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

With the different AMD and Intel platforms offering varying numbers of PCI Express lanes, the impact of this bus on graphics performance is often the subject of discussion. Are the PCI Express 2.0 x8 and x16 equivalent in terms of performance on our 3D cards? What’s the impact of a x4 interconnect width, and what are the differences between lanes that are directly linked to processors and those which transit via the southbridge? So many questions that we’re going to try and answer in this article!

PCI Express in brief

The PCI Express is a bus for all seasons. With a relatively simple physical implementation, this point-to-point serial interface is organised in the form of lanes. Each lane is made up of two pairs of links, each of which transmits data in one direction. Physically speaking, PCI Express is a bi-directional serial bus. Bandwidth can be increased by bonding (or training) several lanes so that they can be used together (up to 32). In our PCs, standard implementation is however 16 lanes, known as x16.

One of the particularities of PCI Express is that the number of lanes used is automatically negotiated between the host and the downstream device. A x16 peripheral, such as a graphics card, can still function at x1. The size of a slot on a motherboard isn’t what determines the running speed of the device connected to it. A x1 device in an x16 slot will will run at x1. A x16 device in an x16 slot (physically speaking) but wired electrically at x4 will however run at x4. Motherboard manuals indicate how slots are wired and this increasingly depends on the processor and the chipset used.

With simple and flexible physical implementation, PCI Express has become the spinal column for motherboards. The Intel DMI bus used to connect the processor to the motherboard southbridge is in reality a x4 PCI Express interconnect (2.0 on the SandyBridges and chipsets that accompany it). On laptops, the ExpressCard exports a PCI Express lane, and the recently announced Intel Thunderbolt is also an extension cable directly based on PCI Express. A bus for all seasons.

In practice

We chose to look at performance in the different modes on a motherboard based on the Intel Z68 chipset. The card used for this test was an MSI Z68A-GD80 B3 (not to be confused with the G3 version presented at Computex). We'll come back to this in detail later but it has been designed for Intel Sandy Bridge processors, which have a 16 lane on-die PCI Express controller. Three x16 PCI Express ports (in blue on the photo below) are nevertheless included on the motherboard.

The first slot at the top is wired at x16. The second slot is only wired at x8. In practice if a card is inserted into this slot, the first slot then also drops to x8. Four Pericom chips serve in effect as a switch to divide the 16 lanes into two groups of 8. The last slot at the bottom is linked to the chipset with four lanes. It therefore shares the x4 link which connects the chipset to the processor with the rest of the peripherals (network, drives and so on) connected to the southbridge, introducing another potential bottleneck in terms of bandwidth. To carry out our test we used two high-end GPUs from AMD and NVIDIA with relatively equivalent performance, the Radeon HD 6970 and the GeForce GTX 570. Thanks go to Nicolas et Fils for the loan of certain products.

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