Originally Posted by Mahigan
That's actually not true. Fiji uses HBM like embeded cache, so even if a game uses more than 4GB of VRAM, it doesn't impact performance. Read up: http://m.hardocp.com/article/2016/02/29/rise_tomb_raider_graphics_features_performance/14#.VtWOyeTS9oM
This myth has been dispelled. For AMD, 4GB HBM is enough for 4K.
HBM is different than GDDR5. Its bandwidth is used like an embeded cache. Anything that requires quick access, sits in the HBM, other lower bandwidth sensitive storage needs go into the System RAM.
It works, 0 performance hit. The myth has been finally put to rest.
All VRAM is used for anything that "requires quick access". That's the entire point of VRAM (local, high-speed GPU memory), because discrete GPU use of system RAM has both horrendously worse latency and bandwidth. All discrete GPUs can access system RAM (equally quickly too), it doesn't lend HBM-equipped cards any exclusive advantage.
4 GBs is 4 GBs and HBM is not utilized any different from GDDR5 just like various GDDR5 implementations of disparate bus sizes and frequencies don't inherently differ from one another in capacity due to resulting bandwidth.
Considering the 512 GB/s of Fiji's HBM-provided bandwidth refers only to communication speed between the GPU and VRAM directly, it provides no advantage whatsoever for Fiji's ability to access system RAM and therefore does not reduce the amount of data Fiji must hold in VRAM at any given time. AMD's direct quote skirts around capacity by simply highlighting HBM's bandwidth/performance (which is flat-out irrelevant to capacity; their analogy of SSDs and HDDs of equivalent sizes are limited to the same amount of stored data for instance) and by referencing aforementioned driver-based optimizations AMD are doing to avoid the capacity's limitations such as this part:
Memory compression, buffer allocations, and caching architectures all impact a game’s memory footprint, and we are tuning to ensure 4GB will always be sufficient for 4K gaming.
Fiji's bandwidth due to HBM is literally irrelevant to system RAM accessibility (which is instead governed by PCI-E speeds) and any optimizations AMD make in where they allocate data (VRAM vs system RAM) would be equally as possible on any GDDR5-based GPU.
Whether Fiji's VRAM capacity is particularly relevant to RoTR, I don't know, but 4 GBs really is 4 GBs (a hard limit on the amount of data that can be stored locally; i.e. in VRAM).Edited by Serandur - 3/1/16 at 6:05am