Radeon RX 7900 XT GPU Dissection Reveals A Hidden Surprise, 3D V-Cache Anyone?
Previous rumors have suggested that AMD could boost its GPU portfolio by adding a stack of 3D V-Cache, a performance enhancing technique established with its Ryzen CPUs. Now, an engineer with access to micrographic infrared imaging equipment has discovered that the existing Radeon RX 7900 XT GPU appears to be 3D V-Cache ready. Specifically, it was observed that the silicon features an array of ‘spots’ very similar to the TSV (Through Silicon Via) connector array on an AMD X3D CPU.
Over the weekend, semiconductor packaging engineering professional Tom Wassick commented upon his initial findings as he put an AMD Radeon RX 7900 XT GPU “under the knife.” Thankfully, there would be no scalpels involved, as he was really talking about infrared imaging, but the operation was still very revealing.
Currently, the Radeon RX 7900 XT GPU (also sometimes called the Navi 31 XT GPU) has a large central GCD (Graphics Compute Die) with all the processing components within, flanked by six MCDs (Memory Cache Dies). Wassick’s IR investigations saw that the RX 7900 XT had one dummy MCD. However, more interestingly and as per our headline, an MCD studied using IR featured “a linear array of ‘spots’ that look remarkably like the keep out zones on X3D, and that are on the same 17-18 um pitch,” according to Wassick. Wassick does not appear to have shared any images beyond the board-level view we have reproduced up top.
Wisely, Wassick doesn’t assert that this is conclusive evidence indicating AMD is planning to vertically stack MCDs in some future product refresh. At the time of writing, though, that is all AMD uses this 3D stacking technique using TSVs for—adding cache. Additionally, we must consider that the ‘spots’ could have been used in prototyping and testing, but may never be used as TSVs in a shipping product, at least for this generation.
AMD has previously stated that it will leverage the best technologies seen in its highly successful Zen CPU architectures within its GPU designs. We saw this happen in RDNA 3 with the adoption of chiplets, and it would not be very surprising to see AMD’s successful foray into adding 3D V-Cache to Ryzen and Epyc CPUs repeated in the GPU lines.
AMD CPUs with 3D V-Cache have seen some impressive gains in specific workloads. On the Ryzen 7 5800X3D, 7800X3D and Epyc Milan-X processors the total L3 cache was tripled thanks to a generous slice being stacked onto the chip. In AMD’s latest Ryzen 9 7900X3D and 7950X3D processors, the L3 cache was doubled using this technique. Performance gains observed with CPUs wielding 3D V-Cache can be as high as 50%, depending upon workloads, and if even half such gains could be mirrored in GPUs it would be an irresistible improvement for AMD to implement.
At the current time, we don’t have any solid data about the scale of improvements versus extra cost the addition of 3D V-Cache could mean for AMD’s RDNA 3 GPUs. We must also remember that stacking this cache on top of the CPUs has a thermal impact, with X3D chips running slower and hotter than their ‘ordinary’ brethren. All things considered, while we remain in the dark about AMD’s tests and plans with GPUs and 3D V-Cache, it is gratifying to know what the firm is very likely testing these ideas to find ways to disrupt the GPU business.