Testing a x16 graphics card in x8 results in no performance difference?

[CoolJosh3k]

Because I may need to use my second x8 PCIe slot, I did some testing with Time Spy for my RX 6800. However I noticed no appreciable difference in score.

 

I did this testing using the bifurcation setting in my bios and verified the lanes with HwInfo.

 

I expected at least a loss of 10% performance or something, but I only recorded a loss of less than 0.5%. Is this to be expected, or did I make some naïve mistake in testing?

 

My scores were:

 

@x16:

16,596

16,640

16,645

 

@x8:

16,589

16,584

16,569

 

My system for those curious is:

AMD Ryzen 7 5800X3D
Gigabyte Radeon RX 6800 GAMING OC 16G
G.skill Trident Z Neo 4x16GB DDR4-3600 CL14

Gigabyte X570S AORUS MASTER

[Zando_]

Set to PCIe 4.0 or 3.0? 4.0 x8 is the same bandwidth as 3.0 x16, and you need a 3090 to see a 1-3% hit (depends on resolution) at that bandwidth. 

[RONOTHAN##]

4 minutes ago, CoolJosh3k said:

Is this to be expected, or did I make some naïve mistake in testing?

That's expected, in games GPUs are very rarely bus bottlenecked. 3090 Tis only really drop 3% at the most when using x8 compared to x16 Gen 4. 

 

If you dropped down to gen 3 you might see a bit more substantial of a difference, but with a 6800 I wouldn't really expect any different. 

 

6 minutes ago, CoolJosh3k said:

G.skill Trident Z Neo 4x16GB DDR4-3600 CL14

Just curious, are you even able to get XMP to run on that CPU? The IMC usually gives up when trying to run speeds that aggressive with quad rank memory, 3466MT/s is usually the most you can hope for with 4 dual rank DIMMs on a Ryzen 3000/5000 CPU (16GB 3600 CL14 DIMMs are one of the few left that are guaranteed dual rank). 

[Eigenvektor]

35 minutes ago, CoolJosh3k said:

Is this to be expected, or did I make some naïve mistake in testing?

To be expected, see:

https://www.techpowerup.com/review/nvidia-geforce-rtx-3080-pci-express-scaling/

 

It makes much more of a difference on some low end GPUs that have (too) little VRAM and have a smaller number of PCIe lanes to begin with, e.g.

https://www.techpowerup.com/review/amd-radeon-rx-6500-xt-pci-express-scaling/

[CoolJosh3k]

56 minutes ago, Zando_ said:

Set to PCIe 4.0 or 3.0? 4.0 x8 is the same bandwidth as 3.0 x16, and you need a 3090 to see a 1-3% hit (depends on resolution) at that bandwidth. 

I didn’t test 3.0 @ x16, since I was able to test 4.0 @ x8 using the bifurcation setting.

[CoolJosh3k]

53 minutes ago, RONOTHAN## said:

That's expected, in games GPUs are very rarely bus bottlenecked. 3090 Tis only really drop 3% at the most when using x8 compared to x16 Gen 4. 

 

If you dropped down to gen 3 you might see a bit more substantial of a difference, but with a 6800 I wouldn't really expect any different. 

 

Just curious, are you even able to get XMP to run on that CPU? The IMC usually gives up when trying to run speeds that aggressive with quad rank memory, 3466MT/s is usually the most you can hope for with 4 dual rank DIMMs on a Ryzen 3000/5000 CPU (16GB 3600 CL14 DIMMs are one of the few left that are guaranteed dual rank). 

Everything is very much stable and verified with a 1:1 infinity fabric clock. Nothing special going on either.

[Shzzit]

If your gpu is 4.0 and your motherboard is pci 5.0.  Putting the gpu in top slot, would that auto change to pci 4.0 because that’s what the gpu is?

 

If so would that mean all of your pci 5.0 lanes are disabled?  So really you can’t use any?

[CoolJosh3k]

20 minutes ago, Shzzit said:

If your gpu is 4.0 and your motherboard is pci 5.0.  Putting the gpu in top slot, would that auto change to pci 4.0 because that’s what the gpu is?

 

If so would that mean all of your pci 5.0 lanes are disabled?  So really you can’t use any?

No idea, but I’d expect that slot to run a the highest the card can do.

[PaulFCB]

30 minutes ago, Shzzit said:

If your gpu is 4.0 and your motherboard is pci 5.0.  Putting the gpu in top slot, would that auto change to pci 4.0 because that’s what the gpu is?

 

If so would that mean all of your pci 5.0 lanes are disabled?  So really you can’t use any?

 It works at 4.0.

 

What I’ve noticed with the new AM5 boards:

 

Those X670E boards, even the top models mention that the PCI Express slot will work at 5.0 x8 speed if you populate the M2 slots. That means that a 4090 which is 4.0 will run at 4.0 x8 on a mobo like that that uses an M2 SSD on the CPU lanes. 
 

Example X670e Aorus Extreme:

 

Expansion Slots

CPU:

1. 1 x PCI Express x16 slot, supporting PCIe 5.0* and running at x16 (PCIEX16)
   * Actual support may vary by CPU.
   * The M2B_CPU and M2C_CPU slots share bandwidth with the PCIEX16 slot. When the M2B_CPU or M2C_CPU slot is populated, the PCIEX16 slot operates at up to x8 mode.

 

 

[Tegneren]

52 minutes ago, PaulFCB said:

That means that a 4090 which is 4.0 will run at 4.0 x8 on a mobo like that that uses an M2 SSD on the CPU lanes. 

Only if the b and c M.2 slots are populated, so if you have a m.2 only in the top(A) slot the GPU slot is still x16

[PaulFCB]

 Which is bad because why would I buy such a mobo and limit myself to 1 M2 SSD /use more but have my GPU lanes cut to half.

 

 Rn I’m using 2 on Z690 Master and planning for a while to get a 3rd. But 1x is on the CPU lanes and the rest are chipset lanes, Extreme wants to be the flagship for Giga and this solution sucks bad.

 

 

[Aragorn-]

Because there are not infinity lanes.

 

When designing a board, you need to make choices as to where you allocate those lanes. That particular board vendor has elected to bifurcate the main 16x CPU link to provide extra m.2's. Other manufacturers (or other boards from the same manufacturer) will take alternative approaches and distribute things differently.

 

Not every X670E board will be configured in the same manner.

 

As a random example, the X670E ACE from MSI has 4 m.2 slots, one from the CPU and three from the chipset. And they have instead bifurcated the GPU connection to one of the other PCIe slots.

[Shzzit]

21 minutes ago, Aragorn- said:

Because there are not infinity lanes.

 

When designing a board, you need to make choices as to where you allocate those lanes. That particular board vendor has elected to bifurcate the main 16x CPU link to provide extra m.2's. Other manufacturers (or other boards from the same manufacturer) will take alternative approaches and distribute things differently.

 

Not every X670E board will be configured in the same manner.

 

As a random example, the X670E ACE from MSI has 4 m.2 slots, one from the CPU and three from the chipset. And they have instead bifurcated the GPU connection to one of the other PCIe slots.

Reading m.2 instruction on motherboards gives me brain cancer. 

[Zando_]

5 hours ago, CoolJosh3k said:

I didn’t test 3.0 @ x16, since I was able to test 4.0 @ x8 using the bifurcation setting.

Yeah, point is that's the same bandwidth. 4.0 x8 = 3.0 x16, and you need a 3090 or higher to have a barely measurable performance hit in games (time spy runs closer to a game than a workstation load or something).