GPU vs CPU Bound Performance and why

[trparky]

We know that when you game at 720p or 1080p games are more CPU-bound when it comes to game performance and when you upgrade 1440p or even 4K suddenly you're now GPU-bound. But why? What makes 720p or 1080p CPU-bound and 1440p and 4K GPU-bound?

 

[jboydgolfer]

At lower resolutions the GPU renders frames quicker and so the CPU is generally what is the culprit for slowing down the system, as the GPU finishes its work fast.

At higher resolution it takes longer and it's more work for the GPU to render those same frames ,so the CPUs inadequacy's are less apparent because now it has more time because the gpu is working harder and longer to render a greater amount of pixels or resolution

its best to keep in mind that its not a "bottleneck" that causes this, but rather, at lower Res your not fully using the abilities of strong GPU's, when swicthing out to higher Res, you ARE using that GPU's abilities, so i would say at lower Res a CPU isnt a bottleneck, but rather at lower Res, your GPU isnt performing to its abilities, but when res is increased, that is no longer the case.

 

[Shihab]

Bottlenecks.

To put it very, VERY simply: The CPU's role in the pipeline usually is independent of the resolution. Resolution becomes a limiting factor after we start rasterizing stuff (converting the purely numerical 3d representation of vertices into fixed arrays of colours -> pixels), and this process is entirely carried out on the GPU in hardware rendering.

So, the CPU's work remains the same at low resolution or high resolution. The metric usually used for this work is "draw calls." Draw calls is dependent mainly on how much stuff goes into your frame (verts/models, shaders, UI elements), this remains the same no matter the resolution.

Now, the GPU's job can be harder the more the res goes up, more tests to carry, more times to run the pixel shaders, more stuff to sample for the AA, etc. So if you push that pixel number high enough, the GPU starts to lag behind the CPU (whose job is mostly issuing orders).

GPUs are processing monstrosities, however, high resolution can put a pressure on them, but cut that number low enough, and they process through it lightning speed! Faster even than the CPU, hence the bottleneck shifts to the CPU, and the pipeline becomes "CPU-bound."

 

[Bill_Bright]

We know that when you game at 720p or 1080p games are more CPU-bound when it comes to game performance and when you upgrade 1440p or even 4K suddenly you're now GPU-bound. But why? What makes 720p or 1080p CPU-bound and 1440p and 4K GPU-bound?

It doesn't. That is, there is no Law of Physics or the like that "makes" this game more CPU bound than that game. It is simply a program development decision, based on what hardware capability the developers want to take advantage of, or which disadvantage (bottleneck, for example) they want to avoid.

If you want to pull travel trailer, you would use a truck. If you want to race around a race track, you would use a sports car. That does not mean you cannot pull a trailer with a car, or race a truck, but to take maximum advantage, you use the tool designed for the job.