AMD Ryzen 9 3900X Linux Memory Scaling Performance

[existensil]

most of them continued scaling to DDR4-3800 nicely though in cases like some of the RAMspeed runs the DDR4-3800 performance came in slower than DDR4-3600 due to higher latencies past DDR4-3733

You are misidentifying the performance issue above DDR4-3600. The regression isn't related to increased latency in memory above that speed, but is instead caused by the dramatic reduction in the speed of the infinity fabric above DDR4-3600. From Anandtech:

AMD had published an interesting slide in regards to the new faster DDR support that went well above the officially supported 3200 speeds, with AMD claiming that the new controllers are able to support up to DDR4-4200 with ease and overclocking being possible to achieve ever higher speeds. However there’s a catch: in order to support DDR4 above 3600, the chip will automatically change the memory controller to infinity fabric clock ratio from being 1:1 to 2:1.

The big reduction in performance with Apache Siege leads me to believe that benchmark involves a heavy amount of cross-core communication and is saturating the infinity fabric, especially when it's nearly halved in speed.

[tildearrow]

Possible typo:

most of them continued scaling to DDR4-3800 nicely

(it only did until 3600)

[phoronix]

Phoronix: AMD Ryzen 9 3900X Linux Memory Scaling Performance

For those wondering if upgrading your RAM to higher frequency DIMMs is worthwhile when moving to AMD X570 and a new Zen 2 processor like the Ryzen 9 3900X, here are some reference benchmarks at different frequencies while maintaining the same timings.

http://www.phoronix.com/vr.php?view=28044