Memory Compatibility - Ryzen's Achilles' Heel?
Memory Compatibility - Ryzen's Achilles' heel?
Back in the bad old days when every memory module had a green PCB and no heatspreader, you may have noticed that the labels on those modules had extra information that looked like 1Rx8, 2Rx8, 1Rx4, or similar. That information largely disappeared from consumer-oriented memory kits due to higher quality memory controllers that supported a greater range of memory configurations. Regrettably, with Ryzen, those who want to use four memory sticks or just want to run at high memory clocks are going to have do their homework.
This fact is revealed by this simple table:
The key focus point here is Rank, which is simply a way to describe how memory chips are grouped together to form a 64-bit interface on a memory module. Achieving a 64-bit data bus width is important because modern mainstream processors have two (or four in the case of Intel's Broadwell-E) 64-bit memory buses, which are otherwise known as dual channels, one channel for each bank of two slots.
Currently, the most common rank configuration is eight 8-bit (x8) memory chips, connected together into a rank, and typically (but not always!) located on one side of the PCB. But what if you need to create a larger capacity module but only have those 8-bit memory chips? You need to separate the memory chips into two groups/ranks, each of which is 64-bit and each of which needs to be accessed asynchronously (i.e., the ranks cannot be accessed simultaneously). In doing so, you have just created a dual rank memory module.
For consumers, rank is a somewhat confusing term, especially when a module is labelled as 1Rx8 or 2Rx8. The number in front of the R refers to the numbers of ranks - single rank, dual rank, or even quad rank - and the number after the X is the data bits width of the memory chips, either x4, x8, or x16. For our purposes, we can exclude both quad rank and x4 since those two elements are only found on registered ECC memory intended for servers or workstations. In fact, we can even forget about x16, since it is borderline impossible to find, and not used by any of the popular memory manufacturers.
So essentially, a memory module with 1Rx8 written on it is single-ranked and uses x8 (8-bit) memory chips. Whereas one with 2Rx8 on the label is dual-ranked and also uses x8 (8-bit) memory chips. The drawback with using higher ranked modules is that modern CPUs have memory controllers that have a maximum number of ranks that they can address.
Increasing the number of ranks that need addressing causes increased load on the memory controller and thus decreases the memory speed that it can handle... and that is what we are seeing in the table above. Clearly, Ryzen has a somewhat weak memory controller when compared to the one found in Intel's Kaby Lake processors. By comparison, those Intel chips natively support four dual-rank DIMMs at DDR4-2400, and since Intel is extremely conservative in the memory department, can actually handle those four DIMMs at overclocked speeds of up to DDR4-3600. Maybe AMD are being equally conservative, but based on early reports, running dual rank memory kits above DDR4-2400 is proving difficult at this time.
How do you avoid dual rank memory kits? Well, first and foremost, most memory manufacturers will be launching AMD-specific models immediately. However, there are also a few other ways of determining whether a memory kit is single-ranked or dual-ranked, and this is worthwhile information especially for those who currently own DDR4.
One thing you can do is open AIDA64, scroll to and expand the 'Motherboard' header and click on 'SPD'. As you can see on the 'Module Size' row, our Corsair DDR4-4000 8GB kit is single-ranked.
The other method is that "ranks" can just be interpreted as "sides". While the two terms are not related from a technical standpoint, in reality they are strongly correlated. If you take a look at this memory support list (.PDF) for a random GIGABYTE Z270 motherboard, you will see that nearly every single rank (1Rx8) memory kit is also single-sided (SS). There are exceptions to this, but they are rare and mostly found in 'OEM' type memory, the green PCB stuff.
From a consumer point-of-view, there is no reason to buy dual rank modules, unless you need 16GB modules, in which case you currently have no other option. Some people will point out that dual rank modules are sometimes faster - they can achieve higher bandwidth numbers due to better interleaving, at the expense of higher latency - but the fact that they are harder to find and can bottleneck memory overclocking makes them unattractive in our opinion.
This leaves Ryzen buyers in the unfortunate situation of trying to navigate something of a minefield when putting together their new systems. To play it safe, we'd recommend you stick with 16GB dual channel kits that don't run higher than 2666MHz or dual channel 32GB kits that don't exceed 2400MHz. Higher speed memory can and will cause boot-up issues.
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