780Ti SLI problems

[Shixon]

I have been reading these forums for quite some time, lurking. I just created an account to make my first post. I am having troubles with my new GTX 780Ti SLI setup.

To put it shortly, I seem to be having worse performance when running the 780Ti in SLI than I am with only one card. I have test across multiple games including Metro LL, Alien Isolation, FarCry 4, and GTA5. I am playing at 4k resolution. At 4K resolution, and in SLI mode, all of my games start to bug out and have strange artifacts after 10 to 20 seconds of game play. This doesn't occur when I bump it down to 1440p. It also doesn't occur if I play at 4K but with SLI disabled.

MOBO: ASrock z87 Killer
GPU: GIGABYTE 780Ti SLI

Also, a little back story to when I first installed the GPUs. At first, I booted my PC after installing the second GPU and my soundcard wasn't working. It was powered on, but didn't work. So I moved it to another X1 slot and the sound card worked but I couldn't enable SLI in the control panel. So a friend recommended setting the link speed of the PCI slot that my sound card is in to "gen1." I did this and was finally able to have my soundcard work AND have the GPUs SLI'ed. Only reason I tell this is in case it might have something to do with my SLI problems.

 

[Armenius]

I doubt the sound card's position is going to affect performance. The PCI-E x1 slots are Gen2 and controlled by the PCH, while the Gen3 slots are controlled by the CPU.

There could be an issue with the other PCI-E x16 slots on your motherboard that you wouldn't have known about without using expansion cards in those slots. I would test the other x16 slots individually by installing one graphic card at a time in them.

Some questions:

What CPU do you have?
What Power Supply do you have?
Do you have your video cards installed in PCI-E slots 2 and 5?
Did you just drop the second video card in without uninstalling the drivers first?
Do you have the SLI bridge connected?

 

[290X]

At 4K resolution, and in SLI mode, all of my games start to bug out and have strange artifacts after 10 to 20 seconds of game play.

Have you tested each card individually? That sort of sounds like a hardware problem.

 

[Shixon]

Have you tested each card individually? That sort of sounds like a hardware problem.

I have not. That was going to be my next troubleshoot test.

I'm not sure if I should test it alone in the PCI slot it is in now (slot 5) or in PCI slot 2 (The one I usually use when only using one card).

Armenius,
CPU: Intel Core i7-4770K Haswell Quad-Core 3.5GHz LGA 1150 84W
PSU: CORSAIR Professional Series Gold AX850 (CMPSU-850AX) 850
2 and 5
Dropped it in without uninstalling drivers first. (But since then have uninstalled and re-installed using DDU multiple times.)
Yes, SLI bridge is connected.

 

[Artificiary]

In addition to the excellent suggestions already provided, I would also encourage you to run a program like Afterburner while you are gaming in SLI to monitor your GPU usage. You can also use this to see if the cards are doing anything strange like not boosting to their normal, single GPU clocks.

If your two cards perform normally individually it also worth a try, while in SLI, to go into Nvidia Control Panel and set your default profile to "Prefer Maximum Performance" (I can't look up the exact instructions for this at work but it should be easy to google).

Good luck!

 

[dragonageinquisition]

Since it only occurs at 4K it might be a temperature or VRM issue? 4K inordinately stresses pixel fill. You could try a lower temperature target if you are using Afterburner or Precision.

 

[TroyX]

actually if you look at game benchmarks, SLI scaling at 4k is abysmal.

especially 700 series and down, the 900 series somewhat manages to do better.

todays gpus are just not designed for 4k gaming yet, even if NVidia claims they are.

NVidia's next gen gpu called Pascal and MS's DX12 is going to change all that, making 4k gaming to run with ease, due to next-gen memory architecture functions and low cpu overhead.

 

[Artificiary]

actually if you look at game benchmarks, SLI scaling at 4k is abysmal.

I won't dispute that, but it does not account for the OP getting worse performance in SLI than a single card. I've seen poor 4K SLI scaling myself but it's still > a single card.

 

[cortexodus]

I won't dispute that, but it does not account for the OP getting worse performance in SLI than a single card. I've seen poor 4K SLI scaling myself but it's still > a single card.

SLI has to swap data between cards across the bridge, and 4k is a LOT of data to swap through that supposedly 1GB/s bridge. Maybe the Kepler cards just aren't up to the task?

My math might be shit here...
3840x2560 = 9,830,400 pixels * 8 bits = 78,643,200 bytes. So, 78MB per frame multiplied by even 30 fps = 2.3GB per second?

Granted, I imagine they're doing some compression magic in there, but I don't know for sure. Someone more knowledgeable than me might know if what I'm calc'ing here is crazy or not.

 

[Armenius]

SLI has to swap data between cards across the bridge, and 4k is a LOT of data to swap through that supposedly 1GB/s bridge. Maybe the Kepler cards just aren't up to the task?

My math might be shit here...
3840x2560 = 9,830,400 pixels * 8 bits = 78,643,200 bytes. So, 78MB per frame multiplied by even 30 fps = 2.3GB per second?

Granted, I imagine they're doing some compression magic in there, but I don't know for sure. Someone more knowledgeable than me might know if what I'm calc'ing here is crazy or not.

Your math is shit .

4k = 3840 x 2160 = 8,294,400 pixels. Each pixel has 4 color channels (Red, Green, Blue, Alpha) at 8 bits each.

So...
8,294,400 * 32 bits/pixel = 265,420,800 bits = 33,177,600 bytes ≈ 32 MB of pixel data.
30 FPS would therefore approximately equal 0.927 GB/s.

However, I don't know exactly how data transfer works with SLI and how the bridge factors into the equation. Or even if the frame data just contains the final pixel color output. Just correcting your math .

 

[cortexodus]

Your math is shit .

4k = 3840 x 2160 = 8,294,400 pixels. Each pixel has 4 color channels (Red, Green, Blue, Alpha) at 8 bits each.

So...
8,294,400 * 32 bits/pixel = 265,420,800 bits = 33,177,600 bytes ≈ 32 MB of pixel data.
30 FPS would therefore approximately equal 0.927 GB/s.

However, I don't know exactly how data transfer works with SLI and how the bridge factors into the equation. Or even if the frame data just contains the final pixel color output. Just correcting your math .

I had a feeling it was shit I don't really have answers either, but I will note that your math does make the per-second data transfer rate ram right up against the 1GB/s limit of the SLI bridge. However, I think even that needs to be sliced in half since the cards are (ideally) each doing half the work, so the secondary card would be passing data up the bridge to the primary card for compositing. So, let's say it's .927GB/s for 60fps then, and then yea... we'd be running up against the bridge's bandwidth limitations (assuming no compression magix)

 

[Shixon]

Well to wrap this issue up, a friend of mine showed me how to use afterburner a little better. We noticed the GPUs clocks were not anywhere close to being the same. We used nvidia inspector and noticed my newer card had a newer BIOS on it. So we copied it to my PC and used nvflash to flash my older GPU with same latest BIOS. The GPU clocks were still out of sync after trying that.

The next thing we did was to take out my older GPU that I knew was working. I moved my new GPU to PCI slot 2 and tested using that GPU all by itself. What do you know, every game was a complete laser light show. So it seems something is definitely wrong with the GPU. I've never really seen a GPU do what this one was doing. Very crazy looking.

What I learned from this experience is to always test out a new GPU by itself before throwing it into a SLI configuration.

 

[cortexodus]

So it seems something is definitely wrong with the GPU. I've never really seen a GPU do what this one was doing. Very crazy looking.

What I learned from this experience is to always test out a new GPU by itself before throwing it into a SLI configuration.

Sorry you got a bum card It is always good to confirm a card is fully operational on its own before hitching it up to another card.