CPU vs chipset PCI-E lanes

[_Drahcir_]

My question is about PCI-E lanes and I haven#t quite figured out how they work:

An i7 8700k has 16 PCI-E and the Z370 chipset 24, so how many are there in total?

I thought 24, but the i9 7980xe and i9 9980xe both have 44 PCI-E lanes and the X299 chipset 24, but in the table from Intel for the 7th gen extreme edition processors the PCI-E lane count was 44 and for the 9th gen there are 68 showed.

So now I'm confused. Do you have to add CPU and chipset lanes?

And when yes does that mean that Cooffe-Lake has got 40 PCI-E lanes to share?

[Swealteek]

[Fasauceome]

the PCIe lanes from the chipset aren't quite capable of the same things the CPU lanes are, such as running a graphics card. However, it allows for HSIO, high speed input/output, so the "lane" count from the chipset is also responsible for SATA, USB, PCIe storage, etc. i9 CPUs have loads of lanes from the CPU, AMD threadripper even more so, so they can handle multiple graphics cards straight from the CPU.

[_Drahcir_]

3 minutes ago, Swealteek said:

I'm aware of that diagram

I only wasn't quite sure if that means that you could put 2 GPU's and 6 M.2 devices in one z370 system

[_Drahcir_]

4 minutes ago, fasauceome said:

the PCIe lanes from the chipset aren't quite capable of the same things the CPU lanes are, such as running a graphics card. However, it allows for HSIO, high speed input/output, so the "lane" count from the chipset is also responsible for SATA, USB, PCIe storage, etc. i9 CPUs have loads of lanes from the CPU, AMD threadripper even more so, so they can handle multiple graphics cards straight from the CPU.

yeah I know, but does that mean that i would have 68 lanes in total? 44 CPU and 24 chipset. I know theri are doing different stuff but could i like put like 4 GPU's (16x8x8x8) on the CPU and 6 M.2 SSD's on the chipset (4+4+4+4+4+4)

[Fasauceome]

1 minute ago, _Drahcir_ said:

yeah I know, but does that mean that i would have 68 lanes in total? 44 CPU and 24 chipset. I know theri are doing different stuff but could i like put like 4 GPU's (16x8x8x8) on the CPU and 6 M.2 SSD's on the chipset (4+4+4+4+4+4)

Like I said above, Can't run a GPU off chipset lanes. A CPU with a ton of lanes would do 8x8x8x8 on GPUs and yeah you could do a ton of M.2 storage.

[_Drahcir_]

Just now, fasauceome said:

Like I said above, Can't run a GPU off chipset lanes.

I know I read that. You haven't understood my question. Does that mean that I have 44 lanes from the CPU for things like GPU's and 24 lanes from the chipset for like M.2 SSD's?

So would that mean that I have got 68 in total? Yes or no?

[Coaxialgamer]

As you noted , both the CPU and chipset/PCH provide provide pcie lanes that add-in cards can interface with .

The 8700k provides 16 usable lanes at pcie 3.0 , with the z370 chipset providing 24 as well .

It might seem like both those solutions are equivalent for a grand total of 40 lanes . Damn !

 

But the reality is that you should only consider these chipset lanes as mere backups for when you run out of cpu lanes.

 

For starters , latency on the chipset pcie lanes is higher than those attached directly to the cpu . This is because all traffic needs to go through the chipset . This is fine for US or ethernet cards , but not ideal for NVME storage , and certainly not for GPUs.

Plus , if you look at the diagram above , you can notice that the chipset and CPU communicate through a bus called DMI 3.0.

DMI handles all communication between the cpu and pch , including the pcie lanes.

But the problem is that this DMI link consists of 4 serial links running at 8GT/s .

With 4 links , this gives you a total bandwidth of ... 4GB/s.

That's far less than the theoretical maximum 24GB/s provided by 24 pcie 3.0 lanes.

 

So what's the deal ? While you will get the total 24GB/s between all devices communicating through the chipset , any communication to the CPU through these lanes will be capped to a lowly 4GB/s . In reality , it will be even less because that bandwidth needs to be shared with other devices as well .

 

 

[Fasauceome]

Just now, _Drahcir_ said:

I know I read that. You haven't understood my question. Does that mean that I have 44 lanes from the CPU for things like GPU's and 24 lanes from the chipset for like M.2 SSD's?

So would that mean that I have got 68 in total? Yes or no?

Oh, yes that will be the case. However, the chipset has to cut lanes off for other forms of I/O so unless you don't need any USB devices or SATA it won't be perfect. It'll end up sharing bandwidth (although some manufacturers say it disables it outright, I actually run some SATA devices on my "disabled" ports)

[_Drahcir_]

1 minute ago, Coaxialgamer said:

As you noted , both the CPU and chipset/PCH provide provide pcie lanes that add-in cards can interface with .

The 8700k provides 16 usable lanes at pcie 3.0 , with the z370 chipset providing 24 as well .

It might seem like both those solutions are equivalent for a grand total of 40 lanes . Damn !

 

But the reality is that you should only consider these chipset lanes as mere backups for when you run out of cpu lanes.

 

For starters , latency on the chipset pcie lanes is higher than those attached directly to the cpu . This is because all traffic needs to go through the chipset . This is fine for US or ethernet cards , but not ideal for NVME storage , and certainly not for GPUs.

Plus , if you look at the diagram above , you can notice that the chipset and CPU communicate through a bus called DMI 3.0.

DMI handles all communication between the cpu and pch , including the pcie lanes.

But the problem is that this DMI link consists of 4 serial links running at 8GT/s .

With 4 links , this gives you a total bandwidth of ... 4GB/s.

That's far less than the theoretical maximum 24GB/s provided by 24 pcie 3.0 lanes.

 

So what's the deal ? While you will get the total 24GB/s between all devices communicating through the chipset , any communication to the CPU through these lanes will be capped to a lowly 4GB/s . In reality , it will be even less because that bandwidth needs to be shared with other devices as well .

 

 

Thank you that's all and even more I wanted to know

[_Drahcir_]

3 minutes ago, VegetableStu said:

  Reveal hidden contents

 

16 straight to the CPU, 4 that that receives chipset connections over DMI. the chipset can handle up to 24 lanes worth of connections, but will only have 4 lanes to work with when it's communicating to the CPU

44 or 28 straight to the CPU, 4 that that receives chipset connections over DMI. the chipset can handle up to 24 lanes worth of connections, but will only have 4 lanes to work with when it's communicating to the CPU

no. ideally you'd just count the lanes you can connect straight to the CPU.

 

read the spec pages and manuals of each motherboard carefully. how each motherboard handles m.2 is different. In general most intel boards connect m.2 devices over the chipset.

 

16 for Mainstream. 28 for low end HEDT. 44 for high-end HEDT.

 

depends on the motherboard.

as for the m.2 part, good luck finding one with six NVMe M.2 sockets, LOL

 

I... might need to check. can't remember if the chipset will just recognise anything else connected to the m.2 socket as a PCIe device.

(I think there are m.2 capture cards that works over the chipset o_o)

 

it'd be extremely bottlenecked if it does run though, LOL

 

oh wow didn't expect the thread to be over this quick ._. I'll just leave my reply here

yeah but it was a good one. Opened my eyes

[Ic3d]

so out of interest, my cpu has 16 lanes and my chipset has 6 (h81), im running a gtx 1060 in the 16x slot and a wifi card in the 3rd 1x slot, will my gpu be running at 16x or 8x speeds and will my wifi card be using the chipset lanes or the cpu ones? thanks for the help:)

[Fasauceome]

12 minutes ago, Ic3d said:

so out of interest, my cpu has 16 lanes and my chipset has 6 (h81), im running a gtx 1060 in the 16x slot and a wifi card in the 3rd 1x slot, will my gpu be running at 16x or 8x speeds and will my wifi card be using the chipset lanes or the cpu ones? thanks for the help:)

Wifi card will use chipset and graphics gets full 16

[Ic3d]

21 minutes ago, fasauceome said:

Wifi card will use chipset and graphics gets full 16

thanks a bunch:)

[Coaxialgamer]

50 minutes ago, Ic3d said:

so out of interest, my cpu has 16 lanes and my chipset has 6 (h81), im running a gtx 1060 in the 16x slot and a wifi card in the 3rd 1x slot, will my gpu be running at 16x or 8x speeds and will my wifi card be using the chipset lanes or the cpu ones? thanks for the help:)

That depends on how your board is wired.

if the 3rd slot comes from the chipset , the gpu gets 16 lanes

otherwise ,it gets 8