Does x16 SLI actually matter? on a Z270 cpu

[ProRules]

Hello, ive been wondering for a while if its worth taking a motherboard that supports X16 x2 pcie slots, if the cpu only has 16 pcie lanes?

Beacause if it has only 16 pcie lanes, doesn't this mean it will be x8 anyways on both cards?

[Bananasplit_00]

your chipset will also supply lanes but you wont have two cards running 16x on the consumer platform seeing as they are used up for a load of other things

[Lurick]

x16 vs x8 doesn't really matter. You're looking at maybe 1-3 FPS difference

[ProRules]

1 hour ago, Lurick said:

x16 vs x8 doesn't really matter. You're looking at maybe 1-3 FPS difference

Even with GTX 1080TI sli?

[Guest]

1 minute ago, ProRules said:

Even with GTX 1080TI sli?

Few percent

[agent_x007]

5 minutes ago, Bananasplit_00 said:

your chipset will also supply lanes but you wont have two cards running 16x on the consumer platform seeing as they are used up for a load of other things

Actually Z170 chipset has more PCI-e lanes than any LGA1151 CPU (20) while Z270 has 24 PCI-e lanes.
Z97 has 8 lanes... of PCI-e 2.0 variety

[done12many2]

2 hours ago, ProRules said:

Hello, ive been wondering for a while if its worth taking a motherboard that supports X16 x2 pcie slots, if the cpu only has 16 pcie lanes?

Beacause if it has only 16 pcie lanes, doesn't this mean it will be x8 anyways on both cards?

 

It depends on the scenario, but it's always a better choice to go with x16 on higher end GPUs if the option is available.  While you want see the benefit of x16 in everything, you will see its benefits in some things nicely.  Especially with higher end GPUs in SLI.

 

https://www.pugetsystems.com/labs/articles/Titan-X-Performance-PCI-E-3-0-x8-vs-x16-851/

 

Quote

Overall, the results of our testing is pretty mixed. With a single Titan X, we saw a wide range of results between using a PCI-E 3.0 slot at x8 and x16. Some applications (Unigine Heaven Pro and Octane Render) showed no difference, while others (Ashes of the Singularity, GRID Autosport, and Davinci Resolve) showed up to ~5% difference in performance. 

 

With dual GPUs, the results actually got a bit more confusing. Although Unigine Heaven Pro didn't see much of a difference with a single card, with two cards in SLI driving three 4K displays in surround we saw roughly a 15% drop in performance running at x16/x8 and a massive 30% drop in performance running at x8/x8. On the other hand, Ashes of the Singularity only showed minimal differences, and GRID Autosport was actually faster at 1080p when running in x8/x8 - although it was about 8% slower at 4K and 4K surround. On the professional side, Octane Render still didn't show a difference when using two cards but Davinci Resolve did see up to a ~10% drop in performance with both x16/8 and x8/x8.

While our results are somewhat inconsistent, there are still a couple of conclusions we can draw:

1. Whether you will see lower performance with x8 versus x16 is going to highly depend on the application. Some may see a difference, others won't.
2. The higher the load of the card(s) - either through a higher resolution in games or a large number of accelerated effects in professional applications - the higher the chance of there being a difference in performance

At this point, we would say that if you are using a high end video card like the Titan X (Pascal) or possibly even a GTX 1080, it is probably a good idea to try to use a PCI-E 3.0 x16 slot - especially in multi GPU configurations. Depending on the applications you use it may not make much of a difference, but the fact that we saw a 10-30% drop in performance with x8/x8 compared to x16/x16 in a couple of our tests just goes to show how large of a difference it can make in certain situations.

 

[SpaceGhostC2C]

4 hours ago, ProRules said:

Hello, ive been wondering for a while if its worth taking a motherboard that supports X16 x2 pcie slots, if the cpu only has 16 pcie lanes?

Beacause if it has only 16 pcie lanes, doesn't this mean it will be x8 anyways on both cards?

It depends on the wiring. If the 16x slots are linked directly to the CPU, you won't get any more active lanes than the CPU can handle. But if the PCIe slots are routed through the chipset, you'll be limited by the bandwidth between CPU and chipset.

 

Typically, PCIe slots aimed at GPUs are meant to use the lanes coming from the CPU, so you are always limited by the CPU lanes, regardless of what the chipset provides (you can use those extra lanes for other things plugged in the other slots). But there are some exceptions to the rule (notice that, in any case, PCIe devices routed through the chipset will be even more limited in terms of bandwidth than an x8 slot).

[ProRules]

1 hour ago, SpaceGhostC2C said:

It depends on the wiring. If the 16x slots are linked directly to the CPU, you won't get any more active lanes than the CPU can handle. But if the PCIe slots are routed through the chipset, you'll be limited by the bandwidth between CPU and chipset.

 

Typically, PCIe slots aimed at GPUs are meant to use the lanes coming from the CPU, so you are always limited by the CPU lanes, regardless of what the chipset provides (you can use those extra lanes for other things plugged in the other slots). But there are some exceptions to the rule (notice that, in any case, PCIe devices routed through the chipset will be even more limited in terms of bandwidth than an x8 slot).

Wow thank you and the rest for the detailed replies, i got all the info i needed, thank you.

And i thought you know what i should maybe then get just a GTX 480 reference, atleast i can cook my food right next to my pc and be lazy more than ever.

[Bananasplit_00]

6 hours ago, agent_x007 said:

Actually Z170 chipset has more PCI-e lanes than any LGA1151 CPU (20) while Z270 has 24 PCI-e lanes.
Z97 has 8 lanes... of PCI-e 2.0 variety

yes but my point is still valid, you dont have 32 lanes cleared up for GPUs. you dont NEED that either seeing as 8x for both will sufice just fine