PCIe 3.0 x16 vs. x8 vs. x4 & Graphics Cards

[Iron124]

Hi all, I realize this question has likely been posed before, but I've found conflicting answers so far and am wonder if anyone else has any experience relating to this specific topic.

Now, the current generation of PCIe is 3.0, and it is usually recommended that you install it in the x16 slot. Most motherboards nowadays come with at least 2 PCIe 3.0 x16 slots and support crossfire at the very least, the more expensive ones supporting SLI.

Now for my question/theory. It seems most of the inexpensive motherboards come with a PCIe 3.0 x16 slot, and a PCIe 3.0 x4 slot (Physically x16, but not fully utilized). This x4 slot is what has me worried, because that's 1/4th of the bandwidth of a standard speed slot. If you were to utilize two high-end GPU's in a motherboard such as this in crossfire, would you see a performance drop? If so how much?

If my understanding of PCIe generations is correct, the revisions have doubled the bandwidth rate every time, therefore:
PCIe 3.0 x16 = PCIe 2.0 x 32 (theoretical)
PCIe 3.0 x8 = PCIe 2.0 x16
PCIe 3.0 x4 = PCIe 1.0 x16

Most of the benchmarks I've found comparing these two showed very little performance drop between the differing speeds, but most of the data was collected years ago (2010 - 2012). Have the recent improvements to graphics cards in recent years, such as Polaris and Pascal, made these slower PCI slots useless for crossfire utilization?

 

[Gam3r01]

I personally have not seen recent tests, however you are correct that modern hardware will fill the bandwidth more so than previous tests.

At this point in time, its not really worth considering the FPS differences on bandwidths. Anything will run perfectly fine on an x8 lane, only x4 starts to have impacts. Even then its hard to really tell and document, so nobody does so.
Xfire supports x4 lanes with multiple cards, SLI however requires at minimum x8 lanes.

 

[Iron124]

I personally have not seen recent tests, however you are correct that modern hardware will fill the bandwidth more so than previous tests.

At this point in time, its not really worth considering the FPS differences on bandwidths. Anything will run perfectly fine on an x8 lane, only x4 starts to have impacts. Even then its hard to really tell and document, so nobody does so.
Xfire supports x4 lanes with multiple cards, SLI however requires at minimum x8 lanes.

So in your opinion, the performance drop by utilizing a x4 lane in crossfire would be negligible at best, even with an R9 480 or similar dual-card setup?

 

[Rogue Leader]

He is saying at x4 it does start to impact performance, I tend to agree. x4 is the minimum for Crossfire, but Crossfire (and SLI) at best only adds about a 50% performance increase tops, then at x4 would be even less.

If you are going to spend the money (and effort as Crossfire is a pain, trust me I have it) on it, really x8 is where you want to be to make it worth it. Beyond x8 the performance difference is negligible (and as well between 2.0 and 3.0 its negligible).

 

[Gam3r01]

With modern cards I would be skeptical of using an x4 lane.
Older cards, like the 7750, were not impacted heavily (a few FPS at most). Given how superior the 480 will be to those generation of cards, it should be a reasonably impact using an x4 lane.

 

[iamacow]

He is saying at x4 it does start to impact performance, I tend to agree. x4 is the minimum for Crossfire, but Crossfire (and SLI) at best only adds about a 50% performance increase tops, then at x4 would be even less.

If you are going to spend the money (and effort as Crossfire is a pain, trust me I have it) on it, really x8 is where you want to be to make it worth it. Beyond x8 the performance difference is negligible (and as well between 2.0 and 3.0 its negligible).

SLI/Crossfire has been shown with upwards of 80 to 90% performance over a single card. 50% is on the extremely low side and none-optimized games.

Now if you are setting up crossfire on 4x, I could see 50% or less. Since SLI doesn't support anything less than 8x, I couldn't say what that would be like.

 

[Rogue Leader]

He is saying at x4 it does start to impact performance, I tend to agree. x4 is the minimum for Crossfire, but Crossfire (and SLI) at best only adds about a 50% performance increase tops, then at x4 would be even less.

If you are going to spend the money (and effort as Crossfire is a pain, trust me I have it) on it, really x8 is where you want to be to make it worth it. Beyond x8 the performance difference is negligible (and as well between 2.0 and 3.0 its negligible).

SLI/Crossfire has been shown with upwards of 80 to 90% performance over a single card. 50% is on the extremely low side and none-optimized games.

Now if you are setting up crossfire on 4x, I could see 50% or less. Since SLI doesn't support anything less than 8x, I couldn't say what that would be like.

I may have been a bit too tough with 50% but 80-90% is not every game. There are some games that will scale in the 80's, but mid to low 70's is more normal. Depending on the price of the card is whether or not its worth it to you for that change in performance.

And then of course I agree at x4 it can be more like 50% which becomes maybe not worth the money/effort vs buying a new better card.

 

[Iron124]

This seems like something I'll have to give a bit more of a test in the future. In the meantime, I'll just look into spending the extra cash on something with at least a x8 secondary PCIe slot.

 

[Viking2121]

On my system, PCI-E x8 at 2.0 was the same as PCI-E at 3.0. The only reason I had it at 2.0 was because I was learning where everything was in my bios and didn't know there was a setting for Gen2 or Gen3. Every game I played was the same, maybe the 780ti's are not saturating the bandwidth enough at 1080p, I don't know.

I had some ASRock 970 motherboard that would do crossfire at PCI-E 2.0 at 16 and x4, my HD 5850's at x16 x4 was horrible and most games lower FPS than with just 1 card, That last PCI-E slot was only wired to do at most x4 speeds at Gen2, it ran through the southbridge. Got a 990FX board that would run crossfire at x16 x16 at Gen2 and the HD5850's were just amazing at the time, it was a night and day difference.

I would avoid x4 in both 2.0 and 3.0 speeds for any kind of gaming, but to display something even at 4k, a 4x 1.1 would be just fine if the CPU can handle it of course.

 

[iamacow]

16x PCIe 2.0 / 8x PCIe only hits max saturation in 4k and that's only some games. Most likely you won't have a card powerful enough to even hit that anyways.

 

[HarveyMatthew]

16x PCIe 2.0 / 8x PCIe only hits max saturation in 4k and that's only some games. Most likely you won't have a card powerful enough to even hit that anyways.

I just got a GTX 1080 card for my system. Seems ridiculous to put it on a motherboard slot that is PCIe 3.0 x4.

But that's where I'm stuck.

I'm trying to juice my RAW video editing capabilities (I don't do gaming) and think I need to consider a different motherboard?? Not sure.

Currently I have a ASUS H170. The gray slot is x16 holding a GTX 970, and the second PCIe slot is x4 holding the GTX 1080..

 

[Rogue Leader]

16x PCIe 2.0 / 8x PCIe only hits max saturation in 4k and that's only some games. Most likely you won't have a card powerful enough to even hit that anyways.

I just got a GTX 1080 card for my system. Seems ridiculous to put it on a motherboard slot that is PCIe 3.0 x4.

But that's where I'm stuck.

I'm trying to juice my RAW video editing capabilities (I don't do gaming) and think I need to consider a different motherboard?? Not sure.

Currently I have a ASUS H170. The gray slot is x16 holding a GTX 970, and the second PCIe slot is x4 holding the GTX 1080..

You should start a separate thread for your issue. A GTX 1080 is a waste for Video editing (unless you are running Davinci Resolve), there is nothing it can do that a $130 GTX 950 can't do.

 

[HarveyMatthew]

You should start a separate thread for your issue. A GTX 1080 is a waste for Video editing (unless you are running Davinci Resolve), there is nothing it can do that a $130 GTX 950 can't do.

Yes, I'm using Resolve.

Some articles I've read suggest that even with a high performance card in a x4 slot, the bandwidth is still okay.

But the 1080 is a whole new level, so I'd guess I'm throttling data, and I'd rather not do that.

I'll post in a new topic.