With the release of the budget RX 5500 XT, AMD started to fill out the bottom two-thirds of its product stack, bringing much-needed competition into that segment. The graphics card (opens in new tab) arrived with 4GB and 8GB variants ($169 and $199 respectively), and in our RX 5500 XT review (opens in new tab), some titles--namely Forza Horizon 4, Battlefield V, Far Cry 5 and Shadow of the Tomb Raider--the 4GB variant performed considerably slower when running Ultra settings. The initial takeaway from this was the 4GB capacity is simply not enough for those titles, which is correct. However, there's more to it.
German website pcgameshardware.de (opens in new tab) recently tested both RX 5500 XT versions using PCIe (opens in new tab)3.0 and PCIe 4.0. The results were a bit surprising. They found that when the 4GB card's VRAM buffer was full, running on a PCIe 4.0 slot improved performance by reducing the impact of using VRAM. The 8GB also displayed minor improvements from PCIe 3.0 to 4.0, but the 8GB capacity is what makes that card shine, since the extra memory means the card didn't have to dump data off of the faster VRAM as often.
So why does this happen? Once the VRAM is filled up, the extra data is sent to the system RAM (opens in new tab)across the PCIe bus. The faster the bus (opens in new tab), the faster the swapped data will get where it needs to go and not delay the next frame. The 5500 XT is wired as a PCIe 4.0 x8 card (physical tab is x16), which yields the same theoretical bandwidth as PCIe 3.0 x16. According to the article, AMD said that is sufficient for a GPU of this performance class. For all intents and purposes, this is correct. However, for games that eclipse the installed VRAM we see it can fall short.
What pcgameshardware.de showed was that during memory reads and writes, the transfer rate was effectively cut in half. Whereas PCIe 4.0 x16 would reach 12.5 GBps, x8 only reaches around 6.5-6.7 GBps -- half the throughput. Memory copy speeds are not affected, as this is the transfer rate from the memory itself.
So what does this mean for performance? The improvements varied by title (and settings), but pcgameshardware.de tested Assassin’s Creed Odyssey, Battlefield V, Far Cry: New Dawn, Wolfenstein Youngblood and Shadow of the Tomb Raider. In each test, there were improvements in the 4GB card when it was running on a PCIe 4.0 system. Some were significant, others not so much. Assassin's Creed and Far Cry appeared to benefit the most, along with Wolfenstein Youngblood. The 8GB variant also showed slight improvements across the board, but not nearly as much.
In the end, it's a curious choice for AMD to wire these cards in an x8 configuration, particularly with the 4GB card, which has a greater chance of running into VRAM limits. While many titles will not run into these issues, those that do can take a severe performance hit.
With all of this information, I’m left wondering why AMD chose to configure the card in this manner? Cost perhaps? Increasing the performance of both cards can only help the optics, especially on the 4GB card, which is the most affected. This could have been spun as a reason to buy into the AMD Ryzen 3000 X570 ecosystem as a bright spot for PCIe 4.0. But instead, we are left wondering, why?