PCI Express 3.0: impact on performance (page 5: Conclusion)

Conclusion

When we look at the theoretical performance, we can see that some of the PCI Express 3.0 promise has been fulfilled. Firstly, the increase in bandwidth does have an impact, particularly with GPU to CPU transfers, though this isn't as significant in the opposite direction even if there are theoretical gains of (just) 50%. The reason for this more limited gain is difficult to measure as we only have a single PCI Express 3.0 compatible platform for now, Intel’s LGA 2011, and one PCI-E 3.0 graphics card, the Radeon 7970. Whether it comes from the interface, platform or card, it’s difficult for us to say as yet.

In practice the OpenCL applications that we were able to test are for now far from being limited by memory bandwidth. In already offering almost 7 GB/s, PCI Express 2.0 x16 covers most usages, even if certain very specific pro applications will certainly be able to take advantage of the theoretical gains.

On the gaming side, the increases in bandwidth aren’t of much more use. While there’s still a difference between PCI Express 2.0 x16 and x8 (a difference that is even more marked with PCI Express 1.0 x16 in CrossFire), in practice this boils down to no more than one percent between PCI Express 3.0 x16 and x8 modes with a single card and half a percent in CrossFire. This is positive for those who are hoping to use CrossFire on future Ivy Bridge platforms, whether with two cards at x8/x8 or three cards at x8/x4/x4 as in PCI-Express 3.0 x4 mode doesn't bring performance down too much.