Memory bandwidth vs. size

[OrionAntares]

I've been trying to find a clear explanation of how the memory channel bandwidth vs. the memory size effects game performance but haven't really been able to nail down a good enough understanding of the full effect. I know that if the channel size is too low it will effect performance but what I'm trying to figure out is once past a certain point for the channel width what the effect of having more bandwidth or more memory is.

A simple example being a 448-bit channel with 896MB verse a 256-bit channel with 1GB of memory.

 

[theevilsharpie]

Memory bandwidth has a direct effect on all aspects of the performance of the video card. The amount of memory on the card doesn't directly impact performance as long as the data the video card needs to process fits entirely within the video card's onboard memory.

 

[OrionAntares]

Would the size of the memory have a greater impact than the channel width if it would be the difference between the memory being able to hold all the information the GPU needed?

 

[theevilsharpie]

Yes. If the data set is too large to fit in the card's onboard memory, it has to use system memory, which has substantially less bandwidth than even the cheapest discrete video cards.

 

[betasub]

Memory bandwidth is generally defined by interface width (bits) multiplied by effective clcok speed (Hz). Thus an 256bit interface paired with very fast clock speed (e.g. 2+GHz DDR5) can provide as much bandwidth as a wider interface with a slower clock speed (e.g. 1+GHz GDDR3). Although more bandwidth is better, most high-end cards have more than sufficient (diminishing returns, "core" limited).

As theevilsharpie describes, there should be sufficient memory capacity/size (bytes) to allow all details (e.g. textures) to fit within it - otherwise system memory will be required (slower and with much higher access latency). The memory size required depends on the app/game and detail/image settings, but for most cases there is unlikely to be a difference between 896MB and 1GB.

 

[Denithor]

Think of bandwidth as the speed limit of the video card while the memory size is the number of lanes. Bandwidth affects how fast the stuff is rendered while memory size only comes into play if capacity is exceeded (traffic jam).