PC performance issues (CPU/GPU interaction).

jabokai

jabokai

jabokai

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Hi.

I'm having issues with my PC. The important specs are -

Code: 
Operating System
	Microsoft Windows 7 Home Premium 64-bit SP1
CPU
	Intel Core i7 920 @ 2.67GHz	(OCed to 4GHz)
	Bloomfield 45nm Technology
RAM
	6.00 GB Triple-Channel DDR3 
Motherboard
	Gigabyte Technology Co., Ltd. X58A-UD7 (Socket 1366)
Graphics
	ATI Radeon HD 7950
Drives
	119GB SAMSUNG SSD 830 Series ATA Device (SSD)
	1397GB SAMSUNG HD154UI ATA Device (SATA)
PSU
	XFX 650

The issues revolve around GPU usage and framerates and I'm not sure where the problem lies. Basically, most games will use the GPU at full whack, all the time (unless I turn VSYNC on), but there are some where both the framerate AND the GPU usage drop. I've seen fps get lower than 30 at the same time the GPU is working at less than 40%. Why? Is there a weird bottleneck going on that only affects some things?

I did an experiment... I ran Kombuster (GPU stress test) and Prime95 at the same time with various changes on the CPU and Prime95 side, see results below -

Code: 
CPU Ghz		Prime95 Cores		GPU usage %
4		8			49
4		7			94
4		6 (or less)		100
2.67		8			5 - 30 (it chops and changes)
2.67		7			81
2.67		6			94
2.67		5			98
2.67		4			95
2.67		3			100

Games - Crysis 3 and Far Cry both use all the GPU all the time. Elite Dangerous (beta) will go lower than 30fps even while using less than 40% GPU.

I thought it might be a power thing, but with the CPU restored to stock it should be using less power and yet the GPU performance gets even worse.

The PC was originally built with an ATI 5870, replaced with the 7950 a year or so ago. I don't recall any such issues with the 5870 but can't be sure.

Any insights appreciated! :)
 
First of all, put the OC back on your i7 920.

Secondly, there are games which their engine are simply not written to use more than one or two CPU cores (Crysis 3 and FryCry 3 would use 6 CPU cores+), and what it means is in CPU demanding games (i.e. mmos and RTS), it is very common for the CPU to bottleneck the graphic card.

The reason for the low GPU usage is that the CPU is not fast enough to give enough workload to the graphic card (aka not keeping up with it thus holding it back), thus the GPU usage being low. For example, the 7950 itself at 100% may be capable of hitting 60fps, however if the game is CPU demanding and the CPU itself can only hit 30fps, what would happen is that the 7950 would only be able to output the same frame rate as the CPU also at around 30fps, thus with the GPU usage would be at only around 50%.
 
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'I've seen fps get lower than 30 at the same time the GPU is working at less than 40%'.,'but with the CPU restored to stock it should be using less power and yet the GPU performance gets even worse.'
here the game is cpu bound, gpu performance is related to cpu speed ie harder cpu works harder gpu works.
Games - Crysis 3 and Far Cry both use all the GPU all the time.
here the game is gpu bound, ie cpu waits for gpu all the time
 
Ah, okay, I get it now and it makes sense. I was a bit confused about the results with Kombustor and Prime95 running side by side but they make sense too in that Kombustor would still need some CPU.

The only problem is that some things are definitely not stressing the CPU and this effect still happens. The aforementioned Elite Dangerous, for example. I know it requires a half-decent CPU (they state a quad core as a recommended minimum) but my 920 runs at about 40-50% and I still get that GPU fps/usage slowdown effect... how would that relate to this situation? When I thought it was maybe power related I did try using Process Lasso to take back a couple of threads from Elite Dangerous and it didn't have any effect at all.

It has also happened on 2 or 3 Oculus Rift demos - even ones that didn't seem that graphically intensive - so CPU at 40-50%, GPU at 40% and the fps down around 30 (VSYNC off).

I game at 1920x1200, generally use VSYNC but switched it off for all the tests I've mentioned.

Thanks for the replies. :)
 
Crysis 3, generally, is a good test. So if Crysis is working as expected I wouldn't be too concerned about a hardware issue.

Most of the time under utilised GPU issues are the fault of the game or video driver(power control). An overclocked 920 is not a significant hindrance for a 7950.
 
jabokai said:
The only problem is that some things are definitely not stressing the CPU and this effect still happens. The aforementioned Elite Dangerous, for example. I know it requires a half-decent CPU (they state a quad core as a recommended minimum) but my 920 runs at about 40-50% and I still get that GPU fps/usage slowdown effect... how would that relate to this situation?
Click to expand...
This is actually exactly the issue I mentioned. While Elite Dangerous "recommend" quad-core CPU, but the truth is probably their game engine would only utilise two threads or cores of the CPU. What it means is that with your i7 920 which has 4 physical cores and 4 logic cores, out of all of that the game might only utilise 2 cores out of those "8" cores.

This frustration of CPU demanding games being light-thread existed for a long time (especially for mmo and RTS players). Very often the game engine would use only 1 or 2 threads, not because it is sufficient for game's need, but smaller developers generally don't have the resources to design the game engine, but to license existing engine from someone else to use. So whatever limitation with the game engine they choose to use, it will reflect in the game their make as well. Also it is far easier to code a game to run with 1 or 2 threads rather than efficiently around 4-8 threads.

Why Crysis 3 and FarCry 3 can support 6 cores or above you might wonder? Because those games are by some of the biggest developers in the industrial who have the money and resources to design, develop and make their own engine for their games (and the engines itself often would be licensed off for the smaller developers to use).

I have looked up what Elite Dangerous is, and it definitely looks like a VERY CPU demanding game. As I said, bring your overclock back up on your i7 920, and you will see immediate improvement comparing to your current stock clock.

And just for reference, even my i5 2500K overclocked to 4.80GHz would bottleneck even my old 5850 and frame rate drop to around 25fps in Guild Wars 2, because of how CPU demanding the game is (at world bosses and world vs world with like 40-60 players nuking it out at each other or at the bosses and its minions).

Also the inefficiency of the current directx with it's high-level access and huge CPU overheads also partly responsible for games being more CPU demanding than it needed to be. Mantle has already proven low-level access and low CPU overheads can be done, so next up is directx 12 which should hopefully be able to do the same and address the issue.
 
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