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The impact of PCI Express on GPUs
by Guillaume Louel
Published on September 26, 2011

We used the most recent drivers available on the date of our test, namely the AMD Catalyst 11.5s and the NVIDIA GeForce 270.61s. We tested at three resolutions to measure the impact of each: 1280 x 1024, 1680 x 1050 and 1920 x 1200.

The test configuration was as follows:
  • MSI Z68A-GD80 B3 motherboard
  • 2x2 GB DDR3 1333 MHz
  • Intel Core i7 2600K
  • Nvidia GeForce GTX 570 / Radeon HD 6970
  • Windows 7 Ultimate 64 bit

Crysis Warhead

We used version 1.1 of Crysis 64-bit.

Hold the mouse over the graph to view the performance index.

The way the NVIDIA and AMD cards are managed by their respective drivers can be subject to significant differences. While the game is responsible for DirectX calls, the driver has the last word on how these calls and transfers are executed. So while there’s only a small difference between x16 and x8 with the AMD card (around 2%), this goes up with the NVIDIA card (around 4%). As we’ll see, this can vary significantly from one game to another.

While the impact at x8 is slight, it starts to make itself felt more strongly at x4, once again with disparities between AMD and NVIDIA. There’s an 8% cost with the AMD card and up to 18% with NVIDIA at 1280. This is no surprise as the variations are highest when most frames are displayed on screen.

Interestingly, using the x4 port on the southbridge shows the opposite trend. Where there’s a 2.5% performance difference with the NVIDIA card between the x4 CPU interconnect and the x4 southbridge interconnect, this rises to 5.5% for AMD. In the theoretical tests we remarked on the fact that there was a slightly higher cost to performance on the southbridge with nonpaged CPU to GPU transfers on the AMD platform. This was borne out in practice here.

FarCry 2

We used version 1.03 of Far Cry 2.

Hold the mouse over the graph to view the performance index.

Certain tendencies were inversed here, with the NVIDIA cards being more efficient at PCI Express x8. The loss in performance was nevertheless higher than for Crysis, because of the higher framerate. It came to 5.5% on the GeForce and a little over 7.5% on the Radeon. At PCI Express x4, we can see the same trend in terms of efficiency, with a performance cost of up to 12% for NVIDIA and a little over 16.5% for AMD at 1280.

One thing remained unchanged however, the difference in efficiency between the PCI Express x4 CPU interconnect and the southbridge. There’s only a 4.5% difference with the NVIDIA card and more than 8% for AMD.

Metro 2033

We used the latest version to date of Metro 2033 (patch 2 via Steam) in DirectX 11 (AAA).

Hold the mouse over the graph to view the performance index.

With a lower frames per second count in what is a very resource hungry graphics engine, the performance differences were lower. Switching to x8 only implies a performance cost of 2.5% with a slight trend in AMD’s favour, which is confirmed at x4, where the cost is “just” 7% against practically 10% for NVIDIA.

NVIDIA cards still do better with the x4 southbridge interconnect, with a performance loss of just 3% against a little over 5.5% for AMD.

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