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Understanding CPU and Video card bottlenecks

Written By:
Date Posted: October 16, 2002

I'm writing this because I still get an awful lot of emails asking me to explain how a video card or CPU can be a bottleneck in a PC setup. As I've said in several reviews and articles, a video card bottleneck occurs when the CPU is providing more information than a video card can handle. A CPU bottleneck happens when the video card doesn't get enough info from the CPU.

To understand how video cards work, almost all modern video cards need a decent CPU to go with it. If the video card is too powerful (for example, a GeForce 4 ti4600), and the CPU is too slow (a Pentium 2 266), the video card is wasted because the CPU can't deliver info as fast as a video card can take it. We're not saying that the video card won't be an improvement over an ATI 16MB Rage, but don't expect 300fps, that's for sure.

An easier analogy is two people loading a truck. Say the guy at the truck is 6'6, 280lbs of muscle (keep the ghey comments to yourself). He can lift a lot, but the guy bringing stuff to him is 5'1, and 100lbs. Well, long story short, they're gonna be there all day loading the truck.

The same theory applies to a powerful CPU and weaker video card. I get some questions about our testing, where a reader has an identical CPU, but scores lower in the game benchmaks. Let's say their setup was previously a Pentium 2 400 w/Riva TNT. They then upgraded to an Athlon XP 1800+ but kept the same video card. Productivity programs will certainly be faster, and any low resolution game (*cough*, any pre Warcraft 3, Blizzard game, *cough*), but framerates in action games at 1024 resolutions seemed unchanged. This time around, the video card is doing all it can, but the CPU is pumping out more info than it can handle. Use the moving truck example, and reverse the roles to get an idea.

In order to illustrate the above examples even further, I ran a couple of quick tests. Here are the testbeds...

Pentium 3 500, 512MB PC100
AMD Athlon XP 1600+, 512MB PC2100
Pentium 4 2.4, 512MB PC2700

We'll be testing with the following videocards...

ATI Radeon 9700 Pro
MSI GeForce 4 MX440

Now, I can go on forever with different resolutions, and different games, but for now we'll do only 640x480 (to test for CPU bottlenecks) and 1600x1200 (to test for videocard bottlenecks) with Quake 3. Low resolution tests will be with the "Fastest" settings (16bit colour, low textures), and high resolution tests will be run with high quality settings...

Radeon 9700 Pro

At the lowest resolutions, the differences are slight between the faster processors. Given the close results, the Pentium 4 still holds a lead over the Athlon 1600+, which would indicate that the CPU is the still the bottleneck for ATI's latest. I think it's pretty obvious that the Radeon 9700 should not be paired up with a 500MHz CPU.

GeForce 4 MX440

Using a lower end videocard doesn't help the P3 500. I don't really think enthusiasts would be using such a CPU for their rigs today, but it's obvious who is the bottleneck here.

For the faster CPUs, the differences at the lower resolutions are normal, but there is almost no difference at the highest resolution. This is a scenario where the videocard is the bottleneck.

In conclusion, it never hurts to have too much power, but be wary of what you decide to upgrade first if you can only afford one upgrade at a time. My general rule of thumb is stick with identical video card and CPU generations to get the most out of your purchases. The GeForce 3 for example was released around the same time as the Athlon XP. Clock speeds aside, you'll generally get the best CPU/GPU scaling from this setup. With the Thoroughbreds and Northwoods, the GeForce 4 Titaniums and Radeon 9700. I'm just generalizing, but you get the idea.

Update: Nothing like fan mail to make you happy. We received a lot of emails the last couple days, but one that generalizes what most people have been asking... what video card now then?

"I'm running a Pentium 3 933Mhz CPU on a 133 MHz FSB, Geforce 2 UltraVideo card with 64 MB DDR VRAM and 256 MB of PC 700 RDRAM. I'm considering upgrading my video card sometime soon, but don't want to buy one that's too powerful for my present CPU. An upgrade of the CPU I have would be very expensive, and I could only go to 1.13 GHz as I don't want to upgrade my motherboard. If I buy a Radeon PRO 9700 will it be too much for my current CPU IE: will I be wasting its power? Should I try a less powerful Video card such as one of the Geforce 4 Ti series or maybe even a Geforce 3 if the Geforce 4's would bog down my CPU?"

Honestly, a GeForce 3, GeForce 4 Ti4200 would give you more bang for the buck. A Radeon 9700 or GF4 Ti4600 would be fine, but not much faster than the other two on your system. On the flip side, a faster video card now would save you an upgrade if you do plan on upgrading the CPU/motherboard in the near future.

It's important to note that I am not discouraging you from getting a faster CPU or video card now. Even if you go with a top end part, and although there may be a bottleneck somewhere, you will save yourself a possible upgrade in the next 6-12 months.

Agree? Disagree? Discuss it in our forums




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