PCIE gen 4.0 to gen 3.0 plx bifurcation

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fudzishtorm

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Feb 8, 2020
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Hello, is it possible (and is there any such product already on the market) to use 16 pcie gen 4.0 lanes to provide full bandwidth 2 x 16 pcie gen 3.0 lanes (with some latency drawbacks obviously)? Can we expect a dual socket epyc rome server that is capable to serve 8 full-bandwidth pcie 3.0 gpus? Isn't g292-z40 the one (the plx model is not provided by the manual)?
 
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i386

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Mar 18, 2016
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It would be possible with pcie switches, but pcie switches are expensive and you have to route the additional lanes through pcb (mainboard, risers or daughterboards) increasing the complexity
To be honest I don't think there is a viable market for such a system anymore; there are other interconnect technologies that provide more bandwidth than pcie (even 4.0) eg nvidia nvlink
 
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zir_blazer

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Dec 5, 2016
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It is technically possible, but at the very least the PCIe 4.0 Switch chips have to exist first. I don't know whenever Avago refreshed the PLX series with at least a PCIe 4.0 uplink.

Then you need someone that uses such PCIe Switches either integrated to the Motherboard itself, or in a card of sorts. Just check this beauty, a riser with a PLX chip in it.
 
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inf3rno

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Mar 24, 2021
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It is technically possible, but at the very least the PCIe 4.0 Switch chips have to exist first. I don't know whenever Avago refreshed the PLX series with at least a PCIe 4.0 uplink.

Then you need someone that uses such PCIe Switches either integrated to the Motherboard itself, or in a card of sorts. Just check this beauty, a riser with a PLX chip in it.
What about M2? I read on the boards that they support PCIE gen4 M2 x4, so in theory those can be converted to gen3 x8. It would be nice to have an ITX board with 4x8 gen3 lanes all coming from a single x16 gen4 slot plus some x8 coming from M2 too. :D I don't know if the standards allow such downgrading, probably it is too complicated and better to buy a server grade ITX board with many slots. Not sure if there is any small form factor alternative. Thunderbolt is interesting, but it is only x4 just as M2. I would get rid of these big connectors for memory and videocards, I think it would be possible to have smaller boards without them with the current technology.
 
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pixelBit

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Jul 16, 2019
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Hello, is it possible (and is there any such product already on the market) to use 16 pcie gen 4.0 lanes to provide full bandwidth 2 x 16 pcie gen 3.0 lanes (with some latency drawbacks obviously)? Can we expect a dual socket epyc rome server that is capable to serve 8 full-bandwidth pcie 3.0 gpus? Isn't g292-z40 the one (the plx model is not provided by the manual)?
You can get a custom riser.





 
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inf3rno

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Mar 24, 2021
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C_Payne writes here sometimes: PCIE Bifurcation you can ask him if you want a custom card. As far as I remember he was working on PCIE 4 support, but I don't know if he managed to do it. I don't think the PCIE 4 -> PCIE 3 conversion is possible with keeping the bandwidth and doubling the lane number. I believe it requires a lot more than a simple riser card. Something similar to PLX chips, which is expensive and it might be a non-existent technology currently.
 
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