Please help find my mystery bottleneck [demonstrated in Skyrim] - why do I get areas of low fps despite only 70s% GPU/CPU use?

[elsnerj]

Hello, and thanks for taking a look at my issue!

So I recently re-installed Skyrim with a handful of favorite graphics mods, and saw that most of the time, I was getting straight 60fps (vysnc on) and using 80% or less of my GPU and CPU (monitored through MSI Afterburner). Makes sense, I'm happy.

However, in some areas I get a sharp FPS drop - which I assumed must be because there was more going on there, forcing some part of my system to 100% and creating a bottleneck.

Much to my surprise, when I looked at my hardware usage, I saw that I was still only using 80% or less of my CPU/GPU despite my framerate dropping from 60 into 40s or even 30s.

So what the heck? Is there some other part of my system limiting my framerate in these areas? Or are my GPU and CPU somehow maxed out at 70-80% reported use?

Some illustrative images:

Out in the wilderness, I get 60fps with GPU use at ~80% and CPU in the 70's:

Yet when I go to some spots, like Riverrun:

I see the exact same GPU and CPU usage (mid 70's) but now get 47fps! Why are my CPU and GPU not increasing their output to raise up and maintain the framerate at 60?

And more in Whiterun:

It gets even stranger! Now my CPU is around the same (80) and my GPU much lower (59) but the framerate is the lowest yet at 37!

Another Whiterun shot:

Same deal here, CPU at the typical 70s, GPU relaxing at an easy 55, and yet it ought to be working hard to raise my 42fps framerate!

Note that at all times I am using far less than max RAM (I have 3gb video memory, using 2gb at most when in-game, and 8gb system, using 4gb at most in-game). Also, I ran some stress testing (eg. furmark) as well as some other games (Crysis 2 especially) and confirmed my GPU and CPU are both capable of rising up to and maintaining 99% usage to keep framerate as elevated as possible. Why would they cease this behavior in certain spots of Skyrim?

My system specs (don't hesitate to ask for more specific info!):

Motherboard GIGABYTE GA-Z77X-UD3H
Memory G.SKILL Ares Series 8GB (2 x 4GB) 240-Pin DDR3 1600
Hard Drive WD Black WD1002FAEX 1TB 7200 RPM 64MB Cache SATA 6.0Gb/s 3.5"
Power Supply XFX P1-750X-XXB9 750W 80+ Bronze Certified, SLI Ready and Modular
Graphics Card Sapphire HD 7970 OC Edition
CPU Intel Core i5-3570K (Socket 1155) at 3.4Ghz with boost to 3.8Ghz
Optical Drive Asus 24xDVD-RW Serial ATA Internal OEM Drive DRW-24B1ST (Black)
Monitor Asus VS247H-P 23.6-Inch 1080p
Windows 7 Windows 7 Home Premium 64bit

 

[chris987]

such cpu usage shouldnt be taking place! clearly you run a very heavily modded version of it. are you not? clearly you really testing your pc but also the games engine cababilities! through gpu z ensure you arent actually close to 3gb vram limit or even over it? if you have capable cooling, try overclocking to see if you can gain better performance!

 

[elsnerj]

I'm only running a few graphics mods (and nothing ENB), but I did change a lot of the graphics settings in the .ini files to make the game engine draw more and farther grass, trees, rocks, etc. So you're right I'm really trying to push both the game and my build!

I double-checked the numbers using gpuz, cpuz and a few benchmarks. My graphics card and CPU are both absolutely capable of going all the way up to 99% usage, which they do under stress testing - so I know they aren't limited at the 70s and 80s numbers I'm seeing. The highest video RAM usage I ever reached in a quick session of playing was just under 2gb, which is nowhere near the 3gb limit. So the issue isn't in my build lacking muscle, and even if I overclocked the hardware limits they wouldn't ever be pushed up to the overclock speeds.

The only things I can think of to explain this are

1) there is something software-side that is limiting framerate (e.g. the Skyrim engine cannot supply directions to my hardware fast enough) but I don't know anything about this subject. Is such a situation even possible, where the processors sit eagerly awaiting to draw more trees and AI paths etc. and the game software somehow fails to instruct them fast enough?

2) Maybe there is some hardware bottleneck at the transfer of info end rather than the processing end (that is, the GPU and CPU finish crunching all the numbers but cannot send the finished frames fast enough)? Again, I have no knowledge of how such things work.

3) There is some subset of processing tasks which are maxed but not reflected in overall GPU use. Do graphics cards have a subsystem or component dedicated only to drawing shadows, for example, such that the GPU could process all necessary calculations but then has to wait for the shadows to finish being drawn by that subsystem before it can release the frame? Or can GPU's sort of "do anything" with their processing power, such that a 55% GPU usage means every bit of every frame (shadows, meshes, particles, etc) is being easily drawn before being requested for display?

In other words, is it possible for a GPU to report only 50% use, while some dedicated subsystem (such as for shadow drawing) is actually being pushed to the max?

 

[James Mason]

Turn down the shadows 1 level.

Shadows destroy FPS.

 

[elsnerj]

I reset all the shadow settings to default (cutting their resolution by over half as well as reducing their draw distance). My GPU is now even more relaxed all the time, going into 70% usage at most.

But I still have the same issue, where in towns and villages my framerate will drop down to 30s and 40s and my CPU/GPU stay at the same usage %s.

 

[chris987]

while gaming gpu should be at 100% of usage but cpu shouldnt really pass the numbers you are having. 70-80% is close of being bottlenecked. re tweak .ini files to something better in terms of performance. skyrim can get a lot better graphically but there are limits. you have tweaked things they shouldnt be tweaked. after all its quite old know(graphically speaking)

 

[Rationale]

Check the use of each CPU core, rather than for your CPU overall. If, for example, 2 cores are at 98-100% and the other 2 are at 50%, averaging to 70% overall, that could cause these problems. Skyrim puts most of its load on 2 cores at a time, if I remember correctly.