Pci lanes

[SleepingGiant]

Can someone enlighten me on how cpu pci lanes affect how many things you can put on your pc

[mariushm]

It doesn't.

 

You have a CPU and you have a chipset ... the chipset is connected to the CPU through a dedicated link.

The cpu creates a number of pci-e lanes and the chipset also creates a number of pci lanes.

These lanes can be grouped together so you can have 1 lane, 2 lanes, 4 lanes, 8 lanes or 16 lanes

 

The lanes created by the CPU are a bit of a special case: there are usually 16 pci-e lanes which are reserved for video cards. Depending on chipset, these 16 pci-e lanes may be split between two slots to have 8 lanes in each slot.  However, it usually can't be split further in smaller groups.

On Ryzen processors, the CPU also creates 4 pci-e lanes which go to the M.2 connector

 

The chipset has various connections to onboard sound, onboard network card and creates usb ports, sata ports etc

Besides this, it creates a number of pci-e lanes, how many it depends on chipset. For example B450 chipset creates 6 pci-e lanes, X470 creates 8 pci-e lanes, x570 creates 8 pci-e lanes if my memory is correct.

 

These are more flexible, for example you can have a pci-e x4 slot and 3 pci-e x1 slots, or you can have a pci-e x4 slot and 2 lanes going to a 2nd M.2 connector and several x1 slots (which become disabled if you actually install a SSD in the m.2 connector)

Some motherboards also use switch chips, which are like network switches that have one incoming 10g port and 16-48 1g ports - they connect to the chipset or cpu using a few pci-e lanes and create more pci-e lanes. So you can have more slots but if the cards in those slots use a lot of bandwidth you're still limited by the total bandwidth of the lanes coming into that chip.

 

[SleepingGiant]

1 hour ago, mariushm said:

It doesn't.

 

You have a CPU and you have a chipset ... the chipset is connected to the CPU through a dedicated link.

The cpu creates a number of pci-e lanes and the chipset also creates a number of pci lanes.

These lanes can be grouped together so you can have 1 lane, 2 lanes, 4 lanes, 8 lanes or 16 lanes

 

The lanes created by the CPU are a bit of a special case: there are usually 16 pci-e lanes which are reserved for video cards. Depending on chipset, these 16 pci-e lanes may be split between two slots to have 8 lanes in each slot.  However, it usually can't be split further in smaller groups.

On Ryzen processors, the CPU also creates 4 pci-e lanes which go to the M.2 connector

 

The chipset has various connections to onboard sound, onboard network card and creates usb ports, sata ports etc

Besides this, it creates a number of pci-e lanes, how many it depends on chipset. For example B450 chipset creates 6 pci-e lanes, X470 creates 8 pci-e lanes, x570 creates 8 pci-e lanes if my memory is correct.

 

These are more flexible, for example you can have a pci-e x4 slot and 3 pci-e x1 slots, or you can have a pci-e x4 slot and 2 lanes going to a 2nd M.2 connector and several x1 slots (which become disabled if you actually install a SSD in the m.2 connector)

Some motherboards also use switch chips, which are like network switches that have one incoming 10g port and 16-48 1g ports - they connect to the chipset or cpu using a few pci-e lanes and create more pci-e lanes. So you can have more slots but if the cards in those slots use a lot of bandwidth you're still limited by the total bandwidth of the lanes coming into that chip.

 

Ok i get the gits of it. Thank you

 

If i recall correctly thread reaper has 44 pcie lanes, how does that work in world scenarios?

For ex: i used my pcie 16x slot for the gpu does that mean 44-16=28 available lanes

 

Is that how it works?

[mariushm]

Threadripper processors have 64 pci-e lanes.

4 lanes are always used to connect the chipset to the CPU, leaving 60 pci-e lanes to be used for whatever you want.

The recommended or "default" way is to use 48 lanes for video cards (pci-e x16 or pci-e x8 slots) and 12 pci-e lanes for IO (m.2 connectors, 10g ethernet, extra usb 3 controllers etc)

 

As an example, here's a layout used by a motherboard with Threadripper 1950x :

You can see they use 3 "splitters" - if the 2nd slot is not populated, the lanes for that slot are routed to first slot to make it x16.

So, you can have 3 x16 cards and 2 empty slots or 5 x8 cards

You can see the chipset also creates a number of pci-e lanes to which they connect eSata, extra usb 3.1 controller and so on...

 

 

The newest Threadripper still has 64 pci-e lanes, but they add the 12 pci-e lanes from the chipset to make it sound better, so they say it's 72 pci-e 4.0 lanes:

It's also a bit less flexible, but still flexible enough.

You can see 48 lanes are still "General" meaning you can put them in x1, x4, x8 or x16 slots but now chipset gets 8 pci-e lanes just for it, for more speed. 

You also have 2 groups, where in each group you can pick between having 4 pci-e lanes, a m.2 connector with 4 pci-e lanes in it, or 4 sata connectors. So you could have 48 pci-e lanes and 8 sata ports from cpu, or 48 pci-e lanes and 2 m.2 connectors from cpu... customizable.

The chipset then also has 8 pci-e lanes that can be put in slots OR again you have 2 groups, in each group you can pick one.

 

 

 

 

[Kilrah]

To summarize, the way lanes are assigned totally depends on a particular motherboard's layout. 

 

So if you have particular requirements you need to look at the doc of the mobos you're interested in to see if they do what you want.