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CPU bottleneck, how can you tell?

post #1 of 23
3/14/12 at 8:34pm
Thread Starter 
Kinda a noob question and I feel somewhat stupid for asking it. I know what bottle-necking means. But I've never knew what to look for to determine if it was happening.

What I'm trying to figure out is, is there an accurate factual way to tell your gpu's are being BN by your cpu?
Does the cpu get pegged at max load when running furmark or something? I've never experienced it so I'm asking what are the symptoms.

For example when running a game on a bottle-necked machine will your gpu not run at its full potential like running half throttle?

Thanks guys
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post #2 of 23
3/14/12 at 8:36pm
when your CPU is at 100% and your GPU isn't.

EDIT: didn't read sorry yes and no in future mark it will pin everything you only know if you have a bottle neck in games whichever is hitting max first is the bottleneck
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post #3 of 23
3/14/12 at 8:41pm
gpu use does not indicate bottlenecks since every game is very different

but there is some sort of knowledge than cards of certain gens, should only be paired with some cpus
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post #4 of 23
3/14/12 at 8:48pm

The only certain way to determine whether a CPU bottleneck exists (and how severe it is) is to do a CPU scaling test.  That is, plot a graph of framerate (fps) vs CPU frequency (GHz) at your resolution with a certain game.

cpu scaling test one.jpg

 

If you see the curve steadily increasing (ie with a slope), then there is a bottleneck.  The steeper the slope is, the more severe is the BN.

 

If the graph is a plateau (ie level off), then there is no BN.

 

Of course, this varies from game to game and with the game settings.

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post #5 of 23
3/14/12 at 8:58pm
Thread Starter 
Ok awesome, that makes sense.

So the reason I'm asking this is I don't know if you guys have heard of Tera online or not but there seems to be a odd performance problem were the cpu is running nearly half throttle on all cores and the gpu is running half throttle.

However overclocking does net you substantial fps gains.
I'm thinking the cpu is occupied streaming the huge amounts or art data in game (expensive shaders and textures maps are used everywhere) while dealing with the server data leaving little left for the gpu. But what kills the theory is the cpu isn't pegged at all.

I have a i7-860 oc @4.0 running at around 55% in game.
I have another rig with a Phenom II x4 840 stock at 3.2 running at 80%

Seems odd to me.
Edited by DigitrevX - 3/14/12 at 9:02pm
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post #6 of 23
3/14/12 at 10:35pm
The easiest way to determine a CPU bottleneck is running a game at the highest resolution you can, check the frame rates. Then switch to a lower resolution and see if frame rates go up. If frame rates don't go up and stay the same, then typically that's a CPU bottleneck. But keeping in mind it's hard to tell because its going to vary from game to game. For example, people will hit a CPU bottleneck in starcraft 2 simply because it uses only 2 threads, in which people with quad cores (regardless of amd or intel) you won't see it pegged at 100%.

From what I've seen in the general consensus though is that you will more than likely be GPU bottlenecked before you will hit a cpu bottleneck. People don't really hit CPU bottlenecks unless they run super high end GPU's, Crossfire, and SLI in which you would need a fast CPU to keep up with all the GPU power. But of course going back to what I was saying, if a game is designed to use only 1 or 2 cores, then you will see it CPU bottleneck high end GPU set ups.

I think the overall key is to balance out, don't buy a $600 CPU and pair it with a $60 pentium and expect uber performance. And vice versa, don't spend $1000 on an i7-3960X and pair it with a Radeon 6570.

Just my personal opinion, I think the most balanced set up out there that will compliment each other would be an i5-2500K and something like a GTX570 or Radeon 7870. Mid-high stuff that is relatively affordable and will run any modern game very well.
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post #7 of 23
3/14/12 at 10:39pm
Quote:
Originally Posted by DigitrevX View Post

Ok awesome, that makes sense.
So the reason I'm asking this is I don't know if you guys have heard of Tera online or not but there seems to be a odd performance problem were the cpu is running nearly half throttle on all cores and the gpu is running half throttle.
However overclocking does net you substantial fps gains.
I'm thinking the cpu is occupied streaming the huge amounts or art data in game (expensive shaders and textures maps are used everywhere) while dealing with the server data leaving little left for the gpu. But what kills the theory is the cpu isn't pegged at all.
I have a i7-860 oc @4.0 running at around 55% in game.
I have another rig with a Phenom II x4 840 stock at 3.2 running at 80%
Seems odd to me.

I just realized I went on all that explanation without actually addressing anything. redface.gif

It could just be that the game isn't very well coded or optimized. One example is my 2600K and GTX 580 playing Crysis. My CPU never really hits 30% nor does my GPU really ever hit 70-80% and only 100% in certain places. I'd be damned if an i7-2600k is a bottleneck.

But with Battlefield 3, my GPU is basically 90-100% the entire time with my CPU around 30% consistently.
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post #8 of 23
3/14/12 at 10:46pm
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To get rid of it, get a better cpu OR turn up the res. Youd think more res would pressure everything, but no, gpus drop frames (but its higher res) and cpu useage can go down. Dont ask how, i dont know.
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post #9 of 23
3/14/12 at 10:49pm
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But that cpu shouldnt be bottle neckin anything thats a monster. Gpu bottle neck more likly
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post #10 of 23
3/14/12 at 10:52pm
Quote:
Originally Posted by jrl1357 View Post

To get rid of it, get a better cpu OR turn up the res. Youd think more res would pressure everything, but no, gpus drop frames (but its higher res) and cpu useage can go down. Dont ask how, i dont know.

That is what I was thinking. Run a game at max settings at a low resolution like 1024x768. Record your FPS, turn up the res and see if it drops. If it drops significantly your GPU is holding you back.
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