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PCI Express 3.0: impact on performance
by Guillaume Louel
Published on February 24, 2012

With the successive arrivals of the Intel LGA 2011 platform and the AMD Radeon HD 7970, PCI Express 3.0 has become a reality. Have the expected increases in speed also been delivered? Is there an impact on performance in applications and games? What about CrossfireX? So many questions for us to answer!

Version 3.0?

As we explained in a previous article on the subject, the PCI Express bus is a point-to-point serial interface, designed to be relatively low cost in terms of implementation as well as being particularly modular.

Beyond the physical implementation which allows the use of peripherals going from x1 to x32, PCI Express also auto-negotiates across two other parameters with respect to the link-up between a host and a peripheral. Firstly the width of the bus, which can vary independently of physical implementation (a x16 graphics card can run in x1 mode if necessary), and secondly the datarate (2.5 GT/s, 5 GT/s and 8 GT/s for versions 1.0, 2.0 and 3.0 respectively). These parameters can be negotiated in real time if necessary as far as speeds go: the operating system can reduce the speed of the bus when it isn’t being used in order to maximise energy saving.

In practical terms, PCI Express 3.0 doubles the bandwidth of PCI Express 2.0, giving a theoretical 16 GB/s in each direction for a peripheral in x16 mode instead of 8. Note however that we aren’t talking 10 GT/s but rather 8 GT/s. This is because while the first versions of PCI Express encode their data on 10 bits for just 8 bits of data (4 GT/s theoretical), PCI Express 3.0 introduces a more complex 128bit/130 bit encoding scheme which maximises the efficiency of an 8 GB/s link.

For the time being, only the LGA 2011 platform based on the LGA 2011 chipset and Sandy Bridge-E processors allows you to benefit from PCI-Express 3.0. In April, the LGA 1155 Ivy Bridges will also support the new standard. There will be backwards compatibility of course and PCI Express 3.0 cards do run on 2.0 and 1.0 ports and vice-versa.
In practice

To carry out our test, we used an X79 motherboard from Asus, the Maximus IV Gene in the Micro ATX format. As of its latest BIOS, it allows you to choose between 1.0, 2.0 and 3.0 modes for your PCI Express ports.

The first two motherboard slots are cabled at x16 while the third, though x16 physically, is only cabled at x8. We used only the first two motherboard slots for our tests.

We also used two Radeon HD 7970 graphics cards as they introduced PCI Express 3.0 support. The test machine processor was a Core i7 3960X and we used 16 GB of DDR3 clocked at 1600 MHz. The tests were carried out in Windows 7 x64 SP1.

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