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3-way sli slower than 2

jrichemont

jrichemont

jrichemont

Associate
Joined
27 Feb 2014
Posts
15
Hi. I have 3xEVGA classified nvidia 780s running on a Gigabyte sniper motherboard with 3 screens. The performance of three cards over two is noticably poor, even in 2D dragging windows around the desktop is slow.

If I change the display mode to "just turn all the monitors on" the 2D issues go away but of course I can't use more than one monitor for 3D.

Currently I have one monitor plugged into each card, in the digital DVI-D connector.

Is the motherboard at fault here ie PCI-E speeds support 16x + 16x with two cards but 16x 8x 8x with three? If it makes that much difference, can anyone recommend an X58 board with 3 16x slots? I can find more modern boards with that spec but I'm not quite ready to chuck my i7 990X away as you may imagine.

I'd be grateful for any suggestions. Even if the consensus is to stick with 2-way sli I can use the other 780 for something else. The rig does look nice with all three in, though...

Other specs: 1200w PSU, 24Gb RAM @1800Mhz (it's rated at 2.1Ghz but I have never managed to make the machine boot faster than 1800Mhz or so), 256Gb SSD striped RAID.

Thanks!

Jeremy
 
What is your 990X running at? Possibly that could be the reason and it is being bottlenecked.

X8 shouldn't give that drop but I am wondering if your third slot doesn't drop it to X4 for all 3, so that could also be an issue. Just guessing though.

X79 is the way to go for 3 cards IMO.
 
The 990X is running at 4.5Ghz, water cooled (nearly wasn't when I knocked out the pump power installing card three). It really shouldn't be CPU bound I think. Especially just dragging windows around the desktop. My work machine (nvidia 210) is smoother doing that right now.
With the 2-way sli running Unigene Heaven at max settings across all three screens didn't use much CPU though I can't tell if it was i/o bound pushing data across tp the video cards by looking at load.

Is there a utility I can use to find out PCI-E multiplier? And I'm sure you're right about x79 but that 990x is an investment!

Jeremy
 
Haveny ou made sure all the cards are properly seated, power connectors, what psu do you have exactly (e.g. Decent name brand?), bad sli bridge?

Just spit balling really

How about swapping the cards around, trying them one at a time etc.
 
Yeah; of all the things I try, CPU seems the most chilled. That's why I'm fond of it even though I know there's newer things out there. I see 'Graphics Interface' in CPU-Z; I guess that will tell me what the interface runs at. I'll check when I get home.

And, yes, that's the mobo.

The thing is, we're not talking about a small drop off in FPS in a demanding benchmark; simply dragging a window around the screen is slow and jerky in sli. Soon as I un-sli it performance goes back to what I'd expect. This makes me think something else is wrong.

I'm assuming that the PCI slots will work at the same speed regardless of whether I am using sli or not? If that's the case and the problem is shoving bytes to the cards fast enough then what difference should being in sli mode make?

What would happen if the sli connector were faulty? Of course, in 2-way I use a different connector. Is it possible that the cards are trying to fall back on communicating using PCI in sli mode because the sli channel is broken? If so, it might explain the issue. In non-sli mode the CPU will simply talk directly to whichever card is relevant. In sli mode perhaps it talks to one and expects them to share but they can't so take up reams of bandwidth over PCI?

I also notice that the performance during benchmarjking is quite variable. With the Heaven benchmark, the scenes with little data can hit 200 FPS (max everything 5760x1024). Make the scene a bit more data rich and it can fall to <20 FPS.

I do have another 3-way sli connector so I will try that but do you think this idea is possible? I don't know a lot about how sli works, really.

Jeremy
 
Running 3x GTX 780 on X58 platform with good performance is possible -> http://forums.bit-tech.net/showthread.php?t=263634

I am just curious if the cards you are using are ones with the reference cooler ?

Edit NVM just saw they are Classified card with the ACX cooler.
Have you checked the temperatures ? 3 cards with the ACX cooler as opposed to the reference cooler aren't good when stacking 3 cards together. They make a massive heat build up and the hot air just circulates between the cards making them throttle.
When you're using 2 cards there is enough space for both cards to "breathe" and stay cool and not throttle.

What is your airflow and system cooling like ?
If the cards in 3 way SLi get too hot (above 90°c) then it will have a seriously negative impact on performance since the won't boost at all and drastically reduce performance. In 2 way SLi they have enough breathing room to run at their max boost clock and hereby easily match 3 too hot cards.
 
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Consider 1 core on the i7 990X @ 4.5 Ghz is just about matching a i7 2600K @ stock, so there is likely to some significant bottlenecks with 3 780s, depending on the game in question.

Running my 2600 at stock is a bottleneck.

Most games are not going to get any benefit from the 990X extra cores.
 
@Gripen90, they are stock coolers I think. I take the point about cooling but as I said earlier the negative effect of adding the 3rd card is easily seen just dragging windows around the desktop. I use sli and I get massive lag. Use 'all 3 monitors' and it's sweet. Same cooling, setup etc. Only difference is enabling sli. Temps get up to around 35 C so heat is not, I think, an issue.
@Sasahara, the problem is not to do with the CPU. Put 2 way sli in that box and I get great results. Add a 3rd card and it all goes to pot.
I have changed the sli adaptor but it made no difference. I suspect the third card has caused some sort of I/o thrashing when the 16x PCI got knocked down to 8x but I have no idea what to do about it. Other than removing card 3 of course.
Cheers
Jeremy
 
jrichemont said:
@Gripen90, they are stock coolers I think. I take the point about cooling but as I said earlier the negative effect of adding the 3rd card is easily seen just dragging windows around the desktop. I use sli and I get massive lag. Use 'all 3 monitors' and it's sweet. Same cooling, setup etc. Only difference is enabling sli. Temps get up to around 35 C so heat is not, I think, an issue.
@sasahara, the problem is not to do with the CPU. Put 2 way sli in that box and I get great results. Add a 3rd card and it all goes to pot.
I have changed the sli adaptor but it made no difference. I suspect the third card has caused some sort of I/o thrashing when the 16x PCI got knocked down to 8x but I have no idea what to do about it. Other than removing card 3 of course.
Cheers
Jeremy
Click to expand...

What type of cooler is on the cards ? EVGA GTX 780 Classified come with EVGA's own designed ACX cooler.
Here's a picture. The top card is with the nVidia reference cooler - which is much better suited for SLi or 3 way SLi where there's 2-3mm spacing between the cards. It's a radial blower fan that sucks in cool air and spews the hot air out of the back and outside of the case.
The lower is the ACX cooler which is superior when using a single card or in SLI where there's at least 3cm spacing between the cards.
DSC00477_zps398c9d97.jpg


As for the PCI lanes etc that shouldn't be an issue (see my previous link to a person with a ci7 920 and 3x GTX 780's). Running 16x8x8 is fine ! I'm doing that myself on my X79 chipset.
One thing I can think of is perhaps you need a BIOS update for your motherboard - or download the lastest available chipset driver for the motherboard also.

Also have you tried different nVidia drivers to test ?. Once on my old PC with ci7 870, P55 chipset and 3x GTX 580 there was a new driver that totally lagged out my PC in windows. Opening Nvidia control panel would take 8 minutes and my mouse cursor would often lag around in windows. I rolled back to an older driver and all was fine again.
 
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Just to throw this one into the mixer....have you tried as different sli bridge?

Also which 1200w PSU and how old is it? I'd say 1200w could be cutting it fine (under oc load) with an over clocked x58 chip and 3 classifieds. Well unless its one of those superflower monsters in which case it will be fine :D
 
Did you already check how the cards do clock and what the usage of them is? Also the CPU at the same time? When you do this with precision you can also have a look at the power target if they go higher then 100%
Like pgi said, the minimum power supply for a tripple SLI of 780 classified is 1200W. When the cards do overclock good it can be possible that the PSU is not good enough
 
Thanks for the replies, guys. OK:

Yes, I thought of the bridge being bad and read the sli spec on wiki. The cards will drop back to using PCI if the bridge is duff. I was hopeful about this but... no. I have both an Asus and Gigabyte bridge and tried them both. Same result. Can't believe they're both duff.

Using precisionx I see my clocks are 324 on all 3 cards. Temps around 40C. I'm not running anything challenging, just dragging a Chrome window around on my desktop.

I know 1200W is a lower limit but it's an Enermax revolution with 85% efficiency. It might struggle later on when I overclock and hammer the cards but I'm just trying not to get a **** windows experience right now ;)

The BIOS is f4 which is the latest Gigabyte will admit to.

And the cooler type is the lower one with two fans as per your picture. This may cause issues when loaded I know and was thinking about what to do to help that but 1) they're not @40C and 2) my case (big Thermaltake thing) has a great big fan in the door right next to all three.

Finally, I looked at CPU-z for the PCI speeds but it only says "link width: x16" and "max supported: x16". I was sort of expecting one report per slot or something?

Still puzzled;

Jeremy
 
Well then I guess a format: c and a reinstall of windows may help.

Have you tried different nvidia forceware drivers ?.
 
Do you have another PSU laying about you could drop one of the cards to? Just to rule in/out the psu.


Edit: Brainwave!!! Some nv drivers lately have caused a string of random issues for multi card users (my self included). Try running a ddu sweep then reinstalling the latest drivers from scratch.
 
@andybird123; wwhoops! I will download gpu-z and report on what it says.
@Gripen90; harsh. I only installed it a few weeks ago!
@pgi; not really, no. But nothing is loaded much so I really doubt it's a PSU issue. Even if it was though it's hard to find one > 1200w anyway. Not sure how I'd fix it if it was an issue. I will certainly try the ddu/re-install trick. And forceware.
 
pgi947 said:
Do you have another PSU laying about you could drop one of the cards to? Just to rule in/out the psu.


Edit: Brainwave!!! Some nv drivers lately have caused a string of random issues for multi card users (my self included). Try running a ddu sweep then reinstalling the latest drivers from scratch.
Click to expand...

^^^
This first

If no joy - you may have no choice but to do the below option :(

Gripen90 said:
Well then I guess a format: c and a reinstall of windows may help.
Click to expand...
 
BTW I only have low power ssd drives in this system so there's a bit more headroom for the PSU. I used this psu calculator for my setup and it suggested 927w. I'm not taking that as iron clad but even if it's out a bit I should have at least 150w to spare.
 
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