Sandy Bridge E & X79 PCIe 3.0: It Worksby Anand Lal Shimpi on December 22, 2011 3:49 AM EST
At the launch of Intel's LGA-2011 based Sandy Bridge E CPU we finally had a platform capable of supporting PCI Express 3.0, but we lacked GPUs to test it with. That all changed this past week as we worked on our review of the Radeon HD 7970, the world's first 28nm GPU with support for PCIe 3.0.
The move to PCIe 3.0 increases per-lane bandwidth from 500MB/s to 1GB/s. For a x16 slot that means doubling bandwidth from 8GB/s under PCIe 2.1 to 16GB/s with PCIe 3.0. As we've seen in earlier reviews and our own internal tests, there's hardly any difference between PCIe 2.1 x8 and x16 for modern day GPUs. The extra bandwidth of PCIe 3.0 wasn't expected to make any tangible difference in gaming performance and in our 7970 tests, it didn't.
Why implement PCIe 3.0 at all then? For GPU compute. Improving bandwidth and latency between the CPU and the GPU are both key to building a high performance heterogenous computing solution. While good GPU compute benchmarks on the desktop are still hard to come by, we did find one that showed a real improvement from PCIe 3.0 support on the 7970: AMD's AES Encrypt/Decrypt sample application.
Simply enabling PCIe 3.0 on our EVGA X79 SLI motherboard (EVGA provided us with a BIOS that allowed us to toggle PCIe 3.0 mode on/off) resulted in a 9% increase in performance on the Radeon HD 7970. This tells us two things: 1) You can indeed get PCIe 3.0 working on SNB-E/X79, at least with a Radeon HD 7970, and 2) PCIe 3.0 will likely be useful for GPU compute applications, although not so much for gaming anytime soon.