Skyrim GPU Usage

[cadefoster]

I just have a question regarding the gpu usage in skyrim. I can run skyrim on ultra with a fairly constant 60fps. However there are a few areas of the game, such as the big cities, where the fps will drop into the 40s. The weird thing is, both my gpu's are only around 30% usage. Do you think the culprit is poor sli utilization from the game, or something else?
Win 7 64
i7-870 3.0gz
560ti x2 SLI
12 gigs ddr3
750w psu

 

[bwrlane]

Yes, indeed. I had exactly the same with my old system, which was quite similar to yours. Mine was an i7 920 overclocked to 4.1 GHz, together with 2x560ti SLI.

Now, the most straightforward answer would be CPU bottlenecking (although for reasons I will explain, I do not think this is the correct explanation). CPU bottlenecking would fit the facts: low GPU utilisation and low frame-rates suggests a CPU bottleneck. Plus, Skyrim's engine is known to be poorly optimised for quad core CPUs. You will normally see only 2 cores utilised, which in extremis will be around 90%+ on one core and only 30% on another. Cities stress the CPU because the geometry is quite complex, plus there are more NPCs meaning more AI calculations. Both of these are CPU tasks.

So, I had assumed my problem was a CPU bottleneck. But later investigation indicated that this was probably wrong.

My next build was based around a 2700k, which I overclocked to 4.8GHz. Initially, I kept my SLI 560 Tis and the experience was exactly the same: good frame rates out in the open, but lower frame rates in cities. If a CPU bottleneck had been the problem, I should have expected to see the problem somewhat relieved by having a faster CPU, but no. I saw no difference whatsoever.

The proof came when I replaced my 560Tis with a single 680. Most benchmarks will tell you that a single 680 has only slightly more power than 2 560 Tis so I wasn't expecting to see much difference. Indeed my only reason for upgrading to a 680 was with the intention of getting a second 680 later (which I have now done).

I was certainly not expecting to see what I saw. With a single 680, performance in Skyrim was on a different planet from my 560Tis. With all settings maxed (and with the hi res texture pack), frame rates in cities were now super smooth.

What could possibly be the explanation given that my 560s had been only 30 percent utilised while I was seeing choppy frame rates?

I cannot say for sure, but I think the explanation is the size of the VRAM. The 560 Tis only have 1GB while a 680 has 2GB. At max settings, this is not enough for a smooth experience in Skyrim. It would also explain why the GPU is not fully utilised. When the VRAM is the bottleneck, there is GPU power to spare.

So on the basis of my experience, I'd say flog your 560s and replace them with a single 680. You'll probably be glad you did.

 

[cadefoster]

I really appreciate the information, thanks a lot. I kinda figured the issue must be sli related as my cpu did not appear to be maxing out. Like you said, on paper a 560ti sli is almost on par with a 680, so I'm surprised the performace gap is as wide as you described. If I can sell the 560tis for a decent price, I think I'll look into getting a 680. Thanks again.

 

[bwrlane]

Yeah, experiencing skyrim with an number of hardware configurations (including one quite similar to yours) has allowed me to find - by trial and error - where the bottlenecks are. I'd clarify my conclusion by saying that I don't think it is specifically SLI related. It's just that the VRAM in the 560 ti just isn't big enough to handle skyrim on max. This is why the real world performance of the 680 is so far superior, when the paper specs would lead you to believe they are about the same.

 

[cadefoster]

I've just been messing around with skyrim an noticed that my gpu usage actually goes up quite a bit in problem areas like rifton when I lower the shadow settings. I think you're right about vram being the culprit. What I don't understand is between 2 560ti's, I have 2gigs of vram, can the game not use it? Please forgive me if this is a stupid question, this is my first system using sli.

 

[AntiZig]

vram can be the culprit, especially when you're running HD texture pack, however consider the following:

-skyrim has shadows rendered by CPU. Stupid right? well, not if you're gaming on a console it isn't. So, the engine from the start was designed for console. I'm not entirely sure if it has been patched on PC since then to be rendered by GPU (bethesda was saying about doing it, but I don't know if they've delivered)

-Anti-aliasing natively implemented in skyrim engine is hugely inefficient. If you want to use AA, but suffering from performance issues do yourself a favor. Turn off AA in the launcher, then go to your control panel and enable it there. It does a better job visually, and is a fraction of the performance cost.

- skyrim didn't support SLi or CF at launch. I believe vendors have fixed that through driver patches, so make sure you have latest drivers installed. If that isn't the issue, try disabling SLi, it might yield better results.

- lastly, if you set reflections to max (ie check all the boxes) it's a significant load, which requires a lot of work from both cpu and gpu. Not sure if this is affected by vram