It's been over two years since our last PCI-Express scaling article. Not much has happened since then in terms of slot technology and bandwidth. Just like two years ago, almost everybody is using PCI-Express X16 3.0, and while Intel's Enthusiast Platform (Haswell-E) with plenty of lanes for even multiple VGA cards is the exception, every other motherboard still offers to split the x16 3.0 slot into two x8 3.0 slots for a dual-GPU setup.
Nowadays, more and more game engines are streaming textures to the GPU during gameplay, so I added the latest and greatest titles to our test suite in order to investigate how things will go looking forward. Modern consoles have a large amount of unified memory the CPU and GPU share, which keeps the performance penalty for streaming textures very low on such platforms, something game developers embrace and often fail to properly adjust for their PC ports.
PCI-Express 3.0 saw its debut on Intel's Sandy Bridge-E processors, but was adopted to the mainstream in 2012 when the Ivy Bridge architecture was released.
While PCI-Express 1.0 pushes 250 MB/s per direction, PCI-Express 2.0 pushes 500 MB/s, and PCI-Express 3.0 doubles that to 1 GB/s. The resulting absolute bandwidth of PCI-Express 3.0 x16, 32 GB/s, might seem like overkill, but the ability to push that much data per lane could come to the rescue of such configurations as 8-lanes (x8) and 4-lanes (x4).
In this review, we test the impact of running the GeForce GTX 980 on electrical Intel Haswwell PCI-Express x16, x8, and x4 slots. We made a point of testing all three generations of the PCI-Express interface: 1.1, 2.0, and 3.0.
This review is made possible thanks to an awesome BIOS option given to us by the ASUS ROG Maximus VI Hero motherboard which allowed us to toggle the CPU's PCI-Express root complex between PCI-Express 1.0, 2.0, and 3.0. Using common plastic adhesive tape to cover up and disable lanes, we also successfully modified the number of lanes available to the GPU for these tests.
For your reference, we have written similar articles in the past: Ivy Bridge PCIe Scaling, GTX 480 PCIe Scaling, HD 5870 PCIe Scaling.
For the graphs on following pages, we've colored same-bandwith configurations in the same color, as an easy visual reference. PCIe x8 3.0, for example, offers just as much bandwidth as PCIe x16 2.0, which is why both are of the same color.