AMD today launched its 3rd generation Ryzen desktop processor family. Be sure you catch our full reviews of the Ryzen 9 3900X and Ryzen 7 3700X processors. In addition, we bring you a number of feature articles around these processors. This one studies the all-important memory subsystem and investigates how performance on Zen 2 scales with memory performance and timings. Ryzen processors in the AM4 package feature a dual-channel DDR4 memory interface. The 1st and 2nd generations of Ryzen were the first AMD processors to support DDR4 and had many quirks with their memory configuration and overclocking capabilities.
Memory has been both the Achilles heel and the greatest performance contributor for Ryzen so far. On the first two generations of "Zen," memory clock is synchronized with the Infinity Fabric frequency, and overclocking memory beyond a point would destabilize the vital on-die interconnect, which limited memory overclocking headroom compared to Intel platforms. On the other hand, turning up memory clock would tangibly benefit performance since Infinity Fabric bandwidth would proportionately increase, enabling faster data-transfers between the various on-die components—most importantly, the two quad-core compute complexes (CCXs).
With the 3rd generation Ryzen "Zen 2" processors and AMD's decision to disintegrate the memory controller from the die that has CPU cores, the memory finds itself unshackled from other low-frequency clock domains. AMD is also motivated to improve this memory controller since the doubling in CPU core-count over generations increases the demands on memory bandwidth. This dual-channel DDR4 memory controller is located in the processor's 12 nm I/O controller die (highlighted in the picture above). The two 7 nm 8-core CPU chiplets talk to this die over 100 GB/s Infinity Fabric links. At a topology-level, AMD detains the dual DCT approach to dual channel, which the company would refer to as an "unganged" interface, with two independent 64-bit paths to the memory modules instead of a blindly interleaved interface. This greatly benefits high core-count scenarios, particularly with the Ryzen 9 series chips.
The Infinity Fabric frequency can now either be synchronized to memory clock or unlinked from it, using various dividers. This will come in handy when running memory clocks higher than DDR4-3600, at which point the Infinity Fabric link on older generations of Ryzen processors would become unstable—on Zen 2, simply reduce the Infinity Fabric frequency and you're good to go. AMD has also introduced a plethora of new memory timing and voltage control settings that are designed to improve memory compatibility at high clock speeds.
In this memory scaling review, we examine the memory performance of the flagship Ryzen 9 3900X 12-core/24-thread processor across seven memory clock settings: DDR4-2400, DDR4-2666, DDR4-3000, DDR4-3200, DDR4-3600, and DDR4-4000, as well as DDR4-3200 running in single-channel mode. The higher memory frequencies are extremely difficult to achieve and hold on the older generations of Ryzen. DDR4-4000 is also the gateway to the enthusiast frequency band running between 4000 to 5000 MHz. AMD processors now theoretically support memory multipliers all the way up to DDR4-5000, and this speed has also been demonstrated at Computex.
Unfortunately, we were not able to test the DDR4-3733 "sweet spot" configuration recommended by AMD because our CPU simply was not able to run at the required 1867 MHz Infinity Fabric—1800 MHz was the highest, which we're covering with the DDR4-3600 test.
As for the lower frequencies, DDR4-2400 is a baseline frequency that's almost a fail safe even if above 2133 MHz. DDR4-2666 is the JEDEC-standard frequency for DDR4 and currently the spec offering the most affordable $/GB memory kits. DDR4-2933 is the recommended memory OC frequency for Ryzen Threadripper processors. DDR4-3200 has been the recommended single-rank memory OC frequency since 1st generation Ryzen and is what we use throughout all our processor reviews. DDR4-3600 is an achievable single-rank memory OC frequency for 2nd generation Ryzen. DDR4-4000 is the ticket to the enthusiast segment. It will take pure skill, luck, and time to crawl beyond this frequency.