CPU<=>GPU bottleneck explained TL;DR for gaming

[191x7]

Since there is a lot of misinformation about CPU and GPU bottlenecks floating around, here's a short explanation for those who are wondering about CPU and GPU related bottlenecks for games.

 

A CPU prepares frames for the GPU to render. It does object placements, physics and general pre-render.

The GPU takes what the CPU prerendered and does the frame (image) render including the details, shading, textures, particles, "triangles", ...

 

Changing the resolution does not affect the CPU a lot. But it affects the GPU a lot since it changes the number of details.

 

EXAMPLE:

If a CPU can prepare 60 frames and the GPU can render...

1. 100 frames on 1080p, the GPU will be used 60% meaning a 40% bottleneck. A stronger CPU would be needed. Or increasing the details, resolution.

2. 60 frames on 1440p, there won't be a noticeable bottleneck and the CPU&GPU are evenly matched. A stronger GPU would be bottlenecked by the same CPU.

3. 30 frames on 4K, there won't be a bottleneck. And the CPU could power a stronger GPU.

 

Let's take a low end CPU like the Intel i5 10400F or Ryzen 5 5500, a mid-range CPU like the Ryzen 7 5700X or Intel i5 12400f and a higher end CPU like the Ryzen 7 7700 or Intel i7 13700.

And let's take a low end GPU like a Geforce RTX 3050 or Radeon RX 6500, a mid-range GPU like the RTX 3070Ti or the Radeon 6800 and a high-end like the RTX 4080 or the Radeon RX 7900XTX.

The monitor resolution of 1080p is considered low end, 1440p (2K) id mid-range and 4K (2160p) is high end.

Let's call the low end CPU CPU1, the mid-range CPU2 and high end CPU3.

Do the same for the GPU; low end GPU1, mid-range GPU2 and high end GPU3.

And the screen resolutions; RES1, RES2 and RES3.

 

We have 27 (3x3x3) possible combinations.

The low end GPU won't be bottlenecked by any of the CPU-s on any of the resolutions. That removes 9 options.

 

(CPU,GPU,RES) - what usually happens

(1,1,1) - no botttleneck

(1,1,2) - no botttleneck

(1,1,3) - no botttleneck

(1,2,1) - slight CPU bottleneck

(1,2,2) - no botttleneck

(1,2,3) - no botttleneck

(1,3,1) - huge CPU bottleneck

(1,3,2) - slight CPU bottleneck

(1,3,3) - slight to no CPU bottleneck

(2,1,1) - no botttleneck

(2,1,2) - no botttleneck

(2,1,3) - no botttleneck

(2,2,1) - slight to no CPU bottleneck

(2,2,2) - no CPU bottleneck

(2,2,3) - no CPU bottleneck

(2,3,1) - huge CPU bottleneck

(2,3,2) - slight to no CPU bottleneck

(2,3,3) - no CPU bottleneck 

(3,1,1) - no CPU bottleneck

(3,1,2) - no CPU bottleneck

(3,1,3) - no CPU bottleneck

(3,2,1) - no CPU bottleneck

(3,2,2) - no CPU bottleneck

(3,2,3) - no CPU bottleneck

(3,3,1) - huge CPU bottleneck

(3,3,2) - slight to no CPU bottleneck

(3,3,3) - no CPU bottleneck

 

A huge CPU bottleneck means a terrible time gaming (the GPU might enter low power states, having frametime spikes, ...) and a CPU upgrade is needed.

A slight CPU bottleneck means a CPU upgrade would be preferable. Or a monitor upgrade (higher resolution increases the GPU load).

 

And no, in 99.9% of the games a GPU does not do stuff that the CPU has to wait on. So, NO, in gaming a GPU CAN NOT BOTTLENECK A CPU.

 

PSA:

Online bottleneck calculators are usually made by people who do not understand how bottlenecks work.

Made purely to get money from advertisements and people who browse the page. Clickbait.

Worse than useless because it's better not having information than working with false information.

[andrewmp6]

at 1080p your cpu takes more load then the gpu,So the cpu will bottleneck first.At 1440p and 4k the load is pushed more gpu then cpu and the gpu will be the bottleneck.And yes your monitor can be another problem if its 60hz and you try to push 144fps at it.

[191x7]

21 minutes ago, andrewmp6 said:

at 1080p your cpu takes more load then the gpu,So the cpu will bottleneck first.At 1440p and 4k the load is pushed more gpu then cpu and the gpu will be the bottleneck.And yes your monitor can be another problem if its 60hz and you try to push 144fps at it.

It doesn't work like that.

 

The load isn't balanced or transfered between the CPU and the GPU, they do completely different work.

Every component has its own jobs.

 

As I've explained, the CPU does roughly the same always, the resolutions only affect the GPU.

[andrewmp6]

43 minutes ago, 191x7 said:

It doesn't work like that.

 

The load isn't balanced or transfered between the CPU and the GPU, they do completely different work.

Every component has its own jobs.

 

As I've explained, the CPU does roughly the same always, the resolutions only affect the GPU.

Yes it does,Why do you think when they test out cpus gaming they do 1080p and never 4k ?Or why does my cpu run at 5% or less and my gpu is 80% or higher when i game at 1400p ?

[andrewmp6]

 

[Winterlight]

In most case is not CPU bottleneck but game engines bottleneck due at best they can utilization only 4-6 cores. This days 12-16 cores is very common but games still can't take full advanged from all this cores. Btw if you run games with 12-16 core CPU they can't utilization even fully this 4-6 cores they almost never used 100% run about ~70%. Modern CPU this days have more than enough power problem that software are bottlenck too use all this power. That is misleading talk that CPU is bottleneck cause that bottleneck come from software side in most case. Sure 4-6 cores this days is bottleneck but than we talk about modern 12-16 cores bottleneck is not CPU itself but software limitation how it use that CPU. So sad part that most developer not even care too write very complex engine codes to fully take advantage from modern CPU is possible but take too much time and money to do that.

[191x7]

6 hours ago, andrewmp6 said:

Yes it does,Why do you think when they test out cpus gaming they do 1080p and never 4k ?Or why does my cpu run at 5% or less and my gpu is 80% or higher when i game at 1400p ?

Go read the first post again. I have explained it.

[andrewmp6]

6 hours ago, 191x7 said:

Go read the first post again. I have explained it.

No your numbers are backwards is the problem you seem to not understand.At 1080p the cpu load is 60% or higher not the gpu

[191x7]

1 hour ago, andrewmp6 said:

No your numbers are backwards is the problem you seem to not understand.At 1080p the cpu load is 60% or higher not the gpu

Are you telling me your CPU load increases with the decrease in resolution?

If that's the case, something is messed up with your system.

[starsmine]

17 hours ago, andrewmp6 said:

at 1080p your cpu takes more load then the gpu,So the cpu will bottleneck first.At 1440p and 4k the load is pushed more gpu then cpu and the gpu will be the bottleneck.And yes your monitor can be another problem if its 60hz and you try to push 144fps at it.

In RATIO sure. in absolute, no. 

the gpu isnt taking load FROM the CPU, the gpu is just taking on more load. the CPU load does increase with increase of resolution, just not at the same scalar. 

Hypothetical easy oversimplified numbers (or what is being asked of a part) Absolute does not matter in this example and real world numbers will vary on engine and game type.

720p
load on CPU is .9, load on gpu is .5

1080p
Load on CPU 1, load on gpu 1

1440p
load on CPU 1.1, load on gpu 2

4k
load on CPU 1.2, load on gpu 4

ignoring bubbles and stalls/memory bottlenecks.
The CPU never takes load from the GPU when you drop in resolution, its just not asking the GPU to render as much. 

The ratio of the load is what changes, not one taking load/work from each other.
 

@191x7
1440p/1600p is not 2k, and never has been. 

2k, is 1080p, its the 1920 part of the resolution
or rather 2048*1080 DCI

4k is two 2k wide (and also tall), aka 4k. (or rather 4096x2160 DCI)

Also your conclusion of a GPU can not bottleneck a CPU is buck wild. 
If the CPU can produce more frames then the GPU can. the CPU is bottlenecked by the GPU. Which is what happens for most people. If the CPU can do 200FPS, and the GPU can only do 50FPS, so you only display 50FPS, what is being bottlenecked? I understand your point that the CPU isnt being stalled by the GPU for the most part, but the fact is remains that the majority of the work work being done is largely being discarded and wasted. 
And its also a lie. GPUs often do part of the physics calculations, engine dependent. GPGPU is being done. PhysX is a thing. Thats why CUDA C, DirectCompute, OpenCL are things, used in game engines, and we now can leverage AI GPU features for... game AI. 

[Guest]

https://www.intel.com/content/www/us/en/gaming/resources/what-is-bottlenecking-my-pc.html#:~:text=This means the GPU is,happen in the opposite direction.

 

 

[PriitM]

You all talk like you know how game assets are handled. The compression rates, chunk sizes. What task is multi thread or single thread. And you all act like the PCI lanes cannot be a bottle neck which is handled by the motherboard. Either to fetch data from the SSD or pass it through between CPU to GPU.

Its not as clear cut as: get faster CPU, performance goes BRRR.

Hope this brings some of you down to earth and looking at the bigger picture.

[tikker]

On 3/16/2023 at 1:43 PM, 191x7 said:

EXAMPLE:

If a CPU can prepare 60 frames and the GPU can render...

1. 100 frames on 1080p, the GPU will be used 60% meaning a 40% bottleneck. A stronger CPU would be needed. Or increasing the details, resolution.

The bolded part is a little bit confusing I think. In this example the CPU is still limited to pushing out only 60 FPS. Putting more load on the GPU won't increase the CPU's ability to send frames to the GPU, so it's unlikely to improve performace in that aspect, but if you do this sufficiently such that the GPU becomes the bottleneck then I could see some benefit from the CPU not being completely floored.

On 3/16/2023 at 1:43 PM, 191x7 said:

3. 30 frames on 4K, there won't be a bottleneck. And the CPU could power a stronger GPU.

There won't be a CPU bottleneck, but there is still a bottleneck here. The GPU is bottlenecking the CPU, since it can't match the potential 60 FPS that the CPU in this example is capable of.

 

On 3/16/2023 at 1:43 PM, 191x7 said:

A huge CPU bottleneck means a terrible time gaming (the GPU might enter low power states, having frametime spikes, ...) and a CPU upgrade is needed.

A slight CPU bottleneck means a CPU upgrade would be preferable. Or a monitor upgrade (higher resolution increases the GPU load).

You'll only increase GPU load as you say. From the way you describe a CPU bottleneck this will only help if you make it so demanding to render that the CPU cannot reach its 60 FPS anymore and the GPU becomes the bottleneck again.

On 3/16/2023 at 1:43 PM, 191x7 said:

And no, in 99.9% of the games a GPU does not do stuff that the CPU has to wait on. So, NO, in gaming a GPU CAN NOT BOTTLENECK A CPU.

Your 3rd example is that of a GPU bottleneck though. If the GPU cannot render frames at the rate the CPU can provide that means the GPU is the bottleneck. For gaming that just happens to generally be the bottleneck you want, because that means your using everything your GPU has to offer, that less demanding games might benefit from your CPUs ability to run higher FPS or that a GPU upgrade will still benefit you.

On 3/18/2023 at 7:41 PM, 191x7 said:

Are you telling me your CPU load increases with the decrease in resolution?

If that's the case, something is messed up with your system.

While resolution doesn't directly impact CPU load, it does impact framerate which in turn impacts CPU load.

 

I think the most important aspect about bottlenecks is to realise that there will always be one in some regard. It's about where it is and whether that is an acceptable one to you, and balancing the various components to get the performance that you want for your use case.