AMD Zen 2 Memory Performance Scaling with Ryzen 9 3900X 106

AMD Zen 2 Memory Performance Scaling with Ryzen 9 3900X

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Our first impressions with 3rd generation Ryzen's memory overclocking capabilities are very optimistic. The processor is capable of memory frequencies well beyond the DDR4-3600 most 2nd generation Ryzen chips cap out at as stability of the Infinity Fabric interconnect beyond that point starts to buckle. Frequencies such as DDR4-4000 require a lot of skill and a lot more luck with the older Ryzens. Achieving this frequency with Zen 2 is fairly straightforward, which has AMD win with memory overclocking headroom. In our main review of the Ryzen 9 3900X, we dive deep into the processor's architecture, going more into detail on the memory, too. One of the things we explain is that decoupling memory frequency from that of the Infinity Fabric interconnect will pay big dividends when it comes to memory overclocking.

Overall, when looking at the application averages, there's barely a five percent performance gain to be had going from DDR4-2400 to DDR4-3600. Once you take a closer look at the individual tests, it becomes apparent that some apps simply like high memory clock, whereas others prefer tighter timings over the highest clock, and then there are those with barely any performance differences between memory speeds at all.

Games are a different story from applications as they tend to be more memory bandwidth hungry. Maybe counterintuitively, it's the FPS rate that drives the memory requirements instead of the resolution. At higher resolutions like 1440p and 4K, the GPU is the limiting factor, and the CPU is running at a lighter load than at lower resolutions. For each frame, the game has to calculate many things and adjust the scene geometry; as FPS goes up, so does the load. At 720p, there is a 10 percent performance increment to be had when going from DDR4-2133 to DDR4-3200. When looking at the more real-life 1080p resolution, the gap is still 7%. Depending on your choice of graphics card, gaining that much FPS with a memory upgrade could actually end up being cheaper than buying a faster graphics card. There are of course exceptions on both sides. Some games prefer bandwidth to latency, while others prefer latency, with games generally favoring bandwidth.

Just for kicks this time around, we threw in a single-channel DDR4-3200 configuration. This is what you'd end up with if you're only using one module or didn't install your two modules in the proper slots. Much to our surprise, the performance hit is much less than expected. One possible explanation for this could be the "unganged" memory controller topology of AMD processors, which favors physically independent 64-bit wide paths to each memory channel instead of blindly interleaving the two channels like Intel does. We would still definitely recommend you to stick to dual-channel configurations. There's no shame in reading your motherboard's manual to find out which memory slots to use for dual channel. Another demerit of choosing one 16 GB module over two 8 GB modules in dual channel would be that dual-rank modules continue to be a problem area for AMD.

DDR4-4000 and beyond, while easy to do on these processors, isn't worth the effort unless you have decent memory modules that are rated for it. With Infinity Fabric clocks not scaling beyond 1700–1800 MHz, the gains from extremely high memory speed are almost certainly not worth the added cost. With DRAM prices in free fall, you may get lucky, tough. If you have the money, we recommend you pick up a memory kit that's rated for decent frequencies out of the box, just so you can run them at DDR4-3600 on a tight leash, so your machine enjoys both good bandwidth and low latencies. If money is tight, we suggest you aim for a DDR4-3000 kit, which is only marginally more expensive than DDR4-2400 or DDR4-2666. According to our data, DDR4-3000 also emerges as the price-performance sweetspot for Ryzen with at least 2% performance gained over DDR4-2400. If you're a memory overclocking newbie and the sheer number of settings on the AMD platform scare you, TechPowerUp has you covered. We recently published a definitive guide for Ryzen Memory Overclocking which will be updated very soon for the 3rd generation Ryzen processor family.

We did run into some problems when playing with the settings, though. The latest BIOS of our ASRock X570 Taichi motherboard has a strange bug that overrides your CAS latency input sometimes. For example, CL17 could simply not be set, and it would instead apply 18T—the same happened in Ryzen Master. We do, however, appreciate the gargantuan number of settings in the BIOS and the Ryzen Master utility, which we're sure an overclocker with the right skill can make the most of. The most useful of these settings is the ability to toggle DRAM and IF clock domain linking. At lower memory frequencies, you'd ideally want IF and DRAM to run at the same clock speed for best latencies. Only when your DRAM real clock is too high (2000 MHz in pursuit of DDR4-4000, for example) would you need to unlink them. We also hardly ran into any compatibility issues with our memory kits. While our Flare X kit uses AMD-friendly Samsung B-die chips, the Trident-Z doesn't, and we could hardly feel any difference playing with them—seems AMD hugely improved memory compatibility over the years.
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