It’s amazing, but AMD has done it again… They have managed to trick their customer base into believing yet another bit of nonsense about AMD’s hardware.
This time it is about HBM. As we all know, AMD has traded in GDDR5 memory for HBM on their high-end cards, delivering more bandwidth. The downside is that with the current state of technology, it is not feasible to put more than 4 GB on a GPU. This while even AMD’s own GDDR5 cards already have 8 GB on board, and the competing nVidia cards are available with 6 or even 12 GB of memory.
So far, so good. Now, the problem is that AMD somehow brainwashed their followers into believing that more bandwidth can compensate for less capacity. So everywhere on the forums you read people arguing that 4 GB is not a problem because it’s HBM, not GDDR5.
The video memory on a video card acts mostly as a texture and geometry cache. For the GPU to reach its expected level of performance, it needs to be able to access its textures, geometry and other data from the high-speed memory on the video card, rather than from the much slower system memory.
As long as your data fits inside video memory, the bandwidth determines your performance. However, as soon as you run into the capacity limit of your video memory, you need to start paging in data from system memory. Since system memory is generally an order of magnitude slower than video memory, the speed of the video memory is completely irrelevant. The speed at which the data can be transferred to video memory is completely bound by the system memory speed.
So, HBM does in no way make paging data in/out to system memory any faster than any other memory technology would. Therefore the only performance problem we’re dealing with here is the point at which the paging becomes necessary. Which is solely dependent on capacity. A card with 4 GB will hit that point sooner than a card with 6, 8 or 12 GB. And when that point is hit, performance will become erratic, because your game will have to page textures periodically, resulting in frame drops. That should be pretty easy to understand for anyone who bothers to think it through for a few moments.
The one thing you can say is that because the initial performance is higher, it can ‘absorb’ a frame drop slightly better. That is, if you only look at average framerates. If you look at frame times, you’ll still see nasty jitter, and the overall experience will be far from smooth. You will be experiencing stutter every time the system has to wait for a new texture or other data to be loaded.