How do PCI-e slots work?

[Mark_Lawrence]

It states that when u use only a single video card u must insert it to the pci-e slot nearest the cpu and run it at x16 but when u use 2 video cards and insert it to the 1st 2 PCI-e slots it will run at x8 x8 speeds and when 3 video cards, it will run at x8 x8 x4 speeds or something. So does this mean that the speeds of the video card gets slower when multiple video cards are being utilized?

 

[aquielisunari]

It states that when u use only a single video card u must insert it to the pci-e slot nearest the cpu and run it at x16 but when u use 2 video cards and insert it to the 1st 2 PCI-e slots it will run at x8 x8 speeds and when 3 video cards, it will run at x8 x8 x4 speeds or something. So does this mean that the speeds of the video card gets slower when multiple video cards are being utilized?

It depends on the motherboard in question. In the most basic way there are PCIe lanes, electrical needs and MUCH more involved in what a card needs(way above my "pay grade"). As cards are added more resources are being consumed. The card's don't get slower as long as the motherboard is built for the amount of cards being installed. Yes, they will get slower or possibly not work if someone just installed multiple cards without knowing if the motherboard or PSU could handle the load. There are motherboards for most every price-point. Should someone want quad SLI they need cards that support quad SLI and a motherboard that can handle that and a PSU with enough PCIe supplementary power cables to offer the right amount of juice.

 

[Ironsounds]

Hello... YES the Slot data lanes/bandwidth could be reduced... These are physical electrical/data lanes directly to the chipset and CPU... Some are used by other hardware on your MB too... You only have so many and the MB's allow you to share them between plug in cards... the new CPU's from intel have added more lanes for PCIe 3.0 GPU use.

https://www.endpcnoise.com/articles/performance-and-pci-express-bus-lanes

 

[rhysiam]

More PCIe lanes allows more bandwidth. So the fewer lanes each GPU has, the lower the bandwidth between your GPU and the rest of your system. While that sounds bad, in real world situations the even high end GPUs don't need all the bandwidth of a 16 lane PCIe 3 slot.

Assuming you're talking about PCIe 3.0 slots, two slots running with x8 lanes each is generally plenty even for an ultra high end setup. If it has to drop to x4, (or you're looking at PCIe 2.0 x8 slot), then with high end gear you might start to see some bandwidth constraints that could affect frame rates. Also, Nvidia SLI requires x8 slots, so you can't enable SLI for any card in a x4 slot.

If your mobo runs two x8 slots, then it's fine for dual GPU. 3xGPUs wouldn't go well in your motherboard, but it's very rarely a good idea anyway.

 

[Mark_Lawrence]

It states that when u use only a single video card u must insert it to the pci-e slot nearest the cpu and run it at x16 but when u use 2 video cards and insert it to the 1st 2 PCI-e slots it will run at x8 x8 speeds and when 3 video cards, it will run at x8 x8 x4 speeds or something. So does this mean that the speeds of the video card gets slower when multiple video cards are being utilized?

It depends on the motherboard in question. In the most basic way there are PCIe lanes, electrical needs and MUCH more involved in what a card needs(way above my "pay grade"). As cards are added more resources are being consumed. The card's don't get slower as long as the motherboard is built for the amount of cards being installed. Yes, they will get slower or possibly not work if someone just installed multiple cards without knowing if the motherboard or PSU could handle the load. There are motherboards for most every price-point. Should someone want quad SLI they need cards that support quad SLI and a motherboard that can handle that and a PSU with enough PCIe supplementary power cables to offer the right amount of juice.

I'm quite aware of the prerequisites needed for a multi gpu set-up. Im just wondering why do pci-e behave in different speeds when the number of gpus are used

 

[Mark_Lawrence]

Hello... YES the Slot data lanes/bandwidth could be reduced... These are physical electrical/data lanes directly to the chipset and CPU... Some are used by other hardware on your MB too... You only have so many and the MB's allow you to share them between plug in cards... the new CPU's from intel have added more lanes for PCIe 3.0 GPU use.

https://www.endpcnoise.com/articles/performance-and-pci-express-bus-lanes

For now im just using a single powerful gpu and wont opt in in sli unless my high end gaming rig will become a minimum requirements in the future. Just that im wondering why pci-e tend to behave like this.

 

[Mark_Lawrence]

More PCIe lanes allows more bandwidth. So the fewer lanes each GPU has, the lower the bandwidth between your GPU and the rest of your system. While that sounds bad, in real world situations the even high end GPUs don't need all the bandwidth of a 16 lane PCIe 3 slot.

Assuming you're talking about PCIe 3.0 slots, two slots running with x8 lanes each is generally plenty even for an ultra high end setup. If it has to drop to x4, (or you're looking at PCIe 2.0 x8 slot), then with high end gear you might start to see some bandwidth constraints that could affect frame rates. Also, Nvidia SLI requires x8 slots, so you can't enable SLI for any card in a x4 slot.

If your mobo runs two x8 slots, then it's fine for dual GPU. 3xGPUs wouldn't go well in your motherboard, but it's very rarely a good idea anyway.

For now im just into single powerful gpu. Im just preparing to educate myself if ever the time for multi gpu set-up is already a must for me.