I have 2 1080 ti but i still did not attached them with sli bridge yet when i attach them and make it SLI Will i will be able to render faster ? or make it easier in any way ??
i saw couple of forum ppl said that it wont help render faster so if anyone have experince with this please share your knowledge
One more thing in 2.28 i can see that 2 gpu when i attach them with SLI bridge will it read it as 1 GPU or 2 ?
Thank you all !
Hi. Assuming you have an adequate PSU in your PC for the two cards, if you connect the two GPUs you will get faster renders for Cycles (not for Eevee). But you do not connect them using SLI, the performance with SLI is worse for Cycles.
When you connect the second GPU, you enable them properly from Preferences > System
Thank you so much for reply…
The thing is in 2.8 in preference I see it says 2 choices
1080 ti
1080 ti
I put 
mark on both so they both active but as soon as I render something my PC restart… I assumed that I need the SLI and SLI bridge!!
So pls tell me what am doing wrong…
Sounds like you might not have an adequate power supply to handle the load of both cards under load.
You don’t want SLI because you want them to appear as two separate devices as you’re currently seeing so they can each work on different parts of the image at the same time, so that’s all working correctly.
^^ Would SLI improve eevee performance though?
I don’t think it’s power thing because they are both working on the motherboard and they both are attached on PC power supply but only when I render… It restart…
I think potentially? But I don’t really know and don’t have a dual card machine to try it.
What’s your power supply model and/or wattage?
Normally when the cards are idle or you’re only using one of them (playing a game without SLI for example) then you may be fine, but with both cards enabled for Cycles it’s going to drive them both towards their maximum power usage. The 1080Ti is specced at 250 watts for an Nvidia stock version (potentially more if they’re overclocked) so you’re looking at up to 500 watts just for the two video cards, plus what the rest of the system needs.
Having the system reset would be the kind of thing you’d expect to see if the power is inadequate I think. If you’re watching it when it happens and there’s no BSOD, or other error but the system just hard resets then it’s probably more likely power than software. Also I’m not sure but I think some power supplies may need you to use specific connectors to balance the load across the multiple output rails.
When I asked developers about it, they told me that in order to take advantage of SLI, some kind of code should be developed in Blender so that it can properly use SLI configuration. Something that probably never happens.
Which would also tightly bind it to one GPU vendor. Something I can’t see them doing either.
My power supply is 500watt and I really font know anymore 
the performance are still the same with 1 1080ti… It’s so frustrating idk what to do upgrade power or add sli or crossfire I not know
So blender originally are not fir SLI
My guess is you need to replace the power supply with something like 750-800 watts if you want to render with both video cards at the same time. If you do that then it should be fine (you still don’t want SLI) and it should render almost 2x as fast.
Another thing that you should see in your PSU, is if they are true 500W. PSUs usually have a label with an informative picture of currents and voltages. If you can, take a photo of it and share here. Or you share the brand and model of it.
Anyway, when you need to buy a new PSU, you ask first in the forum before buying it.
No, it is not. But as I explained, you can use the two GPUs to render with Cycles without using SLI.
I can tell you that from experience this is precisely the expected behavior for a power issue. GPUs do not draw constant current, but rather will increase load depending on demand. The issue is certainly power-related. Maybe thermal, but that’s pretty unlikely.
The TDP for the 1080ti is 250W. So two of them will consume your entire power supply.
I think that is low for 2 cards plus your cpu. If your system is rebooting you are probably under powered. What you can is to remove one card and try that way to see if you still get reboots.
Also did you overclock your cpu?
In advance of a new power supply, you could investigate setting the power limit on the 1080s, or even underclocking/undervolting them to reduce their power consumption. You might get away with setting both cards to 70% power limit and still get a good render speed boost when using both of them and not affect normal interactive performance.
Details of doing this depend on the card manufacturer I think.
Thank you and i test it more today and you were right ill buy 800 watt soon
yes its 500 watt i am sure of it but mine dose not have label or brand name i am not sure what would you suggest for me to buy which brand and which voltage
thank sm yes i tested it today and you guys were right i am honestly not the best in pc setup so i have simple idea about whats going on in my pc all the time so i always ask for help from you guys