AMD Ryzen 9 3900X Linux Memory Scaling Performance

[microcode]

You can see how proud they are of Zen 2, they shipped out the full review kit to Michael, incl. the Trident Z Royal DIMMs.

[profoundWHALE]

Whoa! What happened to the Apache 3800MHz benchmarks?

[shmerl]

Yes, CAS 14 @ 1.6GHz has lower latency than CAS16 @ 1.8GHz, but that only matters for codes that are bottlenecked on memory latency. Codes that are bandwidth limited will run better on the higher bandwidth memory.

So, games can still benefit from using 3600 MHz / CAS16 rather than 3200 MHz / CAS14? Games usually are RAM bandwidth sensitive.

[BillBroadley]

14 / 1600 * 1000 = 8.75 ns.
16 / 1800 * 1000 ≈ 8.89 ns.

So If I understand it correctly, 3200 MHz RAM with 14 CAS latency should perform better than 3600 MHz one with 16 CAS. Though I've never tested that, would be interesting to confirm.

Yes, CAS 14 @ 1.6GHz has lower latency than CAS16 @ 1.8GHz, but that only matters for codes that are bottlenecked on memory latency. Codes that are bandwidth limited will run better on the higher bandwidth memory.

[audir8]

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.

The same effect is visible for the timed LLVM compilation in the geometric mean chart.

[audir8]

This is very helpful for those of with older boards and RAM considering upgrading. This shows the variance at a fixed timing, and shows the combined effect of IF speed and RAM speed scaling together. At 4-8% on Zstd and LLVM compilation, it definitely makes me feel better about using 2666/2933/3200 RAM with an R9.

I think the internal improvements that AMD has made with IF in reducing latency are really showing here. Great job AMD.

[shmerl]

The point was just to to test memory scaling, which is exactly what was tested and is in fact informative.

I meant more for choosing which RAM to get for better performance. Memory scaling with the CPU is surely informative on its own too.

[schmidtbag]

Seems like Zen2 is less hungry for memory bandwidth than previous generations. Though, part of me questions if that giant cache has something to do with it.

So you used the same memory kit running it at different frequencies? I don't think this is very informative. Since different RAM can also have different latency for different frequencies.

The point was just to to test memory scaling, which is exactly what was tested and is in fact informative. In a controlled experiment, using the same DIMMs at the same frequency makes the most sense, where it shows how much frequency alone has an impact. The point of the test wasn't to show the maximum possible memory performance you could get, because Michael isn't known for pushing hardware to its limits; that's not why we come here.
That being said... it would be nice seeing the difference between timings, too.

[S.Pam]

So you used the same memory kit running it at different frequencies? I don't think this is very informative. Since different RAM can also different latency for different frequencies.

I.e. let's say you have 3200 MHz dual channel RAM with 14 CAS latency, and 3600 MHz one with 16 CAS.

So timing usually is calculated as CL / (single channel frequency) * 1000.

I.e.:

14 / 1600 * 1000 = 8.75 ns.
16 / 1800 * 1000 ≈ 8.89 ns.

So If I understand it correctly, 3200 MHz RAM with 14 CAS latency should perform better than 3600 MHz one with 16 CAS. Though I've never tested that, would be interesting to confirm.

Very spot on. Is really like to know how timings affect too. Most off the cheaper range nodules have much higher timings.

[shmerl]

So you used the same memory kit running it at different frequencies? I don't think this is very informative. Since different RAM can also have different latency for different frequencies.

I.e. let's say you have 3200 MHz dual channel RAM with 14 CAS latency, and 3600 MHz one with 16 CAS.

So timing usually is calculated as CL / (single channel frequency) * 1000.

I.e.:

14 / 1600 * 1000 = 8.75 ns.
16 / 1800 * 1000 ≈ 8.89 ns.

So If I understand it correctly, 3200 MHz RAM with 14 CAS latency should perform better than 3600 MHz one with 16 CAS. Though I've never tested that, would be interesting to confirm.