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What is a reliable, but not too time-consuming way to determine if a certain game is limited by the graphics card or by the processor speed?

I have some preconceptions about whether I'm mostly GPU or CPU limited, but I'd like to verify if I'm right about that. I have some ideas how I would go around and test that but performing accurate benchmarks is notoriously tricky and I don't have much experience with it.

So I'm wondering what would be an easy way to determine the bottleneck, having only one computer with a certain configuration available? What tools would I use for that purpose?

It would also be interesting to determine if the amount of VRAM on my graphics card is a limiting factor.

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I wish I could upvote this question more then once, I would love to see a great answer for this –  Aardvark Dec 2 '10 at 19:47
    
one thing that I noticed was the WC3 insane AI depends on how fast your CPU is. I can usually defeat two of them on my slow computer but not on my fast one :D –  bronzebeard Dec 3 '10 at 6:40
    

5 Answers 5

up vote 15 down vote accepted

Two rules of thumb I used to use:

If increasing the resolution brought about a large drop in the frame rate, it could indicate that the game was GPU-bound, as the increased resolution makes the video card work much harder and, as a result, takes longer to get each frame out the door.

On the other hand, if increasing the resolution brought about only a negligible decrease in frame rate (or no decrease at all), then it was an indicator that the game was CPU-bound, as the additional video complexity was easily handled by spare video card processing capacity whereas the main game processing logic already pegged the CPU. In this situation, the game engine is too busy to be able give the video subsystem enough work.

However, I don't think games nowadays are so simple that the performance is easily limited like that. For many years now, GPUs have gotten more complex and developers are finding ways to offload more work to them, so there's a lot of work that could be done on either side... this is why I don't use these rules of thumb very much anymore - they just don't really apply. The assumption was that raising the resolution did not cause additional work on the CPU.

If you followed those video card benchmarks from Tom's Hardware Guide religiously in those early 2000's years, you could see this sometimes - running e.g. Quake 3 using a "modern" video card and you'd get so many bar charts that just flatlined.

If you can measure the frame rate of your game, give this a try and see what happens.

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IMO this is the best advice so far. –  Jeff Atwood Dec 3 '10 at 8:23

from a purely observational perspective (not checking other applications etc) I believe that if you are CPU bound you'll find the that there is a lot more frame stutter as the GPU waits on the CPU, where as if you are GPU bound there will be a more consistent experience, albeit slow and laggy.

this could all be hooey though.

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As reducing the screen resolution and/or texture details options is pretty much guaranteed to improve the performance of any game, this can't be used to determine whether it's GPU bound or not. You could look to see what the highest resolution/texture detail level is available for your system.

If you could reduce the performance of your CPU (underclocking?) then this might give you some indication of whether the game was CPU bound - but again I don't think it can be 100% reliable.

There's also a blurring of what's done by the game engine (CPU) and what's done by the rendering engine (GPU). In the late 1990s/early 200s game physics used to be done by the CPU, but then graphics cards started being capable of performing these calculations on dedicated hardware thus speeding them up and improving performance. This means that seeing how many objects are moving on the screen (for example) can't be used as a guide to how powerful a CPU you have as a lot of the motion might be being controlled by the GPU based physics engine.

One thing to bear in mind is that due to the wide range of hardware that PC games have to run on game developers will be on the look out for any tricks that can improve performance and also the game will (hopefully) degrade gracefully so that it's playable on the lower end machines. This means that if you have the hardware available it will get used, if not to the utmost, then pretty close.

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Underclocking is the correct term. –  jjnguy Dec 2 '10 at 21:36

You could open the Windows Task Manager (the one you can bring up through Control+Alt+Delete), and then minimize it and play your game for a bit. Then exit your game and look at the CPU usage in the performance tab (you can resize the monitor to see more of the trend). Check to see if your CPU usage seems to stay around 100% or much less.

I know there are other utilities that let you view CPU usage, as well as RAM usage and disk IO, that you could try. I don't know if there is one to monitor graphics card performance though.

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Depends on the graphics cards, but for the reasons mentioned in other questions, this is not a good way to identify where a game is bottlenecking. –  ewanm89 Dec 3 '10 at 1:58

Couple of years ago I would definitely tell that by playing with the resolution: if the performance varies in direct relation to resolution, then it's GPU bound, if not, it's CPU, but it's a different story now, as shaders are in the play. Some of the shader effects can't be just toned down by lowering the resolution, they just grab the same amount of resources no matter what.

Some graphics options concern only the CPU, some only the GPU, and some a mixture of both (smoke effects are a common example).

Lowering the resolution is still a rule of thumb, it just doesn't work as well as it did before.

But if I had the money, I would invest on a better GPU than a faster CPU, because there are extras you will never achieve without more GPU power, like Anti-Aliasing and Anisotropic Filtering.

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