Can the CPU bottleneck the GPU ?

[neoglow]

Can someone explain (in simple-ish terms) if/how a CPU can bottleneck a Graphics Card performance?

I thought the idea of a GPU with its own dedicated memory was to take the graphics processing away from the CPU to make processing power more efficient. So games can be run on high settings as the GPU is doing all the leg work with the rendering.

So, if I had a low end CPU but the latest and greatest GPU available (for example, currently something like a GTX 690), would the CPU be holding back the full potential of the GPU in performance when playing games?

 

[greyfu]

The processor is required to interface with the PCI slot. The slower the processor the slower its ability of handling the exchange between GPU and mainboard.

Happy to be corrected.

 

[snaggster]

The cpu can be a huge bottleneck. Even though the gpu is taking care of the graphic side of things, the cpu is still working on the physics side. For example, working out the bullet path in a fps, or working out how much grip remains whilst cornering in a driving game. Not to be mixed up with nvidias physx, what is used to add special graphical effects eg blowing leaves.
If the cpu takes longer to workout a physics calculation, the game will slow down in order to attach the calculation to the frame its required on. Giving you a cpu bottleneck.

 

[MeMeMeMe]

While the CPU can be a huge bottleneck, usually it isn't.

When reviewers becnhmark different processors, they turn the graphic settings down as far as possible, because if they don't, the differences between different CPUs (for gaming, only) is barely discernible.

Now there are games and situations where the CPU can make a big difference, but even there, people make silly decisions.

For instance, someone might hold off getting a graphic card that on paper will give him double the frame rate, because they know it'll be bottlenecked by the outdated CPU. However, the 'bottleneck' might only be a drop of 10 or 20%, meaning they still would be getting a drastic increase in FPS (and at higher settings to boot).

Since you have an 95 2500k, you aren't going to see a bottleneck worth mentioning for the vast majority of gaming situations you'll be in with any single or dual card setup. (More than dual cards, it might be different.)

caveat: i have no experience with multiplayer shooters, and apparently the CPU is much more important there and you might see a significant bottleneck - I can't speak to that, others should weigh in. But if you aren't a heavy multiplayer gamer, it's not an issue.

 

[sean]

Disclaimer: I'm not a game engine programmer but some of my colleagues are. Here's my layman's understanding.

As snaggster writes, to get a frame on to your display a the CPU must first be used to update the game state: It must respond to your inputs, move characters around the scene, compute physics reactions, receive and act on network data from other players and so on. Only once the game state has been updated can the GPU start working on a frame. If you have a very quick GPU and slow CPU it's entirely possible for the graphics card to receive its instructions from the processor, complete its rendering tasks then sit there doing absolutely nothing while it waits for the processor to finish the next frame's worth of data.

I hope the following illustration helps visualise the situation. It's not 'to scale' or anything but the bottom example has a significantly slower processor which increases the total time taken to generate a frame and thus lowers the framerate

 

[Martini1991]

Actually, it's gotten to the point where the CPU can be a huge bottleneck and is discernible (Mates Phenom II netted 43 FPS average with a 7950, SB system netted him 65 and that was an i3 2120), at least where some CPU's are concerned (Core 2's such as the Q6600, Phenom II's in some games, the FX CPU's) hell a stock i7 920 will bottleneck quite badly a 7970.

 

[jaybee]

a stock i7 920 will bottleneck quite badly a 7970.

Are you sure about that?

 

[MeMeMeMe]

Actually, it's gotten to the point where the CPU can be a huge bottleneck and is discernible (Mates Phenom II netted 43 FPS average with a 7950, SB system netted him 65 and that was an i3 2120), at least where some CPU's are concerned (Core 2's such as the Q6600, Phenom II's in some games, the FX CPU's) hell a stock i7 920 will bottleneck quite badly a 7970.

In what games and situations?

 

[DazQ87]

On the topic of bottlenecks would you think that a phenom II 955 would cause a bottleneck to a geforce 670? My mate has just upgraded his geforce 460, I didnt think there would be a significant bottleneck but he was just a bit curious.

 

[jaybee]

I'll have a good opportunity to really test out all of these obsessions with bottlenecks soon. I just bought from OCUK 1 x 7850, and 1 x 7870. One for my rig (7850) and one for my bros (7870).

My rig: q9450 core2quad + 7850
His rig: i3 2100 + 7870

I will obviously be overclocking my q9450 to as far as I can on air. I will do 3.6ghz I think. His i3 will no way be a bottleneck. Modern day sandy bridge only requiress an i3. A recent review clearly highlighted for gaming this fact, that the difference in frame rates between i3, i5 and i7 modern sandy bridge architecture is non existant in 99% of situations.

 

[arknor]

In what games and situations?

probably starcraft2 seeing as it seems to be about the only game to ever be bottle knecked by a cpu.

where at a point where most cpus are far more powerful than a game needs.

most people game a high resolutions where the graphics card will be the limiting factor anyway

 

[dave_beast]

An I3 sb will bottleneck the high end cards, and anything less than an I3 eg Phenoms c2q will definitely bottleneck the high end cards. I was bottle necked on an oc 6870 on a q9450, jumped 15fps from going i7 2600k

 

[Martini1991]

Are you sure about that?

In some games at 1920x1080, yes I am, it runs 2.6GHZ at stock.

probably starcraft2 seeing as it seems to be about the only game to ever be bottle knecked by a cpu.

where at a point where most cpus are far more powerful than a game needs.

most people game a high resolutions where the graphics card will be the limiting factor anyway

SC2 is one, then you've got DOWII, then you have every other game that shows a difference in FPS when using two different CPU's that aren't in the margin of error (Although some bottleneck say 120 FPS out of a potential 140, so it's not an issue, but there are ones that depending on your CPU will prevent you getting 60 FPS average)

 

[andybird123]

the problem is different people have different ideas on what a bottleneck is

some people say that it's bottlenecking if one CPU gets 120FPS and one gets 200FPS... but to me this isn't a bottleneck - or not one that I'd bother spending money on fixing

if one CPU with a high end GPU gets 15FPS and the other 80FPS then yeah I'd call that a bottleneck, but this is very rarely what happens

people tried to tell me that my Q6600 would already be bottlenecking my 560ti and that there was no point buying a 580... but I went ahead and did it anyway and saw a massive improvement in frame rates in the games I was playing at the time and in those games saw 95%+ GPU utilisation - so there was a bottleneck, but it was tiny... I even played through the starcraft II single player campaign (which is the poster child of "that'll be CPU limited" games) with no issues - I can't tell you what fps I was getting as I saw no problems so didn't check

having said that - if a Q6600 bottlenecks a 580, then it will definitely bottleneck a 670 by a good degree... however if you are having problems playing the games you want to play now, then get the 670, see how it plays and plan to upgrade the CPU if it doesn't give you the results you'd hoped for

when you get in to SLI / crossfire then things get markedly worse as the SLI itself takes up extra CPU resource so if someone thinks they might be CPU limited then definitely avoid a 2 card setup

 

[Martini1991]

Obviously I'm not saying they'll become unplayable on a lesser CPU, I mean games are playable at 30 FPS, but I mean bottleneck in the terms of lowers your potential frame rate, and even gave an example as to why it's sometimes not bother worrying about.
I mean hell, I used an Athlon x2 6000+ with a 4870x2 on Crysis

 

[andybird123]

Obviously I'm not saying they'll become unplayable on a lesser CPU, I mean games are playable at 30 FPS, but I mean bottleneck in the terms of lowers your potential frame rate, and even gave an example as to why it's sometimes not bother worrying about.
I mean hell, I used an Athlon x2 6000+ with a 4870x2 on Crysis

sorry, I wasn't replying to you directly, more in general to the thread

 

[Martini1991]

Way I see it, if you're running a high end GPU, you're running it for a reason, if I wanted a lower frame rate, I'd have a different GPU.
I also run a 120HZ monitor so like that full 120 FPS where I can, and it doesn't hurt getting as many frames as I can when playing in 3D.

 

[Robes]

In short their is always going to be a bottleneck whatever the situation, whether it be GPU or CPU. One will always outperform the other even if it's by 1% it could still be called a bottleneck.

I think, as far as gaming is concerned, when your thinking about buying new hardware you shouldn't ask "Will this bottleneck that?" You should instead ask "Which one of them will give me a greater boost in FPS?".

I'll give you my example, I had a core i5 750 cpu and a 5850. If I had bought a top of the line £800 CPU i'd have seen probably a 4-5% increase in FPS. However by buying a 7970 I have seen a 60% increase in FPS. So for me the choice was obvious, but every situation is different.

But in answer to the OP... Yes certainly it can bottleneck the GPU but it depends on what you're doing.

 

[jaybee]

I think my main gripe with this talk of bottlenecks these days is:

1: People make assumptions and state that X will bottleneck Y without any evidence.
2: People make generalisations that older CPUs will generally be a bottleneck.

This is often because they:

A: Spent a lot on upgrading
OR
B: Run a modern day i5/i7 and want to justify it's existence.

Not always the case, and it's obvious modern day SB architecture is clearly way more efficient than older generations and faster. Not everyone games either so as stated, different strokes for different fokes.
I like to get my moneys worth as I'm cheap.
I think if you want to game and need to upgrade to play modern day games, unless you are going for the very highest end GPUs, the bottleneck will be minimal, and only apparent with very CPU dependant games like specific online play 64 man BF3 etc.

Whoever above said that an i3 would bottleneck hgih end GPUs. Let's see the evidence, because I can link to a review where they tested i3/i5/i7 and in most situations fps was the same accross all 3 cpus. When I say the same, literally often identical. It was quite an eye opening review.

 

[Robes]

Whoever above said that an i3 would bottleneck hgih end GPUs. Let's see the evidence, because I can link to a review where they tested i3/i5/i7 and in most situations fps was the same accross all 3 cpus. When I say the same, literally often identical. It was quite an eye opening review.

Well as you're not freely linking to yours here's mine:

http://lab501.ro/placi-video/gigabyte-radeon-hd-7970-oc-partea-iv-scalare-cu-procesorul

In Romainian but you can see the graphs or it you're really interested you can translate in Chrome.

Also

http://www.guru3d.com/article/radeon-hd-7970-cpu-scaling-performance-review/1


Here's the sort of thing people will sometimes highlight as bottlenecking:

the 1280x1024 results show CPU scaling this means at that resolution the lower end CPUs are bottlenecking the GPU. But there are two flaws with saying this, firstly 70fps on the lower end CPU is more than adequate and secondly, who plays at 1280x1024 on a 7970!?

On the more realistic gaming resolutions you can see that the GPU has become the bottleneck as they're all pretty much the same.