaking overclock.

Gonna be a tight fit underneath my monstrously huge Raijintek NEMESIS TISIS cooler, lol, but it should work out.

[richardlawson1489]

Is DDR4 is compatible with any CPU, motherboards?

[Konrad]

lol DDR4 is all you can run on new Intel CPUs and motherboards. It's a couple of years into an expected service life of about a decade. No "DDR5" or other standard has yet been officially defined or scheduled to replace DDR4, there's lots of speculative technical white papers (and actual consumer products!) out there which already use something new and/or better than DDR4, but DDR4 is here now and it'll stay around for a long time.

DDR3 is still widely available for AMD platforms and older Intel stuff. DDR2 is entirely obsolete and unavailable outside of a few very specialized niches.