Bottlenecks

[jamesgoddard]

It seems every other post these days is will xxx cpu bottleneck yyy graphics card, well folks you (yes you the poster) are the only ones who can really answer your own question as there is not black and white answer, everything is dependent on what app / game and what settings you wish to play on. So without further ado, the following is the simple guide to establishing if one component will bottleneck the other (CPU / GPU).


Take the game you wish to play / are having performance issues with and set the graphics setting to the lowest possible option (800x600 with everything turned off if needs be) - and see if you have acceptable performance. At this low setting you will almost always be CPU limited, unless you have a really mismatched GPU, and this can be taken as a base line. If performance at this setting is OK, then your CPU is fine, period, end of.

Next step up the graphics to max you are comfortable with frame rates, as the frames drop you are now GPU limited, and if the performance drops off to far before you get to the resolution you are aiming for, then yes a new graphics card should be on the shopping list.

But - on the other hand if the fames remain stubbornly low whatever resolution / graphics you choose, then the CPU is your next purchase.

Simple!

 

[hallowed_dragon]

But - on the other hand if the fames remain stubbornly low whatever resolution / graphics you choose, then the CPU is your next purchase.

The frames should remain steady to see a CPU bottleneck at any resolution compared to the first 800x600 run.

First run: 800x600 : 100FPS
Second run: 1024x768 : ~100 FPS
Third run 1280x1024: ~100 FPS
....
nth Run: 1920x1200: ~ 90-100 FPS ====> CPU holding back GPU.

 

[jamesgoddard]

But - on the other hand if the fames remain stubbornly low whatever resolution / graphics you choose, then the CPU is your next purchase.

The frames should remain steady to see a CPU bottleneck at any resolution compared to the first 800x600 run.

First run: 800x600 : 100FPS (CPU limited)
Second run: 1024x768 : ~100 FPS (CPU limited)
Third run 1280x1024: ~100 FPS(CPU limited)
....
nth Run: 1920x1200: ~ 90-100 FPS ====> GPU holding back CPU.

Corrected the last bit

 

[hallowed_dragon]

Corrected the last bit

Actually no. If the CPU can't generate more than 100 frames worth of information then the GPU will not output more than 100 frames. If you OC the CPU and get more frames it is clear that the CPU was holding back the GPU.

 

[jamesgoddard]

Actually no. If the CPU can't generate more than 100 frames worth of information then the GPU will not output more than 100 frames. If you OC the CPU and get more frames it is clear that the CPU was holding back the GPU.

I agree with your statement there - but you seem to be confirming what I just posted - confused

 

[hallowed_dragon]

I agree with your statement there - but you seem to be confirming what I just posted - confused

Even at 1920x1200 the CPU only gave ~100 FPS and thus the GPU only rendered ~100 FPS, the result being the same as the 800x600 resolution. I agree that at that high resolution the GPU works harder than the CPU, but that doesnt mean that there is no CPU bottleneck. In most cases the CPU is still bottlenecking the card, although only with 10 FPS max at that resolution, but what I wanted to show was the evolution of the FPS in all resolutions and skipped a few steps. If the FPS dont drop from 800x600 to 1680x1050 or 1920x1200 the CPU is clearly to blame and an OC would solve in most cases minor bottlenecks and a new CPU purchase higher one.

 

[jamesgoddard]

Even at 1920x1200 the CPU only gave ~100 FPS and thus the GPU only rendered ~100 FPS, the result being the same as the 800x600 resolution. I agree that at that high resolution the GPU works harder than the CPU, but that doesnt mean that there is no CPU bottleneck. In most cases the CPU is still bottlenecking the card, although only with 10 FPS max at that resolution, but what I wanted to show was the evolution of the FPS in all resolutions and skipped a few steps. If the FPS dont drop from 800x600 to 1680x1050 or 1920x1200 the CPU is clearly to blame and an OC would solve in most cases minor bottlenecks and a new CPU purchase higher one.

Yes - thats exactly what I said....

'But - on the other hand if the fames remain stubbornly low whatever resolution / graphics you choose, then the CPU is your next purchase.'

 

[hallowed_dragon]

Yes - thats exactly what I said....

'But - on the other hand if the fames remain stubbornly low whatever resolution / graphics you choose, then the CPU is your next purchase.'

Long day at work...my IQ dropped for a few minutes. We were saying the same thing but with different words Nice

 

[jitpublisher]

I have tried this sort of post before, no one listens.

Hmmm, I think the problem is no one really understands all they know about this.........

 

[jamesgoddard]

I just really don't like the sweeping statements that some people make that X cpu will bottleneck Y GPU...

And some people seem to have a fear that if they get it slightly wrong, one componant will not be able to 'feed' the other and they end up slower than were they started... No - logic goes that if you upgrade a componant, the worst case senario is you stay exactly where you were.

 

[4745454b]

Don't forget other things can cause a bottleneck as well. For example, if your using only one or two gigs of ram, then you might not see the performance you were expecting. Going out and buying a new CPU won't help at all. If your thinking you need a new CPU to help with your load times while still using that old 120GB IDE drive, you will again be buying something you don't need. A third problem could be with a bad bios, or one with bad values loaded. Don't forget about improper HSF installation causing thermal throttling. You can't just compare frame rates at different resolutions and say its the CPU or GPU. You have to step back and look at the whole picture as well.

 

[pauldh]

Take the game you wish to play / are having performance issues with and set the graphics setting to the lowest possible option (800x600 with everything turned off if needs be) - and see if you have acceptable performance. At this low setting you will almost always be CPU limited, unless you have a really mismatched GPU, and this can be taken as a base line. If performance at this setting is OK, then your CPU is fine, period, end of.

Simple!

I've seen this stated before, but there's a huge flaw to the logic as a CPU capable of minimum in-game details is NOT fine for maximum in game details. Physics effects, draw distance, number/level of NPC's, Cars, etc. will massively increase demands on the CPU. Shoot, often single-core CPU's would pass your test here, and fail miserably when all sliders go to the right.

I applaud your attempt, and agree on using resolution to test for CPU limitations. Keep in mind though, even different aspect ratios will have some effect on CPU limited situation as what's viewable (and calculated/rendered) changes the wider or narrower the aspect ratio (see last chart here): http://www.tomshardware.com/reviews/amd-cpu-overclock,2304-7.html

 

[pauldh]

I just really don't like the sweeping statements that some people make that X cpu will bottleneck Y GPU....

Agreed!

And some people seem to have a fear that if they get it slightly wrong, one componant will not be able to 'feed' the other and they end up slower than were they started... No - logic goes that if you upgrade a componant, the worst case senario is you stay exactly where you were.

Mostly agree here, although we have something coming up on Tom's soon that will show this isn't necessarily true.


Like Stranger, I'm one who has typically avoided using the work bottleneck. For reasons mentioned, I prefer to talk about CPU/system limited or GPU limited situations rather than a "bottleneck".