Sharing PCIe

[roblesaturcamilo]

Hi guys!

I just got a doubt about this 2 mobos...

 

Asus Maximus viii impact:

 

 

This one has a U.2 connector, and it's using pcie x4 (nvme)

Does this one shares the bandwidth with the pcie x16 slot?

 

 

 

Asus z170i pro gaming:

 

 

This one has a M.2 connector, and it's pcie x4 also nvme

Does this one shares the bandwidth with the graphics card slot?

 

And in case that they share the bandwidth, does this affect the performance of the Graphics card? Does this cause bottlenecks of stuff like that???

[TidaLWaveZ]

Where are the U.2/M.2 connectors? I can't see one on either of these boards.  Either way the U.2 must use PCIe, the M.2 connector can run in either PCIe or sATA mode, unlike my ASUS X99 Deluxe

 

If you run the U.2 drive or the M.2 drive in PCIe and there are not enough CPU lanes, the GPU will switch to running at 8X instead of 16X.  The difference between 8X and 16X, in terms of gaming, is not substantial(maybe a few FPS).

[roblesaturcamilo]

5 hours ago, TidaLWaveZ said:

Where are the U.2/M.2 connectors? I can't see one on either of these boards.  Either way the U.2 must use PCIe, the M.2 connector can run in either PCIe or sATA mode, unlike my ASUS X99 Deluxe

 

If you run the U.2 drive or the M.2 drive in PCIe and there are not enough CPU lanes, the GPU will switch to running at 8X instead of 16X.  The difference between 8X and 16X, in terms of gaming, is not substantial(maybe a few FPS).

well, even if you don't see them, they are there... both of them are using pcie 3.0 x4, but i cannot find any answer from asus... 

[Glenwing]

PCIe storage slots are wired through the auxiliary lanes provided by the chipset. They don't use the primary lanes from the CPU and so won't interfere with the graphics card.

[TidaLWaveZ]

1 hour ago, Glenwing said:

PCIe storage slots are wired through the auxiliary lanes provided by the chipset. They don't use the primary lanes from the CPU and so won't interfere with the graphics card.

 

How does that even work? If the m.2 is running in pcie mode doesn't it have to share bandwidth with the only single pcie slot on the mobo (where the video card must be plugged in?

 

I know for the Asus X99 deluxe the onboard m.2 uses up the CPUs max pcie lanes, it's detailed in the manual.

 

I'm not arguing your comment, I just honestly don't understand how that works.

 

[Glenwing]

3 hours ago, TidaLWaveZ said:

 

How does that even work? If the m.2 is running in pcie mode doesn't it have to share bandwidth with the only single pcie slot on the mobo (where the video card must be plugged in?

 

I know for the Asus X99 deluxe the onboard m.2 uses up the CPUs max pcie lanes, it's detailed in the manual.

No, because the Z170 chipset provides an additional set of PCIe lanes which are completely separate from the 16 lanes provided by the CPU. Z170 provides 20 extra PCIe 3.0 lanes, so M.2 PCIe slots on all Z170 boards are wired through the chipset and don't use any of the 16 primary lanes from the CPU.

 

On X99 and Z97 and below, the chipset only provides an extra 8 PCIe 2.0 lanes, which is not enough for an M.2 PCIe 3.0 x4 slot (since some lanes need to be used for other things). So most Z97/X99 boards either have M.2 PCIe 2.0 x2 slots (10Gbps) instead, or they have PCIe 3.0 x4 slots that use some of the CPU's lanes, which is ok on X99 because you have at least 28 lanes, so you can have an x4 M.2 SSD and still have enough lanes for x8/x8/x8 which is enough for almost any application.

[TidaLWaveZ]

2 hours ago, Glenwing said:

No, because the Z170 chipset provides an additional set of PCIe lanes which are completely separate from the 16 lanes provided by the CPU. Z170 provides 20 extra PCIe 3.0 lanes, so M.2 PCIe slots on all Z170 are wired through the chipset and don't use any of the 16 primary lanes from the CPU.

 

On X99 and Z97 and below, the chipset only provides an extra 8 PCIe 2.0 lanes, which is not enough for an M.2 PCIe 3.0 x4 slot (since some lanes need to be used for other things). So most Z97/X99 boards either have M.2 PCIe 2.0 x2 slots (10Gbps) instead, or they have PCIe 3.0 x4 slots that use some of the CPU's lanes, which is ok on X99 because you have at least 28 lanes, so you can have an x4 M.2 SSD and still have enough lanes for x8/x8/x8 which is enough for anything.

Where do you learn all these technical details from?

[Glenwing]

40 minutes ago, TidaLWaveZ said:

Where do you learn all these technical details from?

Not from any particular source, that's a good question though. It's stuff you pick up over the years. Studying motherboard diagrams helps too