I also use two exactly identical 4x8GB DDR4 kits I purchased together, they even have adjacent serial numbers.
Here's Raja's infamous
Don't combine memory kits! thread. Short version: DDR4 kits are designed (and binned) to work as a unit, they can't even be combined with other identical memory kit(s) without throwing all their fine timings and voltages out of whack. I've learned that "identical" DDR4 kits can each have wildly different tertiary timings, the memory factory programs dozens and dozens of settings into the SPD which an end-user simply cannot improve any further.
I offer a lengthy explanation of
how Haswell-E iMCs work here. I was never really able to get my 32GB DDR4-3000 kits to work well with my i7-5960X proc, I wasted a *lot* of time tweaking and retweaking yet never got it to run stable (let alone to run fast) for long, in hindsight I almost wish I'd just bought a single 32GB or 64GB kit. My E5-1680-3 proc runs at a flat 4.4GHz ("Turbo Boost always ON" = Enabled, lol) and addresses all 64GB of DDR4 at full-rated 1500MHz/3000MTps speeds.
Korth wrote:
While my old i7-5960X processor was capable of a ~1.3V 4.5GHz overclock (on air), it would only do so with one of my DDR4 kits running at 2133 16-18-18-36. XMP was useless.
At stock 3.0-3.5GHz, I could get one DDR4 kit to run at 2866 14-15-15-35, or both kits together to run together at 2133 17-18-18-36.
It turns out my i7 was a mixed bag: a fairly decent overclocker, a pathetic iMC.
Following advice from online guides like this one, I "burned and binned" my 8 DDR4 DIMMs. I found 5 were superior, 1 was decidedly average, 2 (one from each kit) were underspec weaklings - more or less typical results for these kits. Dropping the two loser sticks out allowed me to OC my proc to around 4GHz and still run the other 48GB at rated 3000 15-15-15-35. Much time wasted testing and tweaking and fiddling of voltages.
I later traded my i7 for a similar E5 proc. The guy who got the i7 brags that it runs 4.7GHz (on water) with 4x4GB DDR4-3333 at full spec. The E5 I got can achieve a modest overclock but has a powerful iMC which happily blasts all 64GB of my DDR4 at full spec - basic XMP, no tweaks, no fiddles.
So - for my particular parts, anyhow - the processor determines all. A strong iMC in the proc can even handle borderline memory sticks. A weak iMC in the proc will make it near impossible to run a single high-frequency kit at rated speeds.
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[/Korth]
