2xGTX980 are faster as 2 separate cards than as SLi

I have been trying to find a temporary solution to the low fps on the Normandy map. I know it will improve but I tried a few, to me unusual, ideas.

My configuration is three 27" 1080 screens in surround plus one below for cockpit instruments, all in-game (not Helios). These I prefer to run as 3 screens (Left, Forward, Right) plus instruments (Forward/Down) below. I don't like 1 screen across three monitors. So that's four separate views and a lot of work for the GPUs and I was getting around 15 fps in Normandy.

I took the 2 GTX980s out of SLi, reconnected a DP monitor (Fwd view) and an HDMI (Instruments) to GPU #1 and the Left, Right monitors to GPU #2. Fps jumped to around 24. I then removed the SLi bridge and it jumped again to 28-30. Not great but adequate for now and quite smooth. I re-checked this twice and same result.

I'm really surprised at that. Anyone else experienced the same or can explain it?

btw I still only see on GPU using it's 4Gb memory in 'Nvidia experience', as with SLi the other doesn't show much usage.

I have been trying to find a temporary solution to the low fps on the Normandy map. I know it will improve but I tried a few, to me unusual, ideas.

 

My configuration is three 27" 1080 screens in surround plus one below for cockpit instruments, all in-game (not Helios). These I prefer to run as 3 screens (Left, Forward, Right) plus instruments (Forward/Down) below. I don't like 1 screen across three monitors. So that's four separate views and a lot of work for the GPUs and I was getting around 15 fps in Normandy.

 

I took the 2 GTX980s out of SLi, reconnected a DP monitor (Fwd view) and an HDMI (Instruments) to GPU #1 and the Left, Right monitors to GPU #2. Fps jumped to around 24. I then removed the SLi bridge and it jumped again to 28-30. Not great but adequate for now and quite smooth. I re-checked this twice and same result.

 

I'm really surprised at that. Anyone else experienced the same or can explain it?

 

btw I still only see on GPU using it's 4Gb memory in 'Nvidia experience', as with SLi the other doesn't show much usage.

With 2 cards in SLI the Frame-Buffer that uses most of the GPUs memory needs to be synched, so you effectively use the memory of one card (4GB) while the other 4GB are paid for, but useless.

The PCIe bus then needs to cycle through two GPUs each time and this is extremely timing critical, so any hiccups from other components on the bus add up... Thus SLI often get worse results than even just one card of the same built series.

yeah ran sli through 3 generations and now i always opt for a single card setup. when it works its great but less and less games work well with it now, and then there is the micro stutter.

I have not seen anyone talk very positive about SLI in many years esp here.

Well I was under the impression if the game is not coded and optimized for SLi its a no go. I did a quick search on DCS and SLi and found this 4 pages on geforce.com forum about it. Most of it is about poeple asking for a SLi profile for DCS and then they talk about the game needing patched to make SLi work good. And something about "SLi Bits" I am a nobody to programming but thought it was relevant to the post. Maybe this could help a little.

https://forums.geforce.com/default/topic/903615/sli/dcs-series-sli-/1/

Maybe ED has SLi is on the back burner when they get DCS 2.5 running?

Well I was under the impression if the game is not coded and optimized for SLi its a no go. I did a quick search on DCS and SLi and found this 4 pages on geforce.com forum about it. Most of it is about poeple asking for a SLi profile for DCS and then they talk about the game needing patched to make SLi work good. And something about "SLi Bits" I am a nobody to programming but thought it was relevant to the post. Maybe this could help a little.

https://forums.geforce.com/default/topic/903615/sli/dcs-series-sli-/1/

Maybe ED has SLi is on the back burner when they get DCS 2.5 running?

All that would not solve the Frame-Buffer waste.

If you have 2 cards running in SLI you still need the frames in each cards Frame-Buffer for processing, so the total amount of RAM used effectively is that of one card... So unless you don't want to run a gigantic projector setup with multiple 1080ti, or similar special stuff, you are usually better off with a new single card...

Just my two cents.

All that would not solve the Frame-Buffer waste.

If you have 2 cards running in SLI you still need the frames in each cards Frame-Buffer for processing, so the total amount of RAM used effectively is that of one card... So unless you don't want to run a gigantic projector setup with multiple 1080ti, or similar special stuff, you are usually better off with a new single card...

 

Just my two cents.

Thanks for your posts guys, shagrat especially for the explanations. I'm curious to know why only one card seems to use VRAM (as seen in nvidia inspector) when I am running them separately. Any ideas?

My main hope is that the Normandy map will be improved considerably (I didn't have much trouble in the others). Or maybe they'll remove some of those wheelbarrows, hand carts, old renault cars, piles of dust etc :music_whistling:

It's been an interesting journey.

Thanks for your posts guys, shagrat especially for the explanations. I'm curious to know why only one card seems to use VRAM (as seen in nvidia inspector) when I am running them separately. Any ideas?

 

My main hope is that the Normandy map will be improved considerably (I didn't have much trouble in the others). Or maybe they'll remove some of those wheelbarrows, hand carts, old renault cars, piles of dust etc :music_whistling:

 

It's been an interesting journey.

I need to guess a little bit here. I am assuming this works similar to an additional USB screen, like the "Lilliputs".

In that scenario the one Video Card processes the whole virtual screen and uses its vRAM/Frame-Buffer. For the other Video Card a part of the screen in the Frame-Buffer is grabbed and copied in the normal RAM over the PCI bus and their the second card can load it and display it. As it does not need to buffer (pre-process and render) the frames it just loads them as they come from the primary card.

Basically you use one card and add transfer bandwidth and outputs, without utilizing much of the GPU power in the other card(s).

The bottleneck here may be the PCI-Bus and RAM bandwidth especially with larger screen grabs.

With my old setup before 4K I used two USB displays for the MFD export.

Performance was ok, as the overall screensize was manageable by my GTX770 3GB at that time. The USB screens maybe reduced the fps by 5 fps, overall...

As I said I don't know exactly how Nvidia Cards and their drivers handle a multiple non-SLI setup, but I assume it isn't much different from the USB-Screens. :dunno: