Overclock.net › Forums › Graphics Cards › Graphics Cards - General › GPU usage & processor bottleneck
New Posts  All Forums:Forum Nav:

GPU usage & processor bottleneck

post #1 of 4
Thread Starter 
Hi,

I have been doing some processor testing for my article recently and have a question about GPU usage. So basically it looks like GPU usage 100% is only relative to the current platform. For example i7 3770K + GTX 680 will give me 100 fps in some specific location in Crysis 3, Afterburner reports GPU usage 99%. But if I put my GTX 680 to a different platform with AMD Phenom I get 80 fps in the same location but Afterburner also reports 99% GPU usage.
But it is only the graphics card that produces the frames per second, right? Processor itself can't produce any extra frames it only delivers its calculations to the graphics card. So if it delivers it on time and GPU does not have to wait then I get GPU usage 99%, but if GPU waits for the processor then GPU usage drops and fps drops. In this scenario overclocking my processor increases GPU usage and increases fps, this is how it looks inside every specific platform. But when I put it to a different platform with weaker processor Afterburner also reports the same GPU usage as with the stronger one, but the GPU produces less frames. So basically it is not used in 100%.
Can some expert please explain what does this value 99% GPU usage exactly mean, how it is being read, from where? It looks like this reading is CPU dependent and does not mean the same GPU usage across different platforms with different processors. I would like to understand why both platforms report the same GPU usage but produce different fps?
Edited by Wooojciech1983 - 4/10/13 at 8:13am
post #2 of 4
Read the following to get a good idea of how the flow of a game works: http://www.guru3d.com/articles_pages/fcat_benchmarking_review,1.html
You'll then see how a CPU can bottleneck a vidcard and vice-versa.
The Money Trap
(17 items)
 
  
CPUMotherboardGraphicsRAM
i7-5820K ASUS X99-Deluxe 2 x EVGA GTX980 SLI 4 x 4GB Corsair Vengence DDR4 2400 
Hard DriveHard DriveHard DriveOptical Drive
Games = Crucial 256GB M4 SSD OS = Samsung 256GB 830 SSD Backup Western Digital HDDs  Samsung 223 
CoolingOSMonitorPower
Phanteks PH-TC-14PE  Win 7 x64 Ultimate  Dell U2412M 1920x1200 Antec TPQ 1200W 
CaseOther
Cooler Master HAF 932 UPS: APC BackUPS XS1500 
  hide details  
Reply
The Money Trap
(17 items)
 
  
CPUMotherboardGraphicsRAM
i7-5820K ASUS X99-Deluxe 2 x EVGA GTX980 SLI 4 x 4GB Corsair Vengence DDR4 2400 
Hard DriveHard DriveHard DriveOptical Drive
Games = Crucial 256GB M4 SSD OS = Samsung 256GB 830 SSD Backup Western Digital HDDs  Samsung 223 
CoolingOSMonitorPower
Phanteks PH-TC-14PE  Win 7 x64 Ultimate  Dell U2412M 1920x1200 Antec TPQ 1200W 
CaseOther
Cooler Master HAF 932 UPS: APC BackUPS XS1500 
  hide details  
Reply
post #3 of 4
Thread Starter 
The article is interesting, thanks. But it does not mention anything about the processor role in producing the frames by the GPU and does not bring me closer to an answer for my question.
What I see from my testing is that there are two types of processor bottleneck. Inside every platform when GPU is waiting for a processor then GPU usage drops and CPU is bottlenecking, overclocking CPU helps to stabilize GPU usage to ~99% and fps increases. However when GPU usage is already 99% then CPU is not bottlenecking the GPU inside this specific platform and overclocking this particular CPU does not change fps. In this scenario the graphics card produces as many frames as it can with this specific CPU.
Here is my short test from Crysis 3, map Welcome to the Jungle, test location: big grass field, this location very CPU dependant. Youtube video of a gameplay at 4.5 GHz: gameplay
I have been testing my i7 3770K in different configurations - with only 2 cores active and with 4 cores but Hyper Threading off/on:

And here is how GPU usage looked like:

So as You see the proccesor was bottlenecking my GTX 680 until I overclocked it to 4.5 GHz in this location.
The second type of bottleneck is that a CPU itself can be a bottleneck. So even if the GPU usage is 99% we can still get higher fps with a stronger CPU so the weaker CPU was a bottleneck itself.
But why AF reports the same 99% GPU usage in this case if the weaker CPU is bottlenecking (compared to the stronger CPU) and the card can still produce much more fps with a stronger CPU?
I would really like to understand a processor role and GPU usage role in this scenario from a technical point. Is there any Afterburner/Riva tuner expert on this forum who can give a technical answer?
Edited by Wooojciech1983 - 4/10/13 at 6:41am
post #4 of 4
The CPU simply does the tasks (render a frame) requested by the app, e.g. Crysis 3, then on to the MS DX process, then finally adding the GPU drivers. All this is done by the CPU. A GPU might show 99% active because it is struggling trying to build a frame(s), but the CPU is doing just fine. As mentioned in the article, and the piece of the puzzle you've not mentioned, each frame to be produced by the GPU has a different time to produce, some longer than others. This is somewhat "smoothed" out by the triple buffering and whether or not you have VSync turned on.
The Money Trap
(17 items)
 
  
CPUMotherboardGraphicsRAM
i7-5820K ASUS X99-Deluxe 2 x EVGA GTX980 SLI 4 x 4GB Corsair Vengence DDR4 2400 
Hard DriveHard DriveHard DriveOptical Drive
Games = Crucial 256GB M4 SSD OS = Samsung 256GB 830 SSD Backup Western Digital HDDs  Samsung 223 
CoolingOSMonitorPower
Phanteks PH-TC-14PE  Win 7 x64 Ultimate  Dell U2412M 1920x1200 Antec TPQ 1200W 
CaseOther
Cooler Master HAF 932 UPS: APC BackUPS XS1500 
  hide details  
Reply
The Money Trap
(17 items)
 
  
CPUMotherboardGraphicsRAM
i7-5820K ASUS X99-Deluxe 2 x EVGA GTX980 SLI 4 x 4GB Corsair Vengence DDR4 2400 
Hard DriveHard DriveHard DriveOptical Drive
Games = Crucial 256GB M4 SSD OS = Samsung 256GB 830 SSD Backup Western Digital HDDs  Samsung 223 
CoolingOSMonitorPower
Phanteks PH-TC-14PE  Win 7 x64 Ultimate  Dell U2412M 1920x1200 Antec TPQ 1200W 
CaseOther
Cooler Master HAF 932 UPS: APC BackUPS XS1500 
  hide details  
Reply
New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: Graphics Cards - General
Overclock.net › Forums › Graphics Cards › Graphics Cards - General › GPU usage & processor bottleneck