INTRODUCTION
Is my Graphics card bottlenecked by my CPU ? I have seen this question posted many a time on various gaming forums. A lot of people who ask this question dont even understand what a bottleneck is. I have often thought about doing an article to demonstrate the extent of CPU bottleneck and my first ever analysis was done on an Opteron 165 paired with volt modded and over clocked 7900 GT's. But that article was just a little something for myself to gain a better understanding of CPU bottleneck. This article will expand on my early analytical ideas, explain what a bottleneck is and study how QX 9650 bottlenecks an overclocked pair of GTX 280's while gaming at high resolution.
EXPLAINING BOTTLENECK
In common terminology, bottleneck refers to anything that impedes or limits a process. Whenever a process is going on, be it the construction of a building, driving a car, or a computer running a game, there is always a factor that will limit the end result. When constructing a building, a lot of factors co-exist. There is the number of workers, the availability of construction material, the weather conditions etc. At any given point, one or more of these factors will be the limit as to how fast the building can be constructed. If you have a thousand workers ready to work but there is no cement, or construction tools, then the lack of equipment is the sole bottleneck towards the successful building and completion of the building. Adding more workers is not going to speed up the process. You will not see the benefit of the wrokers until you provide them with equipment.
Let us say you want to travel from home to work in the shortest possible time. You live very far from work so you get a car that can do 300 mph. The car's maximum speed is no good to you since the traffic wont let you go that fast ! You will only go as fast as the rush hour traffic allows you. So the traffic becomes a "partial bottleneck". I say partial because even if there is no traffic you will still only go as fast as the speed limit on the road :)
However, if the roads are empty and there are no speed limits, only then will you be able to see the true potential of the car.
When you are playing a game on your computer, a lot of hardware components are interacting. There is the Motherboard, CPU, The Video Card, Ram, and Hard drive. All these components are constantly sending information between each other and depending on how much load each component is taking, one of them will become the bottleneck or quite possibly there could be multiple partial bottlenecks. Modern games ae very graphics intensive. They have features such as HDR, Complex Shaders, Anti aliasing and other effects that put a lot of stress on your graphics card. However, you CPU is also working hard to render the images. It is sort of like the CPU is painting a black and white piture and the graphics card is adding colors to it. The CPU is also running calculations that are determining how objects interact in the game (physics) and how non player characters (NPCs) react to their surroundings (Artificial Intelligence). Depending on the kind of scene being rendered in the game there is a distribution of work between the CPU and GPU and the slower of the two becomes the bottleneck or partial bottleneck.
It is common knowledge, that high end graphics cards need to be paired up with high end CPUs. These days, with the prevelance of dual, Tri and quad chip graphic card systems it is commonly believed that graphic cards are ahead of CPUs in terms of raw gaming performance. High end cards such at GTX 295 and HD 4870 X2 have immense horsepower whose potential is often hindered by the faster Quad core CPUs. This is especially true at low image quality where the load on the graphics card is low. High resolution and image quality can change things and shift the balance toward the CPU, when the CPU is essentially "waiting on" the graphics card to do its thing. The net affect of CPU and GPU working in tandem is manifested in frames per second. The faster your system is the higher frames it can crunch in a second.
TEST SETUP
Motherboard EVGA 780i SLI
CPU QX9650
RAM 4 GB GEIL Esoteria @ 1000 5-4-4-12-2T
GPU (2) X EVGA GTX 280 SLI @ 720 / 1470 / 2480 (Forceware 180.7)
Sound Card Auzentech Prelude 7.1
Monitor Gateway 30" XHD
OS Vista X64
BENCHMARKING METHODOLOGY
Synthetic tests such as 3DMARK06 are easy to perform as all you have to do is run a test and the results are conveniently output as an overall score and average frame rates. Games however may or may not be so easy to benchmark and there are multiple ways to benchmark a game. For this analysis, I have used two methods :
There has been a follow up to this article...CPU Bottleneck Analysis revisited.
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