SLI Memory Question

[mikey_d05]

I'm running a triple monitor setup for productivity purposes. I'd like to give surround gaming a whirl. This is a relatively simple/stupid question but I don't know how SLI actually uses the GPU's memory. I'm looking to pick up a single 680 and SLI later down the road. So, does SLI utilize the memory of both cards, or just one? i.e. does 2+2=4 or does 2+2=2?

I'd like to think that 4GB between two cards would be more than enough for good frame rates on a 5760x1080 setup, but I'd rather buy the right cards in the first place. If I should wait for the FTW edition, so be it.

 

[Helltech]

2+2=2

The 7970 is a bit better at extremely large resolution setups.

With that said, while the 7970 will be a fit bit better I believe 2GB is enough for that resolution just turn off all AA, if it was 2560x1600*3 it would be a different story.

 

[Rockdpm]

SLI Memory bandwidth works like this, What ever monitor that card is hooked to is the amount of memory that monitor has. When you have two plugged to one card, it shares the memory so probably around 1GB to each monitor. when you have 2 cards and 3 monitors... Well same concept and theory just more memory to go around. But for example if i was to plugg 1 monitor up to one of the Two GTX 680's you'd only have 2GB of vram total because Vram doesn't pool between cards, it spreads between monitors

 

[mikey_d05]

I'm not as dumb as this is about to sound, I promise. What I'm interpreting is:

2GB GTX680/1 monitor = 2 GB/monitor

2GB GTX680/3 monitors = .6 GB/monitor

(2) 2GB GTX680 SLI/3 monitors = 1.3 GB/monitor

OR...are you saying plug the main display into one and the peripheral monitors into the second card, so:

2GB GTX 680/main display = 2GB/main display
AND
2GB GTX 680/2 peripheral displays = 1GB/each peripheral display

I really am a noob when it comes to sli setups. The single 7970 has performed slightly better in the benches I've seen. I'm after a Nvidia card(s) for a couple of completely stupid reasons.

 

[Helltech]

I dont understand Rockdpms explanation at all, I'm not saying its wrong I just don't understand it.

The way I was taught was this.

Both cards are at 2GB of vram, the problem is both cards have to load all the information that is necessary, if one card must load this texture, then so much the other. Thus you technically do have 4GB of ram, the problem is each card is loading the same information at the same time, thus you only have 2GB of effective memory.

That is how it works.

 

[welshmousepk]

its really much more simple than the above poster makes it sounds.

multi card setups clone data in the vRAM. so adding a second 3gb card doesn't add any more vram for it to use. since the same data needs to be held in the vRAM of both cards.

for large resolutions, 3gb or higher is generally worth it. but 4 gb would be better, since the cards still essentially only have 2gb total to work with.

 

[Helltech]

its really much more simple than the above poster makes it sounds.

multi card setups clone data in the vRAM. so adding a second 3gb card doesn't add any more vram for it to use. since the same data needs to be held in the vRAM of both cards.

for large resolutions, 3gb or higher is generally worth it. but 4 gb would be better, since the cards still essentially only have 2gb total to work with.

You mean more simple then Rocky? Right? Not me? I think i explained it well.. maybe not..

 

[welshmousepk]

sorry, helltecks post came in before mine but wasnt there when i replied. i was talking about rockdpms reply.

the total vRAM available is whatever the lowest card in the setup has. if you have a card with 1gb, then your total setup will have only 1gb.

 

[Rockdpm]

VRAM doesn't pool<Blunt summary of my reply....

 

[mikey_d05]

Best answer selected by mikey_d05.

 

[mikey_d05]

Thanks Helltech. That answered my question in a very simple manner. SLI adds processing power, not VRAM.

 

[checkitman22]

don't worry about that monitor explanation, and spanning monitors. this is how it works; it doesn't matter if you have one monitor, two monitors, or three monitors, you will have 4gb of vram but only 2gb of effective ram. it is effective ram because all data going into the first card must also go into the second card to be processed by the second gpu as well. so you really only have 2gb to work with.