Quick question about PCIe ports and max number of cards

So, lets say for sake of argument, I want to avoid any "but why would you?" comments, I am not gaming on my machine, it is for work, and I utilize every last byte of graphics ram, and every last cuda core, if I install a third card using a riser on a machine meant for 2 cards, putting my third card at pcie 3.0 x4, no SLI or NvLink, would using a third card throttle all the cards to x4? That was my understanding a long time ago when I first visited the topic probably close to a decade ago, but at the same time, I really am just not that familiar with the concepts these days.

The bandwidth on a couple cards is relatively inconsequential to me. My main concern is realtime performance drop. What kinds of problems might I run into if the bandwidth does drop? would typical 3D functionality slow, or could an increased number of polygons on a screen in 3D space slow or lag the frame rate due to 3 cards?

This is not a set in stone thing I am chasing right now, but I am very interested to know what kinds of problems I may face if my speeds ARE bottlenecked, and the fastest speed I have is 4gbps.

Thank you so much in advance and I appreciate the time and patience!

Berton said:

I think that unless the riser/adapter card has its own socket for a leg from power supply there will be issues, the PCIe, PCI, AGP, and older ISA slots have a limit of how much power the motherboard can supply, in turn will limit performance. The usual mid-tower, mini-tower cases have limits on motherboard size which in turn limits what resources can be added to them. It's been a long time since I worked with a full-size Server tower, actually in a bowling alley to run the features of the lanes, scoring, etc.

Ultimately the riser would just be an extender from an x16 slot but it is obscured by one of the grpahics cards since the board was technically built for only 2 cards, so my main purpose for the riser is just to make space for a third card.so the main problem your saying would in all likelihood just be the third slot not getting enough power. Would the power loss be at the actual riser or would it be about having 3 graphics cards at all? If it is the riser, say I chopped the heat sync off one of the cards to clear space and went the water cooling route to make room for another card, do you suspect the problem would still exist, and to my original point, do you think operating at 4x speed would bottleneck viewport performance with OGL rendering?

I actually have a 1200 watt power supply right now so then, what your concern is is more of whether the power is there to give and less about if the motherboard can move the power around?

Berton said:

The power issue would most likely apply to all the features of the motherboard and things attached to the power supply such as internal drives and cooling fans, may necessitate getting the largest power supply that will fit the case, maybe over 1000W. But then that may produce more heat and need additional case fans to handle it.

Berton said:

The various 'legs' or wires coming from a power supply can furnish only a limited amount of power to the motherboard and to other devices. The motherboard doesn't 'move' the power around and too many things drawing power can be limiting and everything electrical pulling power to run something creates heat, some times enough to actually melt wires or circuitry. So a first start would be to add up all the power requirements for all the devices and motherboard and fans to assure what you have is sufficient but might be best to go 20% or more larger. All that is for power, there's also possible slowdowns with the data stream to devices, could cause problems with video, sound, data transfer, etc.

Hi see. So realistically, just because I probably could get away with it doesn’t mean I necessarily should. How do you motherboards that are designed to handle three graphics cards deal with this problem? From a mathematical standpoint, it would likely be the same amount of power if I plugged the three cards into one of those as opposed to this one. Is it the physical construction of the board that allows it to handle the conditions like that?

Also, say for shits and giggles that I bit the bullet of reinstalling windows, and upgraded my motherboard to a 4GPU motherboard, would that have the same problems, do you think?

Berton said:

Best I can do is read what is available found by Search, dual cards might be okay and there are some that can feed 4 monitors. But physical size could be problematic.
dual video cards at DuckDuckGo

Yeah the physical size IS problematic. I know because my third PCIEx16 slot is covered by one, LOL. I have 2 cards right now and I need more cuda cores. It is just sorta a reality I am living in right now that 8,000 sum od cuda cores isn't enough anymore and $100 a day for remote PCing into a 6 card system taht is actualy possibly overkill for my day to day needs will quickly cost me more than just upgrading the PC.

No discussion of PCI-E lanes?

I suspect that the power issue may be a red herring. A PCI-E slot is only supposed to deliver 75W max. High powered cards get most of their power through the auxiliary PCI-E connectors. Power may not be an issue if your PSU has enough PCI-E connectors. (I have zero personal experience with that. I haven't run even dual graphics cards in years.)

Maybe you should be considering hardware that is designed for multi-GPU processing. A 2021-Ready Deep Learning Hardware Guide | by Nir Ben-Zvi | Towards Data Science It's only money, after all. The less savory market would be cryptocurrency mining.

Where are you going to get the graphics cards? I see that even the previous generation of nVidia cards tend to be pretty much exclusively available through resellers (aka scalpers) at highly inflated prices.