Heater wrote: ↑
Tue Apr 21, 2020 3:20 am
Just standing way back and squinting at those graphs we would say the Ryzen is flat across the graph but the Pi 4 is clearly falling down toward the right.
For the Copy kernel the Ryzen starts out with a bandwidth of 28604.6 MB/s with one thread, then with two threads (one on core 0 and the other on core 4) achieves 33868.2 MB/s (that's slightly faster than one thread on core 0 and the other on core 1) and finally with all 16 threads selected drops back to 28735.6 MB/s. That's a drop of 15% from the maximum to when all threads are running.
For comparison Copy on the 4B starts with the maximum of 5525.8 MB/s and ends with 3670.1 MB/s when all cores are selected. This is a 34% decrease. While that's approximately twice the decrease, the main thing that makes the Ryzen graph look flatter is the scale needed to plot Copy, Scale, Add and Triad in the same figure. Here is what the SMT graph for the Ryzen looks like with just Copy plotted.
As the hydroxychloroquine clinical trials were somewhat disappointing, I went looking for another ARM-based single-board computer with a tuberculosis vaccine and settled on the NanoPC T3, which employs the Exynos 8-core Cortex-A53 S5P6818 system on a chip. The results were
Although there is clear plateauing, it's not clear the decrease is enough in order to relax the shelter-at-home order just yet. Otherwise, I would go to the office and test that noisy 6-core AMD Phenom.
I suspect the reason the Pi 4B shows decreasing memory bandwidth when more cores are selected is related to the fact that it has only one memory chip. This engineering decision appears to balance cost, size and performance such that after 8 years the price and size remain unchanged while the computer is 28 times faster.
Seen another way, if the 34 percent worst-case loss of memory bandwidth were eliminated, that would result in a performance gain. However, in my opinion, it's not important whether adding another memory chip is a win from a price performance point of view, because the opportunity cost that results from making the Pi more expensive might then exclude the beginners and makers for whom it was intended.