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Your ultimate guide to understanding PCIe 4.0

By ·Categories: Tech Explained·Published On: February 25th, 2020·4.8 min read·

Is PCIe Gen 4 worth the upgrade?

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. We’re here to help, let’s get started.

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. 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

The importance of PCIe encoding

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 it 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 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 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.

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13 Comments

  1. Alwaleed August 22, 2020 at 11:38 am

    That was useful, I really appreciate it.

  2. Jennifer March 13, 2021 at 8:08 am

    My 1070 Ti is on the way out and I’m looking to replace with a more modern GPU (i.e. 3060), however my motherboard has PCIe 3 nd the 3060 uses PCIe 4. Am I able to use the 3060 with the PCIe 3? And thank you, this was very informative!

  3. Sarah Lavoie (OnLogic) March 26, 2021 at 2:35 pm

    Hi Jennifer! I asked a member of our engineering team and his answer was short and sweet – yes, the 3060 will run just fine at PCIe Gen 3.

    Good luck!
    Sarah (OnLogic)

  4. Sammi March 29, 2021 at 9:49 am

    Hey, I got a z490 Mb and a i7 10700k Cpu, I’m waiting on my Rtx 3070, do you think an upgrade to
    an 11700k is worth it for PCIE 4.0? I mean will it really affects the performance?

  5. Paul March 31, 2021 at 7:16 pm

    Hello, I have a similar question to Jennifer’s. I have a 1070ti on a PCIe Gen 3 motherboard but I am looking to upgrade to an RTX 3080 GPU. That being said will the graphics card just use double my PCIe lanes to be able to work at the speeds it’s capable of?

  6. Leah April 5, 2021 at 1:32 am

    Why upgrade to a PCIe 4 GPU only to put it right back into the old PCIe 3 GPU slot? What is the point if the Motherboard is not upgraded too? Is there an adapter for that?

  7. Nika April 8, 2021 at 4:12 pm

    Hi,
    Thank you for sharing information.
    I am wondering, though:
    How can one write “Currently” if there is no date anywhere in the text?
    How can I know in April 2021 if this is written yesterday, one month ago, one or ten years ago?
    Cheers

  8. Darek Fanton April 15, 2021 at 3:55 pm

    Hi Nika, there’s a publish date at the top of all of our blogs. This piece was published February 25th, 2020. We also hate when there’s not a date on articles :)

  9. Shubham April 21, 2021 at 7:09 am

    Hello,
    Thanks for the information.
    I just wanted to ask that , My motherboard supports PCIe Gen3 GPU and if I suppose buy RTX 3070, will the GPU support my motherboard?
    Thnaks

  10. Darek Fanton May 14, 2021 at 2:25 pm

    Leah, as you point out, your performance will only be as good as the lowest performing component, so you’d want to make sure all elements of your setup support PCIe 4.0 to get the most out of it.

  11. Darek Fanton May 14, 2021 at 2:28 pm

    Hi Shubham, PCIe is backward compatible, so the card would work just fine, but you’d be limited in speeds by your PCIe 3.0 motherboard.

  12. Darek Fanton May 14, 2021 at 2:42 pm

    Hi Sammi, if you’re concerned about gaming performance, there will be little to no noticeable difference in performance. In addition, you need to make sure your motherboard supports PCIe Gen 4.0 as not all Z490 motherboards do. Refer to this video if you would like a deeper dive: https://www.youtube.com/watch?v=0DKVVtirNM8

    The video references the AMD X570 platform but if there is a performance hit it will be less than 5% in gaming workloads.

  13. Darek Fanton May 14, 2021 at 2:43 pm

    Hi Paul, the number of PCIe lanes will stay the same from a 1070Ti to an RTX 3080 (16 Lanes) if you have a PCIe Gen 4 compatible motherboard and CPU. The difference will be the data rate each lane can provide (PCIe Gen 3.0 = 8GT/s per lane, PCIe 4.0 = 16GT/s per lane).

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