X99 - Shared PCIe Slots, lanes and total cards?

FlangeMonkey

Limp Gawd
Joined
Sep 11, 2010
Messages
161
Hi Guys,

I feel this is a daft question, but I'm stuck in a loop on this and need some help getting out of it.

I'm wanting the most flexibility in expansion and performance. Ultimately I intend to have the following:

Processor: 5960x
PCIe:
2x GTX 1080 (PCIe 3.0 16x)
1x 10gb ethernet (PCIe 3.0 4x)
1x M.2 950 Pro (PCIe 3.0 4x)
Total Lanes: 40

I've been looking at the Asus X99a II, MSI Carbon Pro, Asus Strix and MSI X99a Tomahawk. I'm specifically looks at future proofing myself with USB 3.1 gen 2.

The problem is most (except MSI's Tomahawk) specify in there manual shared slots that will restrict to 8x for one of the graphics cards and I feel I'm missing something with lanes, total cards in higher multiplexers of PCIe and the reasons for sharing bandwidth between slots.

Can someone please help in out on this and give some information on what I'm missing?

Thanks,
 
I don't think you're going to restrict anything with your setup. A Titan XP will only just barely see any change between 8x and 16x, something in the neighborhood of a few percent difference. So if one of your cards is at 8x, it's not something you'll ever notice.

One guy did testing here: with a pair of Titan XPs and only had a few fps difference. A 1080 would have even less difference.
 
Thanks for the response, I understand that 8x should suffice from a graphics point of view, but I’m lacking the understanding of were 8 lanes go (8, 16, 8 or 16, 16, 0 totaling 32 leaving 8 lanes left) or why if I have 3 cards installed (not specifically graphics), why I cannot run 16, 16, 8 or more appropriately for my situation 16, 16, 4 and 4 for M.2.
 
Thanks for the response, I understand that 8x should suffice from a graphics point of view, but I’m lacking the understanding of were 8 lanes go (8, 16, 8 or 16, 16, 0 totaling 32 leaving 8 lanes left) or why if I have 3 cards installed (not specifically graphics), why I cannot run 16, 16, 8 or more appropriately for my situation 16, 16, 4 and 4 for M.2.

I have a feeling its USB 3.1 gen 2 that is taking some pcie 3.0 lanes?
 
Thanks for the response, I understand that 8x should suffice from a graphics point of view, but I’m lacking the understanding of were 8 lanes go (8, 16, 8 or 16, 16, 0 totaling 32 leaving 8 lanes left) or why if I have 3 cards installed (not specifically graphics), why I cannot run 16, 16, 8 or more appropriately for my situation 16, 16, 4 and 4 for M.2.

In theory, nothing would prevent it. But MB manufacturers have to design for each of the use-cases, and having 2 separate 4x cards is not a typical case, so not something they spend a lot of man-hours on developing. So read the manuals very carefully before you buy to make sure it will support your specific use.
 
In theory, nothing would prevent it. But MB manufacturers have to design for each of the use-cases, and having 2 separate 4x cards is not a typical case, so not something they spend a lot of man-hours on developing. So read the manuals very carefully before you buy to make sure it will support your specific use.

I've been reading the manuals and have just been looking at the MSI X99 Tomahawk, but it doesn't specify conflicts or shared resources. That board is throughing a curve ball at me, I beginning to believe the manual is crap.

Do we know if USB 3.1 gen 2 is taking PCIe 3.0 lanes? it could be taking 2.0, which I think x99 has a different bus for that doesn't affect lanes.
 
I've been reading the manuals and have just been looking at the MSI X99 Tomahawk, but it doesn't specify conflicts or shared resources. That board is throughing a curve ball at me, I beginning to believe the manual is crap.

Do we know if USB 3.1 gen 2 is taking PCIe 3.0 lanes? it could be taking 2.0, which I think x99 has a different bus for that doesn't affect lanes.

It varies from MB to MB. To get full speed from a pair of 3.1 ports, you need x2 PCIe 3.0 lanes. But a MB can be configured with it's USB 3.1 ports tied to either the 3.0 or the 2.0 bus. So I'd have to say again, "it depends"
 
I should have suspected that, so thanks for pointed it out. Anyway, the ASMedia ASM1142 uses either PCIe2.0x2 or PCIe3.0x1, this is on the X99 Tomahawk. I'm going to dig around a little more. Thanks for your input.
 
Nothing is attached to the pcie lanes from the cpu aside from the x16 pcie slots and the m2/u2 slots. Usb 3.1 is x2 pcie 2.0 as shown in the chart below.

07-diagram_pcie_routing_40lane.gif



http://techreport.com/review/30456/asus-x99-a-ii-motherboard-reviewed/2
 
Thanks to the above diagram and digging around including techreport, which has some other boards with block diagrams. I'm finding a lot of boards are only using a maximum of 36 lanes of 40.

I now understand some of the reasoning’s (including compromise, poor design or nerf), but as mentioned above, ultimately it is down to design as the processor’s lanes are broken up into two x16 and one x8 or more accurately five x8 (according to wiki and Intel documentation). Interestingly, some diagrams show the x8 being broken up further into x4 so this might be a misnomer but the principles still apply, as viewable in the above diagram.

At least this understand explaining the mystery of missing lanes and ways to share them, but now I need to find a flexible board so I can have my cake or compromise. MSI is looking positive but as they have poor documentation in this area, I’m still trying to get information on their layout and lane sharing.

If anyone else is running two cards in PCIe 3.0 x16 each, one card in PCIe 3.0 x4 with one M.2 slot in PCIe 3.0 x4 please let me know so I can consider this as a purchase.

Thanks,
 
It's just the practical application of the lanes. You're making it into something more than it actually is. You have 40 lanes from a 40 lane cpu and are free to use those as you wish within the confines of the practical use of the slots and ports. In other words it is what it is and practically all the boards are the same.
 
It's just the practical application of the lanes. You're making it into something more than it actually is. You have 40 lanes from a 40 lane cpu and are free to use those as you wish within the confines of the practical use of the slots and ports. In other words it is what it is and practically all the boards are the same.

I agree with the practical application, however with a lot of boards we don't have the practical use of 40 lanes but rather 36 and I'm finding boards, where the practical application is poor. Agreed, we can only work within the confines of the board design, which has compromise and is were scenario kicks in. If 40 lanes are available you won't be able to us all lanes due to hardware configuration and choice. This is were I was making it more than it is, but as I now have more understand of the design and differences between the boards, as they are not all the same, I want the most flexibility for my money.
 
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