Am I Misunderstanding PCIe I/O Lanes?

[ManUnderMachine]

I've never built a PC before and I'm trying to educate myself enough to make a sound parts list but, I'm a little confused about CPU I/O PCIe lanes.

I'm trying to build a gaming PC and future-proof it as much as possible (assuming "buy once, cry once" applies to computers). To elaborate, I was looking at the Gigabyte Aorus GA-Z270X Gaming 9 because it has two PCIe ×16 expansion slots in which I would run a Geforce GTX 1080 now and SLI a second card in the future as it becomes relevant. I also wanted to use the M.2 for a boot drive and use SSD drives in RAID 10 for storage.

The problem is that the LGA-1151 form CPU's supported by the board have only 20-ish I/O lanes. Wouldn't running one GPU at ×16 and an M.2 boot drive saturate all 20 lanes? Am I correct in thinking that this would leave me nothing for storage or a second ×16 GPU? I can't imagine they'd make a board that runs only CPU's which can't support all of the features that it boasts but I don't want to assume too much and end up making a costly mistake. Is there such a CPU that can support that many lanes for my intended application? Should I be looking for a different board if there is? Or am I simply misunderstanding I/O lanes? Thanks in advance for your help and patience.

 

[Ironsounds]

Hello... good questions... PCIe is a communications standard... and recent intel CPU's will now have physical "direct lanes" for GPU use... there will be other PCIe lane communications through the 'Chipset" for other devices to use too... so what has changed is PCIe 3.0 support direct from the GPU to CPU... you will not lose other or past PCIe use on the other MB devices... we have actually gained more for use... Depending on the CPU and MB chipset models.

https://en.wikipedia.org/wiki/LGA_1151

 

[ManUnderMachine]

Hello... good questions... PCIe is a communications standard... and recent intel CPU's will now have physical "direct lanes" for GPU use... there will be other PCIe lane communications through the 'Chipset" for other devices to use too... so what has changed is PCIe 3.0 support direct from the GPU to CPU... you will not lose other or past PCIe use on the other MB devices... we have actually gained more for use... Depending on the CPU and MB chipset models.

https://en.wikipedia.org/wiki/LGA_1151

Okay so then, the Intel Core i7 7700K for instance, has 20 lanes excluding those used for the GPU(s)?

 

[Ironsounds]

Hello... The 7700k has 16 PCIe 3.0 direct lanes to the MB GPU slots. https://ark.intel.com/products/97129/Intel-Core-i7-7700K-Processor-8M-Cache-up-to-4_50-GHz

So you will have 16 for 0ne GPU or 2x8 for two GPU's... this is more and faster communications than what we have ever had before from a intel consumer CPU. B / ... before all of these PCIe 2.x GPU lanes had to go through the "Chipset".

You still have PCIe x.x lanes through your Chipset for your other MB devices (network/sound/chips/slots/?) basically all the other ones will be hardwired/shared through another chip on the MB model.

PCIe is just a fast serial Communications standard being used at this time. PCIe4.0 could be years or months down the road... or something totally different will eventually become the standard? B /

 

[VincentP]

I've never built a PC before and I'm trying to educate myself enough to make a sound parts list but, I'm a little confused about CPU I/O PCIe lanes.

I'm trying to build a gaming PC and future-proof it as much as possible (assuming "buy once, cry once" applies to computers). To elaborate, I was looking at the Gigabyte Aorus GA-Z270X Gaming 9 because it has two PCIe ×16 expansion slots in which I would run a Geforce GTX 1080 now and SLI a second card in the future as it becomes relevant. I also wanted to use the M.2 for a boot drive and use SSD drives in RAID 10 for storage.

The problem is that the LGA-1151 form CPU's supported by the board have only 20-ish I/O lanes. Wouldn't running one GPU at ×16 and an M.2 boot drive saturate all 20 lanes? Am I correct in thinking that this would leave me nothing for storage or a second ×16 GPU? I can't imagine they'd make a board that runs only CPU's which can't support all of the features that it boasts but I don't want to assume too much and end up making a costly mistake. Is there such a CPU that can support that many lanes for my intended application? Should I be looking for a different board if there is? Or am I simply misunderstanding I/O lanes? Thanks in advance for your help and patience.

The Core i7 7700K has 16 x PCI-E lanes (version 3.0). These are typically used for graphics cards. Boards with a Z series chipset can split these between two cards running at x8 speed (which is still plenty of bandwidth). The spec on the "Gigabyte Aorus GA-Z270X Gaming 9" that it has "2 x PCI Express x16 slots, running at x16 (PCIEX16_1, PCIEX16_2)" is misleading. It isn't possible to have two slots running at x16 speed with any socket 1151 CPU.

Separate to the PCI-E lanes from the CPU, there is a DMI link to the motherboard chipset, and then the motherboard has additional PCI-E lanes used for storage, etc. The limited bandwidth of the DMI means these lanes are generally not useful for graphics cards.

You could run two GTX 1080 cards on any motherboard that supports SLI if you want, and the limitation of x8 speed per slot will make very little difference to performance. By the time one card isn't fast enough though, you would be better to just buy the latest generation of card.

If you want to run two cards at x16 speed, you need to look at a processor like the Core i7 6850K and an appropriate socket 2011-3 motherboard.

 

[Ironsounds]

Hello... I see now that the main concern here is how does the M2 fit into the PCIe use equation... B / am I correct?

 

[ManUnderMachine]

Hello... I see now that the main concern here is how does the M2 fit into the PCIe use equation... B / am I correct?

Hello, Ironsounds. Yes, that is one of my concerns.

 

[Ironsounds]

Hello... Typically in the past the M2 would be shared with one of your SATA chip controllers... thus you could possibly lose use of two SATA ports out of the six or eight available.

The other possibility is that it could take 4 PCIe lanes from the direct/GPU lanes... B / leaving you with 14 for GPU use.

Have you downloaded the PDF for the MB of choice yet? what I'm reading is that it depends on the MB construction, and it should be explained there how the M2 slot is wired... Just a guess here... B / I would expect a consumer level board will be wired for taking two SATA ports from use here... but just a guess B D

And that make sense to me, since you are using another "faster drive" that controller chip will be dedicated to the drive to communicate with it... your manual should explain which SATA drive ports you will lose when setting it up your BIO's.

 

[VincentP]

They did use CPU PCI-E lanes on the first boards to support M.2 in PCI-E mode.
This is now supported through the motherboard chipset instead.

The real limitation for storage bandwidth is in the CPU DMI link to the chipset. This has the same bandwidth as a PCI-E X4 link. A single M.2 SSD could theoretically saturate the link, but in real world situations I don't think it is generally a problem.

 

[ManUnderMachine]

They did use CPU PCI-E lanes on the first boards to support M.2 in PCI-E mode.
This is now supported through the motherboard chipset instead.

The real limitation for storage bandwidth is in the CPU DMI link to the chipset. This has the same bandwidth as a PCI-E X4 link. A single M.2 SSD could theoretically saturate the link, but in real world situations I don't think it is generally a problem.

Yes, I downloaded the PDF. Regarding M.2 it looks like it uses SATA:

"Due to the limited number of lanes provided by the Chipset, the availability of the SATA connectors may be
affected by the type of device installed in the M2M_32G and M2P_32G connectors. The M2M_32G connector
shares bandwidth with the SATA3 4, 5 connectors. The M2P_32G connector shares bandwidth with the
SATA3 0 and U2_32G_1 connectors."

 

[ManUnderMachine]

I think I see now. So, you two are telling me that the 16 lanes are GPU lanes and the DMI uses a separate set of lanes that shares bandwidth between things like storage. Is that correct? If so then is overwhelming DMI something I would ever have to worry about?

 

[Ironsounds]

Hello... Yes...

Not in a consumer or home use system... basically it's not typical to need to access data from more than one drive at the same time, they will take their turn as the APP calls for them. And the chipset will pass the DATA along from them in a fast efficient manner.