I understand CPU overclocking, but not GPU overclocking

Hello,
I have some experience with CPU overclocking, and also experience with undervolting. But now I tried to OC my 270X, and I just don't understand it.
The way I understand CPU overclocking, performance arises from increasing frequency (via FSB or multiplier), and power serves merely to support the increased frequency (to help reach stability), and increasing power does not actually net any performance.

I found a stable OC without meddling with power settings (1155 core, 1460 memory), and then tried to increase both frequency and power, but I could not reach stability (not even 1160 with +20% power). So I decided to keep it at the 1155/1460, and then I figured I could try to decrease temperature and therefore fan noise by decreasing power a bit, if the stability remained. But then I checked FPS at FurMark, and they decreased! I was puzzled, and tried the opposite, I again kept the frequency the same and increased power, and the FPS increased... yeah, increasing power could mean that frequency could be increased and kept stable and that would increase performance, but WHY did FPS increase when the frequencies remained the same?

EDIT: I used CCC to OC.

So, a few things:

1) Typically, the additional voltage you are able to apply to a GPU is not a lot, so there isn't a ton of headroom with some cards (silicon lottery plays in here). This could be the reason you hit 1155 with no voltage increase, but couldn't hit 1160 with the maximum allowable increase (there are also some checkboxes you may need to tick to allow voltage modification depending on the software you're using - I typically use Afterburner, so not sure on CCC).

2) Throttling plays in a bit more "discretely" when overclocking a GPU. Depending on the card, the freq might just decrease a bit at a certain temp or when the card detects it doesn't have enough voltage for its task. You won't see a crash, but your scores may suffer a bit and be lower than when not overclocked because you weren't throttling then. GPU-z has some nice graphs you can track during a benchmark to ensure you aren't throttling. When you increase the voltage and see a higher score, this is likely why. You're giving the card the voltage it needs to not throttle.

3) When comparing, you'll want to use a GPU-specific benchmark like Heaven or 3DMark. If you're comparing FPS in games (or in Furmark - I've never used that one), your CPU and all other components are playing a part (too many variables).

4) Find out your maximum core overclock number before you start working the ram up. Core speed increases matter WAY more than mem increases.

Thank you, you were right - I had to increase power by 12% to get rid of throttling. Drove up my temperatures to 89°C in Furmark, and 74°C in a game that is GPU-limited (=100% GPU usage), which is acceptable.
And Furmark is to GPU like Prime95 is to CPU stresses it to the max, but does nothing else.
Though I found out I can OC core speed at the same time as memory, because hitting the limit manifests in a different way (drivers crash with too high core speed, and too high memory speed causes artifacts)

Use the Heaven benchmark to test for stability on a Gpu, Furmark can be damaging.

Mandrake4565 said:

Use the Heaven benchmark to test for stability on a Gpu, Furmark can be damaging.

Click to expand...

Damaging? How? You mean by the high temperature?

Mandrake4565 said:

http://www.overclockers.com/forums/showthread.php/677416-Does-Furmark-really-kill-video-cards

Click to expand...

You learn something new every day. Cheers.

Though, I have to admit I feel pretty tired with having to tune-up settings with each and every game to maximize graphics quality while keeping FPS above 80... damn, I'll just pass my budget-minded PC to my girlfriend and simply buy a Titan X something. I'll have to cope with economy class tickets for a while though, lol damn. Decisions...

Certainly don't need a Titan X @ 1080P. 270X will definitely require some things to be turned down. GTX 970 is the sweet spot at the moment for 1080P

ninjacore said:

Certainly don't need a Titan X @ 1080P. 270X will definitely require some things to be turned down. GTX 970 is the sweet spot at the moment for 1080P

Click to expand...

That is my point - "require some things to be turned down" - I am tired of tuning... I would like to simply launch a game, set settings to max, and simply play.

GTX 970 it is then

I have to PUT my two cents in here, as this is AMD (ATI) GPU forum
The GTX 970 is a good card but if you are looking to stay with AMD and a cheap card. The R9 290 is still a good card if you are staying at 1080p.
I just checked E-Bay on the price of the cards. The GTX 970 is going for $275+ while the R9 290 is going for $200 - $300 with R9 290x's in the mix also

I can agree with that. 290 will trade blows with a 970 and will be about $20 cheaper on average comparing used to used/new to new. 290X will win out in most games and be about the same price, used and new.

I think it's worth the little bit extra for the decreased power draw, but depending on the rest of your system, it might not even matter. ITX, I would go with Maxwell. Any other build type, whichever is cheaper.