Today's Hard|Forum Post
Today's Hard|Forum Post

GTX 480 SLI PCIe Bandwidth Perf. - x16/x16 vs. x8/x8

In our continuing coverage of Multi-GPU configurations with varying PCIe Bandwidth, we put x16/x16 and x8/x8 PCIe to the test. Does having less PCIe bandwidth make difference in gaming? We know that a "lesser" motherboard can save you money. Using GeForce GTX 480 SLI we show you the real-world differences.

Introduction

Last week we showed you what the real-world gaming performance differences when we used SLI and CFX video card configurations in PCIe x16/x16 operation and x16/x8 operation. Our conclusions were simple, moving the secondary video card of a CFX or SLI configuration into a x8 PCIe slot did not degrade our gameplay experience, or cause significant changes in framerates. So if you have a motherboard that does not support x16/x16 or if you want to space your cards out in your chassis for better airflow, going x16/x8 is not a big deal even on a high end cards like the GeForce GTX 480 SLI.

In light of what we discovered, we wanted to take this testing even further and move to a x8/x8 scenario with two video cards. Why is this important? The fact is that many of today's users will find themselves in this situation if they go to use CFX or SLI. Here is a quote from a P55 chipset motherboard manual that refers to the first two PCIe x16 physical expansion slots on the motherboard.

The PCIEX8_1 slot shares bandwidth with the PCIEX16_1 slot. When the PCIEX8_1 slot is populated with a PCI Express graphics card, the PCIEX16_1 slot will operate at up to x8 mode.

Most P55 motherboard owners are going to find themselves in x8/x8 position when using a new P55 motherboard for SLI or CFX due to PCIe lane limitations of the chipset. This is one of the features that Intel limits in its product stack as you move down from the X58 chipset.

So is a P55 motherboard or any other motherboard with fewer PCIe lanes holding your gaming performance back when you are using SLI or CFX?

How We Tested

Our testing is simple today, we are going to use NVIDIA’s GeForce GTX 480 SLI and run it in x16/x16 mode in our X58 motherboard and record apples-to-apples real-world gaming data. We are then going to use the same taping method we used in last week's evaluation and tape off both video cards this time to create a x8/x8 PCIe 2.0 operating mode on the video cards and record the real-world gaming data again in an apples-to-apples scenario. We will then put data on a graph to compare. As we did in our previous evaluation, we are going to test at both 2560x1600 and triple-display 5760x1200. Keep in mind that we are using the GTX 480 SLI because it is the highest "bandwidth" video card configuration there is and produces the fastest framerates out of all SLI and CFX two-card combinations. Using slower video cards in our last comparison amounted to nothing but extra work so we decided to focus on the faster video cards.

Article Image Article Image

The system setup is as follows: MSI X58 Eclipse (two x16 PCIe slots native), Intel Core i7 920 @ 3.6GHz, 6GB DDR3, Dell 3007WFP, Win7 x64. We are using NVIDIA drivers 258.96 WHQL.

We are comparing x16/x16 versus x8/x8 in three games, Aliens vs. Predator, Bad Company 2 and Metro 2033. Once again, AvP is very texture/bandwidth heavy, Metro 2033 is very shader intensive, and BC2 is more of a mix.


GTX 480 SLI - 2560x1600

Aliens vs. Predator

Article Image

In Aliens vs. Predator at 2560x1600 with 4X AA we experienced absolute identical performance, in fact the average framerate was dead on with the exact 79.9 FPS. It seems that 4X AA at 2560x1600 plays very well on the GTX 480 SLI in this game, as both scenarios were more than playable and there was no difference in the gameplay experience at 2560x1600 with 4X AA. s

Battlefield: Bad Company 2

Article Image

In Battlefield: Bad Company 2 at 2560x1600 with 4X AA, just like AvP, we see no difference in performance at all. There was no change in the gameplay experience at this setting and the game was also more than playable.

Metro 2033

Article Image

In Metro 2033 the same pattern is repeated ad naseum, there is absolutely no change in the gameplay experience or performance. In this game, these settings are the highest playable settings for GTX 480 SLI, this setting was the highest we could set the game and still have acceptable framerates. Therefore, in no way shape or form is x8/x8 harming our gameplay experience here.

All three games above were at 2560x1600 with 4X AA, which in the past has been considered the "benchmark" for high-end resolutions and AA settings. Now let's see if our pattern holds true at 5760x1600 Eyefinity/NV Surround resolution.