When is the CPU a bottleneck?

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CybernatonEvolution

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May 6, 2016
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As in, how can you tell? Is it via CPU usage? I have an i7 4770k at 4.4GHz and a GTX 1080. I ran into serious issues with Fallout 4 and GTA V; the GPU usage is low and the CPU usage is normal (30-50% and 75% at the most demanding areas in FO4).

I have a screenshot of Fallout 4 where the CPU and GPU usage(s) are below 40% and I get 44fps.
2njwHqU.png


There seems to be reports of driver issues and DPC latency in the latest Pascal cards that are related to the utilization issues. I am just asking for a future reference, how do you generally decide that your CPU is the bottleneck? In my case, I am getting 50% CPU usage across all cores, but 60% GPU usage; could it be the speed/architecture of my CPU rather than usage?

 
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ThatVietGuy
The point where you hit 100% CPU usage with far lower GPU usage (for example 50%) that's when you know you have a bottleneck.

In this case it just might very well be driver issues.
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CybernatonEvolution

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Yeah it is weird. Everyone kept on pointing at my CPU even though I never had such fps drops with my old 780. The Nvidia forum is full of utilization complaints, so it is indeed a driver issue that Nvidia is working on. I hope it fixes these issues; it is hard to watch others get 100+fps when you get less than 60 with a high-end GPU. I had doubts about my CPU, but I guess I am fine since I have yet to see a game push all my CPU cores to 100%. Only Forza Apex did but it only utilizes two cores (only the first and last cores of my CPU had very high usage %).
 
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Decends

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The CPU portion is not entirely accurate. A CPU does not need to hit 100% load in order to be a bottle neck, but when it appears the GPU is being under-utilized is accurate. For example, in some areas in GTA V my I7 4790k will start to bottleneck my 1070 causing frames to drop to low 50's high 40's. But, the I7's usage never exceed 50%, but the GPU's usage dropped from 99% to 60-80% usage when the frames dropped, a signal that the CPU couldn't feed the GPU information fast enough.
 
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Decends

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On another note, sometimes it can also be the Optimization of the games engine that causes a bottleneck. So there is a possibility that Fallout 4 and GTA V's Graphics engine just isn't fully optimized and doesn't take full advantage of the hardware it is given.
 
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InvalidError

InvalidError

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You get a CPU bottleneck whenever a performance-critical thread fails to complete execution in a timely manner. Since practically all games do "busy waiting" instead of letting Windows schedule at regular intervals, the main thread and possibly others always use 100% CPU time on whichever cores they run on and that makes it impossible to tell directly if they are truly CPU-bottlenecked or being stalled by other factors.

In a genuine CPU bottleneck situation, you would see no performance gain from reducing graphics details. If driver anomalies are causing stalls, this simple test may be inconclusive. With some luck while experimenting, you may stumble on a setting that will make the stalls go away.
 
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