PCI Express over Fiber [video]
88 points by mmastrac 6 days ago | 28 comments
mmastrac 3 hours ago
This was a super interesting video to watch. I honestly thought SFP required more setup, but this explains why AliExpress is so ripe with USB3 and HDMI over SFP converters that are dirt cheap.
replyjauntywundrkind 9 minutes ago
It's been amazing having 6 years of fiber optic HDMI & DP monitor connections, that work so so so well. I bought some no name one on Amazon in ~2019 and was flabbergasted it was real & worked.
replySuch a huge upgrade from the heavy thick 35 ft HDMI<->dvi cable I've used for so long.
Literally the only downside is figuring out how to roll it up, which I still haven't figured out how to do well with the 150ft cable I have.
It was astoundingly cheap too. I think the first one I got was under $60?! No one really knew the segment existed, they just needed to get some sales, I assume. I heard usb3 has been available but they've been bulky & expensive. Where-as the whole fiber optic cable seamlessly integrates the transceiver on mine. I like Cable Matters, they make some fine ones.
ahepp 2 hours ago
How does this compare to something like RDMA over Converged Ethernet (RoCE)?
replypncnmnp 51 minutes ago
A fun tangent - if someone wants to explore how Azure is performing RDMA over RoCEv2 - check this paper out - https://www.microsoft.com/en-us/research/wp-content/uploads/...
replyThere is an interesting NSDI talk on the paper too - https://www.youtube.com/watch?v=kDJHA7TNtDk (2023)
fl4regun 3 hours ago
Cool project! PCIe itself I think is likely to end up doing something similar soon, there are provisions in the spec now for optical retimers.
replyrussdill 2 hours ago
There's a number of optical modules for TB3 and TB4, might be an easier (but less fun) route as TB3 and TB4 can carry PCIe.
replywhalesalad 2 hours ago
So you're saying I can put a handful of 4090's out in the middle of snowy Michigan with a handful of OM4 cables snaking into my basement to run legit arctic cooling with no noise?
replymyself248 2 hours ago
No part of Michigan is in the arctic, but sure, outside of mosquito season, that would work.
replypreisschild 2 hours ago
Might as well put your entire computer outside and use thunderbolt/usb-4 over fiber docks
replyphendrenad2 2 hours ago
Watercooling loop light be better, the radiator fins will still rust from condensation.
reply
While at a higher level, thunderbolt and https://en.wikipedia.org/wiki/ExpEther can both of course work over fiber too!
(Q|O)SFP are basically just raw high speed serial interfaces to whatever - you see this a lot in FPGAs, you can use the QSFP interfaces for anything high speed - PCIe, SATA, HDMI…
> [snip]
> As explained in the intro, this doesn’t come without a host of compatibility issues, least of all PCIe device detection, side-channel clocking and for PCIe Gen 3 its equalization training feature that falls flat if you try to send it over an SFP link.
So, uh… what’s the benefit? How much overhead does Thunderbolt really introduce, given it solves these other issues?
- Learning : I want to learn about the lower level of PCIe and it's a good project. - Re-use of cabling : I have a bunch of single mode fiber bundle going around already. You can't find thunderbolt that just have a LC connector ... - Isolation : Active thunderbolt cable still often have copper for some low speed signals, they don't offer true galvanic isolation - Avoid dealing with thunderbolt. I want a custom chassis/pcb at one end and chips to convert from TB back to PCIe are not readily available to make custom stuff with ... (not as an individual anyway).
So yeah, if you want a ready to use solution, TB cable is absolutely a good choice, here I'm having some fun, learning in the process and hopefully sharing some of the knowledge.
Thunderbolt 5 offers 80Gbps of bidirectional bandwidth. PCIe 5.0 16x offers 1024Gbps of bidirectional bandwidth. This matters.
TB5 cables can only get so long whereas fiber can go much farther more easily. This means that in a data center type environment, you could virtualize your GPUs and attach them as necessary, putting them in a separate bank (probably on the same rack).
I couldn’t find any optical TB5 cables, but here’s a 4.5m TB4 one: https://www.owc.com/blog/the-new-superlong-40gb-s-owc-active...
And if TB3 is enough, Corning makes them in lengths up to 50m: https://www.corning.com/microsites/coc/oem/documents/ocbc/OE...
As for bandwidth, the medium transition seems to actually limit the author’s capabilities by losing some of the more advanced link-training features that are necessary for the highest-bandwidth PCIe 3 connections, never mind PCIe 5.
Good luck getting a 1Tbit tranceiver. Anydirectional. Also it's 512Gbitish per direction.
One note: I believe the SMF max fiber length is 2km not 1m [1]. The data sheet [2] also says:
> - 2000m max on single mode fiber
[1]: https://www.vitextech.com/products/1-6t-osfp-2fr4
[2]: https://resource.fs.com/mall/resource/cn_osfp-2fr4-16t-data-...
There's already 800Gb transceivers readily available, 1.6 is probably getting preview deploys to some hyperscalers & other early adopters as we speak.
But yes I meant 512Gbps each way, to be clear.