Your ultimate guide to understanding PCIe 4.0
PCIe 4.0 is the next evolution of a widely used, high-speed interface. Because nothing is ever quite fast enough when it comes to technology, it’s no surprise that PCIe 4.0 has been talked about a lot. But with a lot of conversation comes a lot of misconceptions and outstanding questions that need to be addressed, so we are here to do just that. Let’s get started.
What is PCIe Gen 4.0 and how is it different from PCIe Gen 3.0?
Simply put, PCIe 4.0 has double the throughput of PCIe 3.0. The two standards are structurally very similar, with the key difference being the higher transfer rate and, in some cases, the material used to achieve successful transmission of the PCIe signal. PCIe 3.0 offers a data transfer rate 8GT/s the rate of bits (0’s and 1’s) transferred per second that get transmitted from the host to the end device or endpoint. A real world example would be an NVMe SSD operating at PCIe 3.0. Industry standard NVMe drives use 4 PCIe lanes. Translating this, your peak theoretical bit rate via PCIe 3.0 would be: 4 Lanes x 8 (GT/s / Lane) = 32GT/s.
4 Lanes * 8 (GT/s/Lane) = 32GT/s
PCIe 4.0 operates at 16 GT/s per lane.
4 Lanes * 16 (GT/s/Lane)=64GT/s
To determine the actual amount of data that can be transferred, the encoding technique must be understood. PCIe Gen 3.0 and PCIe Gen 4.0 use a 128b/130b encoding technique. Older generations such as PCIe Gen 2.0 use 8b/10b encoding. This encoding technique transforms 128-bit data into 130-bit line code. This allows for reasonable clock recovery (which is the process of extracting timing information from a data stream) and ensures alignment of the datastream. The two extra bits contain a preamble which aids in the clock recovery of the serial data stream to allow the receiver to decode the transmitted signals. This means, the maximum theoretical bandwidth in Gbps (Gigabit per second) of each PCIe Gen 4.0 looks like this:
16GT/s * (128b/130b) =15.754Gbps
Knowing the encoding allows us to calculate the overhead required to transmit PCIe data streams. This is found by:
([130b-128b]/130b)*100 = 1.54%
The increase in bandwidth enables faster data transmission between the PCIe and the endpoint (SSD, GPU, etc). The doubling in bandwidth is made possible through new PCIe 4.0 controllers, like the one included in the AMD X570 chipset, as well as through low-loss dielectrics materials. Low-loss dielectric materials allow higher speed signals to propagate further through PCBs.
To learn more about the math behind the PCIe 4.0 process, check out this article.
The engineering behind PCIe 4.0 gets pretty complicated and involves a lot of science. Check out this article to learn the details behind the development of PCIe 4.0.
How does PCIe 4.0 affect my choice of SSD, NVMe, and GPU?
Like PCIe 3.0, PCIe 4.0 is forward and backward compatible. This means that PCIe 4.0 can be used as a direct replacement for PCIe 3.0, but it also means that if you connect a PCIe 3.0 card to the PCIe 4.0 slot, the card will perform to the PCIe 3.0 specs. That said, PCIe 4.0 offers another key advantage in addition to its higher bandwidth outlined above, and that’s the ability for designers and system integrators to increase the amount of expansion cards on a platform. For example, devices requiring up to 100Gbps of bandwidth now only require 8 lanes with PCIe 4.0 compared to 16 lanes with the older PCIe 3.0. If you are buying chipsets supporting PCIe 4.0, here’s how it will affect your choice and use of GPU and SSD.
PCIe 4.0 GPU
Due to the forward and backward compatibility, a PCIe 3.0 GPU will perform like a PCIe 3.0 GPU card if connected to a PCIe 4.0 (or in the future a PCIe 5.0) slot. The specs of your GPU card do not change. The only benefit to connecting to a PCIe 4.0 would be able to leverage a PCIe 4.0 endpoint such as AMD’s RX 5700XT. PCIe 4.0 would allow quicker transfer of the data being loaded on the GPU’s memory and decreased latency on the PCIe bus. As video games continue to increase in file size and graphical complexity, and Machine Learning applications continue to require larger and larger data sets, PCIe 4.0 will play a large role in increasing frame rates and reducing compute time.
To further see how PCIe 4.0 speeds differ from PCIe 3.0, check out this video that compares the frame rates.
PCIe 4.0 SSD
Similar to PCIe 4.0 GPUs, connecting a PCIe 4.0 NVMe SSD will result in higher data transmission rates and lower latency. However, the forward-backward compatibility still applies, so you can connect any SSD Gen to the PCIe 4.0 interface, and the speed will be determined by the PCIe SSD’s controller generation. The major impact is with SSD read and write speeds as well as the latency required to retrieve or write the data. NVMe drives in PCIe 4.0 SSDs can be twice as fast as their previous Gen 3 counterpart during operation.
Where can I buy chipsets supporting PCIe 4.0?
Currently only AMD offers PCIe 4.0 technology on their x86 platforms, including:
- The Ryzen 3000 Series paired with a motherboard featuring it’s premium X570 chipset
- The Threadripper 3000 Series on the new TRX40 platform
- AMD Epyc Rome server platform
Should I wait for PCIe 5.0?
Future products from both AMD and Intel will support PCIe 4.0, particularly Intel’s Ice Lake Scalable Xeon servers and, likely, subsequent generational releases across their product lines. One common misconception is that PCIe 5.0 is “just around the corner”. Unfortunately, while the specs for PCIe 5.0 are currently available, PCIe 5.0 is unlikely to be commercially available for another few years. Luckily, you can expect some OnLogic AMD products to support PCIe 4.0 in the very near future.