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Your Ultimate Guide to Understanding PCIe Gen 4.0

By ·Categories: Tech Explained·Published On: April 15th, 2022·4.7 min read·

Is PCIe Gen 4 worth the upgrade?

PCIe Gen 4 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. We’re here to help.

To learn more about PCIe in general, be sure to check out our video, PCIe Basics in 60 Seconds.

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 of 8GT/s, or Gigatransfers per second. GT/s is the rate of bits (0’s and 1’s) transferred per second that get transmitted from the host to the end device or endpoint.

Let’s look at a real world example of NVMe drive operating at PCIe 3.0 and PCIe 4.0. 

  • Industry standard NVMe drives use 4 PCIe lanes. 
    • Your peak theoretical bit rate via PCIe 3.0 would be: 
      • 4 Lanes x 16 (GT/s/ Lane) = 32GT/s.
    • Your peak theoretical bit rate via PCIe 4.0 would be
      • 4 Lanes x 32 (GT/s/Lane)= 64GT/s

The Importance of PCIe encoding

A graphic comparing PCIe speeds

Understanding the encoding technique is necessary to determine the actual amount of data that can be transferred. PCIe Gen 3.0 and PCIe Gen 4.0 use a 128b/130b encoding technique. On the other hand, 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.

([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.

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. However, if you connect a PCIe 3.0 card to a 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 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 potential benefit would be leveraging a PCIe 4.0 endpoint such as AMD’s RX 5700XT. This would allow quicker transfer of the data being loaded on the GPU’s memory and decrease 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 key 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

Connecting a PCIe 4.0 NVMe SSD will result in higher data transmission rates and lower latency. 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. This has a major impact on SSD read and write speeds and 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.

What Processors Support PCIe 4.0?

For those looking to take advantage of the most throughput, the following processors support PCIe 4.0:. 

  • 11th Gen Core Processors by Intel 
  • 12th Gen Core Processors by Intel 
  • 3rd Gen Xeon Scalable Processors by Intel 
  • Ryzen 3000 and 5000 Series Processors by AMD
  • EPYC 7002 and 7003 Series Processors by AMD

Check out our newest line of rugged computers, the Karbon 800 Series. The Series is built upon Intel’s 12th gen Core processors and they support PCIe 4.0.  

Should I wait for PCIe 5.0?

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 months if not years. Want to learn more about PCIe? Check out our Ultimate Guide to PCIe 5.0.

Note: We originally posted this blog on Feb 25, 2020. We updated it for content on April 15, 2022. 

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About the Author: OnLogic

OnLogic is a global industrial computer manufacturer that designs highly-configurable, solution-focused computers engineered for reliability for the IoT edge.