Can you change the amount of lanes a GPU will have?

[Carlos1010]

Hi all,

I was just wondering, if you have a 16 lane CPU, and you have a GPU running on x16, can you change it to run to x8 if you want to add a PCIE SSD? Even if you don't add it, how can you change the amount of lanes a GPU will run on your system?

Thanks in advanced!

[Electronics Wizardy]

Only the high end chipset support splitting the cpu lanes. 

 

Virtue ssd just use chipset lanes 

 

You can lower than lanes by putting tape on some pins on the pcie connector. 

[Godlygamer23]

It's bound by how the manufacturer set it up on the board. Some boards might offer the ability to change how lanes are allocated, but if they don't, there's nothing you can do

 

Except for shoddy mods

@Electronics Wizardy

[Max_Settings]

I don't know if you can limit it manually, if you can it's probably in the BIOS. But if you add an SSD, it will adjust the lanes automatically

[RadiatingLight]

You don't need that.

 

CPUs have 16 PCIe lanes direct to the first PCI-e slot, and then dozens more from the chipset. (although it all goes over DMI, so limited to 3.0 X4 speeds total)

the PCI-e SSD will go through the chipset, so no lane problems.

[Carlos1010]

9 minutes ago, Godlygamer23 said:

It's bound by how the manufacturer set it up on the board. Some boards might offer the ability to change how lanes are allocated, but if they don't, there's nothing you can do

 

Except for shoddy mods

@Electronics Wizardy

 

9 minutes ago, Electronics Wizardy said:

Only the high end chipset support splitting the cpu lanes. 

 

Virtue ssd just use chipset lanes 

 

You can lower than lanes by putting tape on some pins on the pcie connector. 

So what happens if you run SLI on 2 GPU's that are on a x16 board/CPU? Will one of them won't work or will the CPU automatically make both of them run on x8?

[mariushm]

Most processors have 16 lanes or more reserved strictly for video cards.

Those 16 lanes can be split by the motherboard in 2 x8 when you insert two video cards in the first two pci-e slots that have pci-e lanes coming directly from the cpu. Some motherboards have a chip which convert this x16 into 2 x 16 (like a network switch combines multiple network cables into a single outgoing network cable)

 

Some processors will allow non-video cards in the second pci-e slot. These 16 lanes are "special" in the sense that there are some limitations, not everything will work in those slots coming directly from the processor.

 

Motherboards also have pci-e slots which have the pci-e lanes coming from chipset.

 

With the pci-e lanes coming from the chipset, those lanes are more "universal".. the chipset arranges the lanes in multiple pci-e slots (x1, x4, x8,  x16) , uses some x1 lanes for onboard things like audio card, network card, extra usb 3 controllers

The pci-e controller in the chipset can also handle multiple devices on pci-e, basically you can insert a "pci-e switch" in one pci-e slot and that device works like a network switch or router, splitting the number of lanes in the slot in multiple slots, or multiplexing the signal from multiple slots into that single slot.

For example, you plug such a card into a x4 slot and the "switch" on the card creates 4 x1 slots.  That's basic splitter.  But such chips can also take on x16 and create 2 x 8 , or 1x8 and 1x4 and 4x1 or whatever configuration ... only problem is such chips are expensive.

 

Ryzen processors have more than 16 pci-e lanes, where 16 pci-e lanes go directly to the first pci-e slot for video cards (or two slots in 2x8 configuration), 4 pci-e lanes go directly to the m.2 connector on the motherboard so one SSD would be very fast, and a few pci-e lanes are inside the processor and used for the SOC part (a sata controller and a usb 3 controller built inside the cpu which get faster speeds). Then, 4 pci-e lanes are used to make the connection between the cpu and the chipsets, and the chipsets have sata controller for more sata ports, usb 2 and 3 controllers for more usb ports and also create a bunch of pci-e 3.0 and 2.0 lanes that can be combined in various slots or used for onboard devices.

 

 

[Electronics Wizardy]

4 minutes ago, Carlos1010 said:

 

So what happens if you run SLI on 2 GPU's that are on a x16 board/CPU? Will one of them won't work or will the CPU automatically make both of them run on x8?

You need a chipset that supports splitting lanes so that's a z or x chipset. Other wise you just can't have them both plugged to. The cpu. 

[manikyath]

if you cant use chipset lanes for, for example, your PCIe SSD, you'd have to put it in a slot that's wired into the cpu lanes, where the mobo & cpu combo will handle where the lanes go.

 

in an example:

forget that this is an asus maximus hero board for a brief moment, and imagine it's a super generic motherboard.

 

it's got a bunch of PCIe slots on it of vareous physical sizes, in order:

slot 1: 1x

slot 2: 16x

slot 3: not there because the GPU will cover it anyways, and it makes room for an M.2 slot.

slot 4: 1x

slot 5: 16x

slot 6: 1x

slot 7: 16x

 

now.. the 1x slots always get hooked into chipset lanes, because they're for stuff like wifi cards, etc. it's never anything high speed.

the first 16x slot is usually supplied with (all) 16 CPU PCIe lanes, and is destined to hold the GPU, if you plan to install one.

the second 16x slot (slot 5) is often 8 lanes electrically (meaning only the first 8 are hooked up), and will in cases where there's only 16 CPU lanes, share with the first slot, causing that one (slot 2) to run at 8x speeds when this slot (slot 5) is used as well. this is something that happens automagically these days.

 

as for the third 16x slot (slot 7).. thats where things get interesting. some board manufacturers make it a 4x slot that taps into (slot 5) in a similar fashion as it taps into (slot 2), meaning that if you use (slot 7), it'll cause (slot 5) to run at 4x, and (slot 2) to run at 8x.

 

some other board manufacturers tho.. take a different route, where that slot is hooked into 4 (or more if available) chipset lanes, and is destined to hold things like PCIe SSD's, dual- or quad NICs, or other medium-high speed perhiperal cards.

 

to know what goes where.. is usually a pretty deep dive into the motherboard manual, and sometimes other pools of information as well. if any mobo manufacturers are listening in, i'd be down for some printing on the board pointing out what goes where.