CPU and GPU Bottleneck Explained

Hi there,

I've recently had a bit more time to devote to PC gaming as a whole and found lost in the huge variety of available CPUs and GPUs and their impact on gaming. Back in the day when I did my share of gaming what mattered most was the GPU, now it seems the CPU matters a lot as well!

I've put the sample below together to try and summarise bottlenecking as I understand it. See what you think and if you agree with conclusions and findings. Perhaps it will help you understand a thing or two as well. Have fun!



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Bottleneck Test Configuaration
Assumes a Mid-Range CPU and a Powerful GPU
***

***
Assumptions:
Changing Graphic Settings in a game does not increase CPU Load. CPU load remains static for any game played, regardless of Settings.
GPU Load increases as Settings are increased.
***

***
Test Games:
Various Low and High CPU and GPU Intensive Games
***


***
Test Settings:
LOW: Details LOW Resolution LOW
MED: Details MEDIUM Resolution MEDIUM
MAX: Details MAX Resolution MAX
***



>CPU Bottleneck - Scenario1:<

Game1 - Very CPU Intesive:
LOW Settings : 15FPS
MED Settings : 15FPS
MAX Settings : 15FPS

Conclusion: Max _CPU_ output in Game1 is around 15FPS. This is limiting the System's performance to 15FPS, even though the GPU in this Scenario is able to output far more FPS.

NOTE: Game is NOT playble with this CPU. CPU bottleneck is too high.
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>CPU BOttleneck - Scenario2:<

Game2 - Both GPU and CPU Intensive:
LOW Settings: 50FPS
MED Settings: 50FPS
MAX Settings: 30FPS

Conclusion: Max CPU output in Game2 is 50 FPS. GPU output in lower resolutions is higher than 50FPS and therefore the CPU is bottlenecking the System in lower resolutions. However with MAX settings enabled, the GPU can no longer match CPU's output of 50FPS and the GPU becomes the bottleneck. Upgrading the GPU will increase performance up to the CPU bottleneck of 50FPS.

NOTE: Overclocking the CPU or replacing it with a higher clocked version would NOT casuse the system to run faster in MAX settings, where GPU remains the bottleneck. It would however increase LOW and MED FPS if more FPS is desired.
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>CPU Bottleneck - Scenario3:<

Game3 - Highly GPU Intensive:
LOW Settings: 30FPS
MED Settings: 20FPS
MAX Settings: 10FPS

Conclusion: Max CPU output in Game 3 is unknown, as the GPU is the limiting factor. Every increase in Graphic Settings brings a decrease in FPS. The GPU is not able to handle the CPU input fast enough. With a faster GPU even performance would increase in every Setting.
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>CPU Bottleneck - Scenario4:<

Game4 - Moderately GPU and CPU instensive:
LOW Settings: 100FPS
MED Settings: 100FPS
MAX Settings: 60FPS

Conclusion: Both CPU and GPU bottleneck occurs. The CPU is unable to output more than 100FPS in this scenario. However when the Resolution and Details are increased to MAX, the GPU is no longer able to process CPU information fast enough and it becomes the bottleneck.

NOTE: Although in lower resolutions the Graphics card can be considered bottlenecked by the CPU, the actual performance of the system is high enough to make any CPU bottleneck irrelevant.
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FINDINGS
When attempting to avoid a CPU bottleneck, it is very important to select a CPU that will provide your desired MINIMUM output in games in terms of FPS. Assuming a CPU is able to output at least 60 FPS in your favourite game, the maxiumum output of your GPU matters less. The game will remain very playable at maximum settings.
As long as the CPU meets your game's Recommended requirements, it will be able to output enough information to the GPU to make the gameplay smooth with Maximum settings enabled, whether it is bottlenecking the GPU or not.


How to use this information in practice to find your bottlenecks?

0. Establish the target FPS you'd like to achieve, 30FPS or 40FPS depending on the game.

1. Choose a scene from your favourite game. Make sure the scene requires a lot of GPU and CPU power to process and that you can control what's on the screen. That means plenty of NPCs (preferably doing something), lots of Lighting effects, and 3D objects to render. I've found the Market Square in Denerim in Dragon Age great for that purpose, or the Werewolf hideout.

2. Reduce all graphic settings you can think of to the absolute minimum. No AF/AF, lowest resolution, everything on Minimum or turned off. Check your FPS. Chances are the FPS you are seeing is the maximum your CPU is able to output in the game. With such low settings, the GPU has hardly any work to do and it will be able to process a LOT of FPS. Anything over 60 means your CPU is not the problem. Even if it is bottlenecking your GPU a little, it still allows for 60+ FPS which is good.

3. Crank up settings to the MAX with high resolution and AA/AF. See if the FPS is as high as in stage 2. Chances are it won't be unless you spent A LOT on the GPU . This means your system is bottlenecked by the GPU.

4. Now you have two options. Start lowering the Resolution or the graphic settings. Let's go with the first option.
We know that the CPU is able to send around 60FPS worth of data to the GPU from our low settings test. Keep lowering the resolution until you reach 60FPS. At this point, with the settings that you have, you're back to being limited by the CPU.

It's not easy to find out exactly how much the CPU is bottlenecking your card in terms of numbers of FPS because a) there is no way to increase the CPU load in a game through settings and b) there is no way to increase CPU output other than overclocking. Overclocking is your best best of course to test exactly how much FPS you're losing due to the CPU bottleneck.

 

Alright this is the easiest to tell if your GPU is the bottleneck.

If turning down Texture, Game details or eye candy in the setting increases your FPS, then it's GPU not CPU.

That simple.

 

So you had to go through all these scenarios to conclude this? Maybe you should explain a bit more of what were you aiming at, otherwise I think you only created a complex logic for a simple problem.

 

Ok more simply. CPU is rarely the bottleneck.

The only time you see this being spammed is when people are using poor GPU. When using a poor GPU, CPU can make a difference. But when you use the "RECOMMENDED" GPU, CPU is rarely the culprit in current modern PC games.

Bad Company 2 has been getting a lot of forum posting time about it's intense CPU usage.

WRONG WRONG WRONG WRONG WRONG WRONG WRONG WRONG WRONG WRONG WRONG WRONG WRONG WRONG WRONG.

And WRONG for one more time.

http://www.techspot.com/article/255-battlefield-bad-company2-performance/page3.html

Using the SAME i7 920 with different GPUs, you can CLEARLY and DEFINITIVELY see that GPU is the culprit for bad performance in BC2.

Test System Specs
- Intel Core i7 920 (Overclocked @ 3.70GHz)
- x3 2GB G.Skill DDR3 PC3-12800 (CAS 9-9-9-24)
- Asus P6T Deluxe (Intel X58)
- OCZ GameXStream (700 watt)
- Seagate 500GB 7200-RPM (Serial ATA300)
- Radeon HD 5870 (1GB) Crossfire
- Radeon HD 5870 (1GB)
- Radeon HD 5850 (1GB)
- Radeon HD 5770 (1GB)
- Radeon HD 5750 (1GB)
- Radeon HD 5670 (512MB)
- Radeon HD 5570 (1GB)
- Radeon HD 5450 (1GB)
- Radeon HD 4890 (1GB)
- Radeon HD 4870 (1GB)
- Radeon HD 4850 (1GB)
- Radeon HD 4830 (512MB)
- Radeon HD 4770 (512MB)
- Radeon HD 3850 (512MB)
- GeForce GTX 285 (1GB)
- GeForce GTX 275 (896MB)
- GeForce GTX 260 (896MB)
- GeForce GT 240 (512MB)
- GeForce 9800 GT (512MB)
- GeForce 9600 GT (512MB)
- GeForce 8600 GTS (256MB)



BC2 CPU Overclocking

As you can see in order to see discernible difference with CPU overclocking, you have to overclock the 920 from a whopping 2.2 to 3.14 Ghz. From 2.2 to 2.9 there is no discernible difference.



But as you see here, with higher res, NO DIFFERENCE.



Dual Core vs Quad Core. As you can see if you have the right GPU, BC2 will not even need the full power of your dual core.

Dual Core CPU Usage



Quad Core CPU Usage. Quad is barely being used...

 

Hey Ziddy, nice graphs there but that doesn't prove BFBC2 isn't getting CPU bottlenecked does it? You've showed one of the best CPUs available on the market right now, what about previous generation lesser clocked CPUs like a 2.33Ghz C2Duo or Quad.

Let's take the Radeon 4850 from your first example, sure it does 35FPS and that's the limit of that card with an i7 CPU @ 3.70Ghz. That only proves this card can push 35FPS Max.

Question is can it still push 35FPS with a 2.33Ghz Core2 Duo? Probably not right, depends how much FPS that CPU is able to achieve regardless of GPU used.

Obviously with a 3.70Ghz core i7 there will only be a GPU bottleneck it's one of the best CPUs available on the market now!


In the last example, of course all the FPS is going to be the same regardless of CPU - a resolution of 2560x1600 is causing an obvious GPU bottleneck of 74FPS. Lower the res a little and the same GPU will get 25FPS more with a 3.70Ghz as opposed to 2.40Ghz for example, thus getting badly CPU bottlenecked when a 2.40Ghz is installed. NOt that it matters as it's all above 60FPS, but still

 

I've done this as much for myself as anybody else, to better understand every aspect in every scenario.

It's easy to understand your CPU is good enough if it meets the recommended specs for a game .

The real question was however, does it make sense to invest in a powerful CPU to utilise the MAX FPS your GPU could potentially provide.

From what I've gathered so far, the answer is no, it doesn't. Sure you get higher MAX FPS but there's not much practical difference between 60 and 120 as far as the human eye is concerned

 

it's a pointless conversation going on here, really. Most laptops, even gaming ones, get GPU bottlenecked way before they hit CPU bottleneck (GTA4 out of equation), as simple as that

 

Very good Spartan82, I applaud you for writing this.

 

Spartan the first CPU graph is a Core 2 Duo Dual Core. There are no issues.

Also there is a graph, I'll have to look where it shows when using a i7 920 and a E series Core 2 Duo (Dual Core), the difference in FPS is TWO!

And if you READ the link I posted, you will see that they saw NO difference in performance when they locked down a i7 Quad down to 2 cores.

So as Njoy said, I believe this is a pointless thread as the CPU will almost never be the bottleneck, especially for Notebooks.
- Intel has done much better job of bringing performance to notebooks than either ATi or Nvidia.

 

chill winstaaan, this may be of help to someone even if it is of no help to you

 

I've read the article carefully, and it's been very educational. It's a fact that top-clocked core i7 gains FPS, however like you say the gain means little in terms of actual performance.

The purpose of the article I wrote is to help people understand how bottleneck works in terms of CPU capability. I've said myself that it doesn't make much sense paying for a powerful CPU to gain more FPS in games. It just doesn't scale well enough to matter.

Here's another set of benchmarks regarding Dual and Quad core CPUs with BFBC2.

http://translate.google.cz/translat...ompany-2-fyzika-predevsim?start=5&sl=cs&tl=en

Basically, in Low to Med resolutions, @ 2.4Ghz a Core2Quad gains around 10 FPS over a Core2Duo.

However, as long as you have ANY Quad Core (or a really fast Dual), your CPU is fine. In High resolutions 1650x1200 and 1920x1200 the FPS difference between a 2.4Ghz Core2Quad and a 3.5Ghz i7 920 is on average 2 (that's TWO) FPS.

 

I do believe this is the general consensus.. Laptop GPU's are just not powerful enough as desktop ones.