PCI riser weirdness

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PCI riser weirdness

Postby doctorzaius » Wed Jun 17, 2020 6:30 am

Okay, my motherboard has the following:

3x PCIe 3.0 x16 slots (support x16/x0/x4, x8/x8/x4 modes)
3x PCIe 3.0 x1 slots

I have two GPUs (both RTX 2070) and I'd like to run one using a riser because that way both of them stay cool without screaming fans. There is some speed hit doing this, even running linux, but its not that big of a deal. I would also like to add a third GPU and this is where the problems start. Adding the third GPU means that I have to attach the riser to one of the 3.0x16 slots. And that works, except that performance drops by 5x or more. So a WU that would take 1 hour to complete on the x1 slot, takes 5 or more hours on the x16 slot. I have a feeling this may be a PCIe issue (and not a driver issue). But I figured I start here. Anyone seen this?
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Re: PCI riser weirdness

Postby PantherX » Wed Jun 17, 2020 8:15 am

Welcome to the F@H Forum doctorzaius,

From what I have read here, as long as you're running Linux, you can use 3x PCIe 3.0 x16 Slots without encountering a a significant performance impact. However, if you're using any of the 3x PCIe 3.0 x1 Slots, you will encounter significant slowdown.

Is this the current setup:
PCIe 3.0 x16 running at x8 -> RTX 2070
PCIe 3.0 x16 running at x8 -> RTX 2070
PCIe 3.0 x16 running at x4 with riser -> Third GPU

Also, is the riser a powered one or not?
Do you have sufficient power from the PSU to drive those GPUs?
What kind of CPU do you have?
What are the operating temperatures of the GPUs?
ETA:
Now ↞ Very Soon ↔ Soon ↔ Soon-ish ↔ Not Soon ↠ End Of Time

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Re: PCI riser weirdness

Postby doctorzaius » Wed Jun 17, 2020 6:09 pm

Additional data: the riser is powered and the PSU is 1600 watts. Processor is Intel Coffee Lake. GPUs are cold. I think they have a target temp of 61C and they both operate around that temp. The remotely mounted GPU achieves that with intermittent fan use. The GPU mounted on the motherboard uses about 30% of fan capacity. Picture is worth a thousand words: https://imgur.com/a/4Ifm9ho
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Re: PCI riser weirdness

Postby foldy » Wed Jun 17, 2020 6:59 pm

Maybe the 3rd GPU just got a work unit which takes more time? Or the 3rd riser is damaged? It should really work in theory - even putting 4 GPUs in.
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Re: PCI riser weirdness

Postby doctorzaius » Wed Jun 17, 2020 7:12 pm

I haven't installed the 3rd GPU yet. This happens just moving the 2nd GPU to what I assumed would be a faster slot. The big problem is that, as it is, there's only one little slot available. And if I can't use the bigger slots, there's no reason to even install the 3rd.
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Re: PCI riser weirdness

Postby PantherX » Wed Jun 17, 2020 8:56 pm

Thanks for the picture! If the sliver PCIe Slot is slow, then look at your motherboard if it is sharing the PCIe lanes with an M.2 SSD or something else. I agree with foldy that it should work without issues.
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Re: PCI riser weirdness

Postby doctorzaius » Wed Jun 17, 2020 10:29 pm

Thanks all. I replaced the little card that plugs into the PCI slot and everything is good. So, electrical problem. Which is weird because it was working--just really, really slowly. Thanks again.
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Re: PCI riser weirdness

Postby bruce » Wed Jun 17, 2020 10:52 pm

I'm not sure exactly what you're describing, but if you have, say, a x4 connection with a bad pin somewhere, the connection might deprecate itself to, say, a x1 connection.
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Re: PCI riser weirdness

Postby doctorzaius » Thu Jun 18, 2020 4:08 am

I think that is exactly what happened.
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