8700k and PCI-E lanes

[Evil Scooter]

been thinking of doing a bit of a side grade to my current system to get a bit more single thread performance with higher clocks. One thing that has me concerned is the state of PCI-E lanes on the 8700k. With only 16 lanes is it possible to run SLI and an M.2 boot drive? I'm thinkin 8x8 for the gpu's is fine.. but that leaves you all outta lanes.. am I wrong here?

 

[/dev/null]

been thinking of doing a bit of a side grade to my current system to get a bit more single thread performance with higher clocks. One thing that has me concerned is the state of PCI-E lanes on the 8700k. With only 16 lanes is it possible to run SLI and an M.2 boot drive? I'm thinkin 8x8 for the gpu's is fine.. but that leaves you all outta lanes.. am I wrong here?

I personally would find a board that does 8/4/4....but that is just me. Any modern m.2 nvme (assuming you go that route) will kill DMI3 under load.

 

[Dan_D]

been thinking of doing a bit of a side grade to my current system to get a bit more single thread performance with higher clocks. One thing that has me concerned is the state of PCI-E lanes on the 8700k. With only 16 lanes is it possible to run SLI and an M.2 boot drive? I'm thinkin 8x8 for the gpu's is fine.. but that leaves you all outta lanes.. am I wrong here?

The Z370 Express platform gives you a total of 36 PCIe lanes. 16 on the CPU and 24 via the chipset. M.2 devices get their PCIe lanes from the chipset and that data passes over the DMI 3.0 bus, which is effectively an x4 link that doesn't count against the 16 or 8x8 allocated to the GPU's.

I personally would find a board that does 8/4/4....but that is just me. Any modern m.2 nvme (assuming you go that route) will kill DMI3 under load.

It doesn't work that way. The Z370 Express platform doesn't work that way. The PCIe lanes provided by the CPU can't be split more than twice. You get x16/x0 or x8/x8. That's it. If you use a device that takes less, you'll get x8/x4 or whatever that secondary device needs. SLI also requires a minimum of 8 PCIe lanes for each GPU to pass SLI certification. x8/x4 isn't a valid configuration for 2-Way SLI. x8/x4/x4 isn't a valid configuration for 3-Way SLI. You can only use x4 lanes for one or more GPU's with AMD's Crossfire.

Lastly, "any modern" M.2 drive does NOT necessarily saturate DMI 3.0. It generally takes a pair of drives in RAID 0 to do that. Even so, the reality is that the only time you really bump into the bandwidth limit is when you are running benchmarks and synthetic tests. In the real world, it works fine.

 

[/dev/null]

The Z370 Express platform gives you a total of 36 PCIe lanes. 16 on the CPU and 24 via the chipset. M.2 devices get their PCIe lanes from the chipset and that data passes over the DMI 3.0 bus, which is effectively an x4 link that doesn't count against the 16 or 8x8 allocated to the GPU's.



It doesn't work that way. The Z370 Express platform doesn't work that way. The PCIe lanes provided by the CPU can't be split more than twice. You get x16/x0 or x8/x8. That's it. If you use a device that takes less, you'll get x8/x4 or whatever that secondary device needs. SLI also requires a minimum of 8 PCIe lanes for each GPU to pass SLI certification. x8/x4 isn't a valid configuration for 2-Way SLI. x8/x4/x4 isn't a valid configuration for 3-Way SLI. You can only use x4 lanes for one or more GPU's with AMD's Crossfire.

Lastly, "any modern" M.2 drive does NOT necessarily saturate DMI 3.0. It generally takes a pair of drives in RAID 0 to do that. Even so, the reality is that the only time you really bump into the bandwidth limit is when you are running benchmarks and synthetic tests. In the real world, it works fine.

I was under the impresion Z370 coudl split 8/4/4. I must be mistaken...

Edit: Unless I'm misunderstanding, ARK says 1x16,2x8 or 1x8+2x4

 

[Dan_D]

I was under the impresion Z370 coudl split 8/4/4. I must be mistaken...

Actually, I just checked the block diagrams and evidently it can. Either way, M.2 slots connect to the chipset and SLI doesn't support mismatched PCIe lane configurations between GPU's.

 

[Evil Scooter]

Actually, I just checked the block diagrams and evidently it can. Either way, M.2 slots connect to the chipset and SLI doesn't support mismatched PCIe lane configurations between GPU's.

Thank you both for your replies. So you do not think a 1TB 970 Pro m.2 will see a performance dip running off the Z370 chipset provided PCI-E (DMI 3.0 bus) lanes?

 

[Dan_D]

Thank you both for your replies. So you do not think a 1TB 970 Pro m.2 will see a performance dip running off the Z370 chipset provided PCI-E (DMI 3.0 bus) lanes?

No. The rated performance of the 970 Pro is 3,500MB/s. That's still below the maximum throughput of DMI 3.0. While other devices do use the DMI 3.0 link, its harder to saturate that bus than you think. You won't see the maximum throughput on the drive outside of very rare circumstances anyway.

 

[Evil Scooter]

No. The rated performance of the 970 Pro is 3,500MB/s. That's still below the maximum throughput of DMI 3.0. While other devices do use the DMI 3.0 link, its harder to saturate that bus than you think. You won't see the maximum throughput on the drive outside of very rare circumstances anyway.

Thanks Dan.. off to do some shopping.

Best
-scoot

 

[/dev/null]

Thank you both for your replies. So you do not think a 1TB 970 Pro m.2 will see a performance dip running off the Z370 chipset provided PCI-E (DMI 3.0 bus) lanes?

I don't think any single SSD will bottleneck DMI 3 at this time.

However, when you have a couple of 10Gbit ethernet ports (or 40Gbit..) etc...then you can run into problems, but this isn't typically a desktop use case.