PC performance issues (CPU/GPU interaction).

[jabokai]

Hi.

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

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 -

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!

 

[jabokai]

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.