PCI Express 4.0 Specs Released Doubling Bi-Directional Bandwidth To 64GB/s

The PCI standards group PCI-SIG (Special Interest Group) comprised of AMD, Intel, NVIDIA, and other technology bigwigs announced it has finalized the specifications for the PCI Express 4.0 standard. It has previously announced in June at the group's DevCon event that the spec was feature complete and undergoing review. Now several months later PCI Express 4.0 is finalized, and fast.

PCIe 4.0 offers a transfer rate of 16GT/s with flexible lane width configurations. That is double the raw bitrate of PCIe 3.0 and more than triple the bitrate of PCIe 2.0. In full duplex mode, that translates to around 64GB/s of bi-direction x16 bandwidth, whereas PCIe 3.0 topped out at around 32GB/s, and PCIe 2.0 at 16GB. Going back to PCIe 1.0, that spec hit a ceiling of just 8GB/s.

PCI Express

What is interesting about those previous specifications is that PCI-SIG initially stuck to a 3-4 year release cadence. PCIe 1.0 debuted in 2003, followed by PCIe 2.0 in 2006 and PCIe 3.0 in 2010. It has been 7 years since the last PCIe spec was finalized before PCIe 4.0. Part of the reason is because it is not easy to double the bandwidth every few years while maintaining backwards compatibility. However, there wasn't a real need for more bandwidth, not until recently anyway.

PCIe 4.0 arrives at a time when more attention is being paid to the PCIe bus. In addition to graphics cards, storage is making a transition to PCIe in the mainstream, with newer M.2 NVMe solid state drives such as Intel's newly minted Optane SSD 900P Series shuttling data through the PCIe bus.

That said, AMD and NVIDIA have de-emphasized running multiple graphics cards, mainly because the support from developers just has not been up to par—in many cases, running two GPUs does not double the performance of games over a single GPU, and sometimes it can even hurt performance. With the new PCIe 4.0 spec, however, it will interesting to see if multiple graphics cards becomes a point of focus.

In the meantime, PCI-SIG says the spec is being received well by early adopters.

"We’ve seen unprecedented early adoption! Prior to publication, we’ve had numerous vendors confirmed with 16GT/s PHYs in silicon and IP vendors already offering 16GT/s controller. Given the interest, we held a pre-publication Compliance Workshop with preliminary FYI Testing Only for PCIe 4.0 architecture that attracted dozens of solutions. We’re continuing to conduct FYI testing in our workshops throughout the remainder of the year," PCI-SIG said.

On top of that, PCI-SIG is already looking ahead to PCIe 5.0. The standards body expects to have an early version available by the second quarter of 2019.