z77 pcie lane allocation

[thelongdivider]

Hi guys,

I'm sure as you all know, z77 comes embedded with 16 pci-e 3.0 lanes. My question is, if I have two of these adapters:

http://www.amazon.com/Intel-1000-Dual-Server-Adapter/dp/B000BMZHX2

with such an amazing price as amtech has this posted for, I couldn't resist buying another for a future system.

Does it consume 8 of the lanes. For one, I note that each of these adapters uses a physical 4x pcie-1.0 interface. Realistically, according to wikipedia, a pci-e 1x interface is enough for this (250 MB/s). Intel probably increased the pci-e physicalsize to give the device ample degrees of overhead as the 4x gigabit version of the same card also uses 4x connectors.

So here is my question: To say that z77 supports 16 pci-e lanes, does that imply that it supports the bandwidth of 16x pci-e lanes (~15760 MB/s) or just the physical inputs worth?

Watching Linus's FAP video on pci-e 3 actually confused me more, because he said that they run at the speed of the lower of the two interfaces, but ends the video by saying you can plug the 4x 4 gigabit card mentioned above into a 1x slot. My would be configuration would be:

3770k

MSI Mpower

7950 (x16 3.0 physically, not in terms of actual bandwidth necessary)

xonar (x1 2.0 physically I think, though the bandwidth here is probably quite minimal in comparison to the other device)

2 x dual gigabit cards (x4 pci-e 1.0 physically)

Why this is important is if I game while massive file transfers are taking place, I don't want the graphics card to drop to a 4x mode.

Thank you guys and I love this community.

[Glenwing]

PCIe x1 slots are wired to the chipset's secondary PCIe lanes (it has enough for 8 PCIe 2.0 lanes), they don't consume any of the 16 PCIe 3.0 lanes that come directly from the CPU's PCIe controller.

[Vitalius]

I want to point out that very very few cards can fully saturate a PCI-e 2.0 x16 bus. 

Only Dual GPU cards can do that. PCI-e 3.0 x8's bandwidth is the exact same as PCI-e 2.0 x16's bandwidth. They are effectively the same thing. 

If it supports PCI-e 3.0 x16 PCI-e lanes, then you will not fully saturate all the lanes, even with all that and so the GPU will not drop down to a lower bandwidth so that the other cards can have some.

So no, you are good.

And Glenwing makes a good point (I did not know that) that basically PCI-e 2.0 and PCI-e 3.0 lanes are separate and do not use the same lanes to the CPU.

[Glenwing]

So here is my question: To say that z77 supports 16 pci-e lanes, does that imply that it supports the bandwidth of 16x pci-e lanes (~15760 MB/s) or just the physical inputs worth?

 

I forgot to actually answer the question... lol.  It means that the CPU itself has 16 lanes of PCI Express bandwidth, which is then divided up between all the expansion devices plugged in.  The board may be wired for more lanes, to give you the option of running a 16x device in any of the 16x physical length slots available, instead of wiring them as 16x/8x/4x like most vendors do... They may wire it for more bandwidth than is available, but in total you only have 16x lanes you can use for all your devices at any one time.

 

It is worth noting though that it is not simply "however much bandwidth the device needs is how much it will use", since PCIe lanes are distributed in clumps, not on an "single lanes as needed".  For example when you plug 2 graphics cards in, they will both run in x8 mode.  If you plug in a third device the will run in 8x/4x/4x mode even if the third device doesn't actually need 4x bandwidth.  If it only needs for example 1x lane worth of bandwidth, the PCIe controller cannot set things up in some kind of "8x/7x/1x" configuration; it dedicates lanes in clumps.  Fortunately 1x slots are wired to secondary chipset lanes for this reason (that, and some I/O ports and devices use PCIe lanes, like Thunderbolt).

 

All that aside though, as was mentioned, PCIe 3.0 provides massive bandwidth.  It is not currently possible to max out even PCIe 3.0 x4 unless you use your card for GPGPU type applications (computational work rather than gaming).

[thelongdivider]

Thank you guys very much. Your answers are more than adequate. I never knew there were secondary pci-e lanes on ivy, so that makes a lot more sense. Thanks again!