Question How does GPU with 225W gets power?

[prakhars962]

NVIDIA RTX A5000 has a TDP of 230W. It supports PCIe 4 which draws 75W and allows you to put a 8-pin connector which draws 150W. So, 75+150=225W. How they are gonna provide 230W to the GPU?

Does dual-lane GPU draw 75*2 from the PCI lane?

As the power draw from the PCI lane is same for PCIe 3 and 4, does that mean RTX A5000 should work with PCIe 3 slot?

I am asking this because I have a PSU supplying 1300W. The PC supports PCIe 3. But whenever I stress the GPU, the PC shuts down. The PC is a prebuilt Dell Precision Tower 7910. There are two 6-pins and one 8-pin to power 3 GPUs. tried both 8 pin to 8 pin connector and a dual 6 pin to 8 pin connector. But the results are the same.

Here is a video of the power failure when stressing the GPU for rendering:

View: https://youtu.be/LTDMlMiNMh8

 

[Eximo]

150W is a guideline, in reality a 6-pin carries two 12V wires, and an 8-pin carries three 12 wires. So by the standard 2 wires is 75W and 3 wires is 150W. No, closer to 60W per wire before there is significant voltage droop, so 180W and more is perfectly acceptable through an 8-pin connector.

Yes, the physical PCIe slot power output is the same across all PCIe revisions. That doesn't mean your motherboard can handle 225W through the slots by themselves. So that is one place to check. What does Dell recommend for GPUs in their purchasable configurations. That is where I would stop.

As to your problem. I don't know enough about Dell 1300W PSUs to say. Could be multi-rail and you are exceeding a rails OCP, or it is a single rail and the overall load has transient spikes that last long enough to trip the overall OCP.

Which specific three GPUs, or are they all A5000? Or is the problem only occurring when you stress the single GPU? (Could be the GPU)

 

[prakhars962]

It is hard to tell, the PSU is custom made Dell and I have never opened a Workstation PC. Here is the PSU:

https://imgur.com/HZsRG0R

View: https://imgur.com/HZsRG0R


The manual says it support it supports 3*225W GPU. I am using one A5000 with either 8 pin to 8 pin or two 6pin to 8pin. In both cases the PC shuts down.

As per the manual. the PC supports a NVIDIA Quadro M6000 24GB released in 2015 with the 8 pin connector. The card has a TDP of 250 W which is more than my GPU (A5000).

I have to remove the motherboard to see the rails. I am not confident that I can replug everything.

 

[DRagor]

The manual says it support it supports 3*225W GPU.

First, the link does not work, it goes to some Dell page instead of pdf.
Second, your claim is impossible. To be able to run 3 225W GPUs the PSU would need 3 cables ending with 8 pin connectors each. While you wrote yours has one 8 pin and two 6 pins. Note that Dell manuals are usually covering a lot of different but similar models so it is possible that that claim only applies to different model then yours. Btw that PSU picture is unusable to us. Please show the top sticker only (with picture quality good enough to be readable).

 

[Eximo]

Well what I can find of the 7910 specs would say it is a multi-rail PSU with a silly number of rails. Up through J (10 rails). And yes, according to Dell up to three 225W GPUs.

However, an Ampere card is likely to have transient power spikes well beyond 225W. And if they last long enough that would easily trip the PSU. Just one of those things you have to deal with. My first suggestion would be to set an artificial power limit of like 180W and see how it goes.

After that you have to look at silly solutions like adding a power supply which carries a lot of risk.

 

[prakhars962]

Here is the PSU's image. Hope its readable.

https://imgur.com/nloHtFw

View: https://imgur.com/nloHtFw


BTW I recorded that the PSU stops at around 39W with two 6-pin to 8-pin connector. Here is the video:

View: https://www.youtube.com/watch?v=4w_H7Cfkcvs


The 8-pin to 8-pin connector stopped at 101W. Here is the video:

View: https://www.youtube.com/watch?v=LTDMlMiNMh8


The output is nvidia-smi dmon where the second column shows the power consumption.

 

[DRagor]

Here is the PSU's image. Hope its readable.

Thanks. So, each 12V rail has a limit of 18 A. Which means it tops at about 216 W. It is perfectly possible for a GPU that has TDP of 230 W and only single PCIe connector to exceed those 216 W. Although it is true that the slot can provide up to 75 W most GPUs will try to draw most of the power from the cable rather then slot. Not saying it has to be the case here but it is possible your GPU overloads PSU rail and this causes shut down. Especially if we consider transient spikes where power draw can double temporarily.
Of course there are other possibilities as well. We can't rule out that your GPU is faulty. To test that you would need to put it in another PC and see if it shows same behavior.

 

[prakhars962]

The GPU was awarded to use by NVIDIA, I mean it was free. The university checked it when it was arrived. It did pass the userbenchmark on another PC which it didn't pass on my PC. So, I can say that the GPU is not faulty.

My university's IT team couldn't conclude anything and they just asked me to buy a new PC. I capped the maximum power consumption at 140W. Still it turns off when stressing the GPU. Is there a way to combine two 6-pin and one 8-pin as the input. I know this is madness.

 

[DRagor]

So, I can say that the GPU is not faulty.

In that case there are only two possibilities: PSU and motherboard. Was the motherboard working with any other GPU that also needs a lot of power?

Is there a way to combine two 6-pin and one 8-pin as the input.

Not sure. Never heard of such adapter.

 

[prakhars962]

Yes somebody was using RTX 2070 8GB (2018 card) before my joining. Seems my card is too much for the PC to handle.