Multi-GPU and CPU overhead

[ShePearl]

To simplify my question, let's just say that GTX Titan 2-Way SLI performs *exactly* same as GTX 680 4-Way SLI in term of frame rate.

Now, 2-Way SLI Titan is 2 GPUs while 4-Way SLI 680 is 4 GPUs.

Does 4 GPUs running in SLI use more CPU resource (overhead) compared 2 GPUs running in 2-Way SLI provided that both setups output exactly same number of frame per second ??

 

[Brackle]

To simplify my question, let's just say that GTX Titan 2-Way SLI performs *exactly* same as GTX 680 4-Way SLI in term of frame rate.

Now, 2-Way SLI Titan is 2 GPUs while 4-Way SLI 680 is 4 GPUs.

Does 4 GPUs running in SLI use more CPU resource (overhead) compared 2 GPUs running in 2-Way SLI provided that both setups output exactly same number of frame per second ??

Its possible (i am not 100% sure), just because ifyou use 4 680's, that uses more PCI-I lanes? so MAYBE possibly it would use more CPU?

The thing is though, with 4 680 gtx's it would be ALOT easier to be CPU bound, since you have so much GPU power.

It is also possible the CPU would be used less, if you have more GPU power.

But it all depends on the game. Some games are very CPU dependent, some are GPU, and some are both.

I would also think the titanx2 in SLI Would be smoother then 4x680 gtx's. 3 and 4 way SLI (and even tri/quadfire) scaling sucks, and is prone to driver problems.

Anyway now im rambling.

 

[ShePearl]

Its possible (i am not 100% sure), just because ifyou use 4 680's, that uses more PCI-I lanes? so MAYBE possibly it would use more CPU?

The thing is though, with 4 680 gtx's it would be ALOT easier to be CPU bound, since you have so much GPU power.

It is also possible the CPU would be used less, if you have more GPU power.

But it all depends on the game. Some games are very CPU dependent, some are GPU, and some are both.

I would also think the titanx2 in SLI Would be smoother then 4x680 gtx's. 3 and 4 way SLI (and even tri/quadfire) scaling sucks, and is prone to driver problems.

Anyway now im rambling.

Ah.... that's why I said the assumption which is the same number of frames per second.
And I don't think PCI-E lanes has much effect on (if at all) CPU overhead.
Well, if I twist my question because you said, PCI-E lane, How about 2x690 in Quad-SLI vs 2x Titan. The same number of physical cards which is 2. But it's 2 GPU vs 4 GPU.

 

[Brackle]

Ah.... that's why I said the assumption which is the same number of frames per second.
And I don't think PCI-E lanes has very small (if at all) CPU overhead.
Well, if I twist my question because you said, PCI-E lane, How about 2x690 in Quad-SLI vs 2x Titan. The same number of physical cards which is 2. But it's 2 GPU vs 4 GPU.

Then no I dont think it would. Then to me it would all come down to software (Drivers and Games).

SLI scaling is better then 3/4 way SLI scaling.

Only reason I said PCI-E lanes is because the X79 chipset has WAY more available lanes then Z68/Z77.

 

[ShePearl]

SLI scaling is better then 3/4 way SLI scaling.

I thought the sweet spot was 3-Way SLI ??

 

[Brackle]

I thought the sweet spot was 3-Way SLI ??

Well yea it can be, but 3 way SLI can be prone to more problems then 2 way SLI (again software).

Also for 3way SLI to scale well, you will need 1 beefy CPU so you arent CPU bound, then you need a monitor to be able to push that power. Be it 1080p 120hz gaming or 1440/1600p gaming, or Multimonitor gaming.

I have had used both AMD and Nvidia for 3 way scaling....and its a problem (more with AMD) while gaming.

 

[demowhc]

Yeah it does becuase if you have Dual SLI in a CPU limited scene, and then 3-way in the same CPU limited scene the 3-way setup is generally a few % slower. Nvidia taxes the CPU more than AMD too.

 

[ShePearl]

Yeah it does becuase if you have Dual SLI in a CPU limited scene, and then 3-way in the same CPU limited scene the 3-way setup is generally a few % slower. Nvidia taxes the CPU more than AMD too.

So, more GPUs = higher CPU overhead, right?