Something I haven't seen mentioned yet is the fact that current-gen gaming CPUs may actually be holding back the RTX 5090, but not the RTX 4090, at least not in any noticeable way.
The TomsHardware GPU testing rig currently in use features an AMD 9800X3D, which is an excellent gaming CPU, but there’s an interesting point to consider when it comes to memory bandwidth. Based on AIDA64 benchmarks, with DDR5 6000 dual-channel RAM, the 9800X3D shows a read speed of around 62 GB/s, a write speed of 84 GB/s, and a copy speed of 59 GB/s.
For PCIe 4.0 x16, which maxes out at 32 GB/s unidirectional and 64 GB/s bi-directional, this isn’t really an issue. The available system memory bandwidth is comfortably above what the PCIe bus can handle, meaning there’s little to no risk of a bottleneck between the PCIe controller and the system memory. The CPU and memory can keep up with the data demands without any major slowdowns.
However, things start to look a little different when you consider PCIe 5.0 x16, which doubles the bandwidth to 64 GB/s unidirectional and 128 GB/s bi-directional. With PCIe 5.0 pushing such high data rates, there's a possibility that the memory subsystem may become a limiting factor.
If the GPU is trying to fully saturate the PCIe 5.0 link, the RAM's read and copy speeds might struggle to keep up, potentially causing a bottleneck between the PCIe controller and the system memory. This could lead to situations where the 5090 isn't able to fully stretch its legs, particularly in scenarios involving high data throughput.
In contrast, the 4090, running on PCIe 4.0, wouldn't face the same challenge because the available system memory bandwidth is nearly equal to what the GPU can demand via the PCIe bus.
What do you all think? Could the memory bandwidth limitations of the 9800X3D be a contributing factor here? Do you think that the 5090 might eke out a few more points if paired with a system with higher RAM bandwidths? Would love to hear others' thoughts on this.