NVIDIA is working to support Smart Memory Access (SAM) on its GeForce RTX 30 series graphics cards, otherwise known as “resizable BAR”. As per Team Green, the feature is being tested internally and the results are similar to what AMD has claimed with its Big Navi GPUs.
This info was shared by GN, according to whom NVIDIA is working on a SAM alternative that will support the RTX GPUs across both AMD and Intel CPUs. However, it’ll still be a while before we see it implemented on the Ampere GPUs. In case you’re wondering, it doesn’t require PCIe 4.0 support and is part of the PCI Express spec.
A while back this info was also shared by Nemez on Twitter. According to the GPU enthusiast, the PCIe spec by default only allows 256MB of memory (or I/O region) per PCIe device, with one of them being the VRAM. You can resize it to the full size of the VRAM, but it requires PCIe BAR resizing (as mentioned above) and 4GB+ decoding.
Furthermore, most modern OS like Windows and Linux already allow you to enable it with some tweaking, and should work with most CPUs and GPUs launched in the last 5-7 years. AMD simply supports it without doing the need for any modifications on its Zen 3 and 500 series board.
The main advantage of a resizeable PCIed BAR is that most applications (games) frequently assign the VRAM buffers into their address space. This is made possible by the driver which constantly swaps or reassigns this 256MB buffer which can be a minor bottleneck in some workloads.