Comment by 0xQSL

Well there's much more to memory performance than bandwidth. Generally applications are relatively cache friendly, thus the X3D helps a fair bit, especially with more intensive games (ones that barely hit 60 fps, not the silly game benchmarks that hit 500 fps).

Generally CPUs have relatively small reorder windows, so a cache miss hurts bad, 80ns latency @ 5 GHz is 400 clock cycles, and something north of 1600 instructions that could have been executed. If one in 20 operations is a cache miss that's a serious impediment to getting any decent fraction of peak performance. The pain of those cache misses is part of why the X3D does so well, even a few less cache misses can increase performance a fair bit.

With 8c/16 threads having only 2 (DDR4) or 4 (DDR5) cache misses pending with a 128 bit wide system means that in any given 80-100ns window only 2 or 4 cores can continue resume after a cache miss. DDR-6000 vs DDR-7800 doesn't change that much, you still wait the 80-100ns, you just get the cache line in 8 (16 for ddr5) cycles @ 7800MT/sec instead of 8 (16 for DDR5) cycles @ 6000MT/sec. So the faster DDR5 means more bandwidth (good for GPUs), but not more cache transactions in flight (good for CPUs).

With better memory systems (like the Apple m3 max) you could have 32 cache misses per 80-100ns. I believe about half of those are reserved for the GPU, but even 16 would mean that all of the 9800X3Ds 16 threads could resolve a cache miss per 80-100ns instead of just 2 or 4.

That's part of why a M4 max does so well on multithreaded code. M4 max does better on geekbench 6 multithread than not only the 9800x3d (with 16 threads) but also a 9950x (with 16c/32 threads). Pretty impressive for a low TDP chip that fits in thin/light laptop with great battery life and competes well against Zen 5 chips with a 170 watt TDP that often use water cooling.

For AMD I think Infinity Fabric is the bottleneck so increasing memory clock without increasing IF clock does nothing. And it's also possible that 8 cores with massive cache simply don't need more bandwidth.