6 ms·

What's the current state of eGPUs these days? I had a quick look, and wow, there aren't many eGPU enclosures going about, and what there are, are expensive. I

What's the current state of eGPUs these days? I had a quick look, and wow, there aren't many eGPU enclosures going about, and what there are, are expensive. I always saw them as intended for laptops rather than desktops - are there any downsides or performance penalties compared to slotting directly into a PCIe slot?

mschuster91 3y ago
> are there any downsides or performance penalties compared to slotting directly into a PCIe slot? Latency, for one. All signals must at least pass through two Thunderbolt/USB4 chipsets on their way to and from the GPU, and at least for anything but high profile AMD CPUs Thunderbolt is connected to the southbridge instead of northbridge (where the x16 GPU slot is).
- solardev 3y ago
  Are there none that still use pcie, just through a cable instead of a slot? That's too bad, if so :(
  - mschuster91 3y ago
    PCIe can only go to like 14 inches in trace length, and that's already challenging enough to do for risers and extender cables without compromising signal integrity. [1] https://www.protoexpress.com/blog/pci-express/ https://www.protoexpress.com/blog/pci-express/
    - solardev 3y ago
      I see. Thanks for the explanation! (Not a hardware guy. Didn't realize there physical/electrical limits to these.)
    - oakwhiz 3y ago
      There is a way to solve this with optical but the cost is still high and it's not terribly compact. Usually you can only get 4 lanes directly into one optic. Most implementations I have seen rely on a 2-port PCIe packet switch chip, and they rely on SRIS/SRNS clocking and hotplug/"surprise removal" support is provided by that switch (don't want a crash if you yank the cable)
    - j16sdiz 3y ago
      otoh, many risers are out of spec and still work
      - mschuster91 3y ago
        There's a difference between "appears to work" and "works at advertised speeds". PCIe is surprisingly tolerant but it will downgrade the link speed during initial training.

mschuster91 3y ago

> are there any downsides or performance penalties compared to slotting directly into a PCIe slot? Latency, for one. All signals must at least pass through two Thunderbolt/USB4 chipsets on their way to and from the GPU, and at least for anything but high profile AMD CPUs Thunderbolt is connected to the southbridge instead of northbridge (where the x16 GPU slot is).

solardev 3y ago
Are there none that still use pcie, just through a cable instead of a slot? That's too bad, if so :(
- mschuster91 3y ago
  PCIe can only go to like 14 inches in trace length, and that's already challenging enough to do for risers and extender cables without compromising signal integrity. [1] https://www.protoexpress.com/blog/pci-express/ https://www.protoexpress.com/blog/pci-express/
  - solardev 3y ago
    I see. Thanks for the explanation! (Not a hardware guy. Didn't realize there physical/electrical limits to these.)
  - oakwhiz 3y ago
    There is a way to solve this with optical but the cost is still high and it's not terribly compact. Usually you can only get 4 lanes directly into one optic. Most implementations I have seen rely on a 2-port PCIe packet switch chip, and they rely on SRIS/SRNS clocking and hotplug/"surprise removal" support is provided by that switch (don't want a crash if you yank the cable)
  - j16sdiz 3y ago
    otoh, many risers are out of spec and still work
    - mschuster91 3y ago
      There's a difference between "appears to work" and "works at advertised speeds". PCIe is surprisingly tolerant but it will downgrade the link speed during initial training.