Who said 2 cards in SLI doesn't double the memory?

There was a very knowledgeable person on these forums who showed me an article he posted about SLI and in that article it states that having 2 cards in SLI does NOT double the amount of available memory, only one card's memory is used.

So how come when I play GTA V it says my available graphics memory is 12 GB? that's two card 2x6GB so is it really making use of the 12GB or what's going on? I'm confused

 

12GB's of memory is available to be used between the two cards, but the memory doesn't add up. Each GPU has its own 6GB's.

SLI allows two, three, or four graphics processing units (GPUs) to share the workload when rendering real-time 3D computer graphics. Ideally, identical GPUs are installed on the motherboard that contains enough PCI-Express slots, set up in a master-slave configuration. All graphics cards are given an equal workload to render, but the final output of each card is sent to the master card via a connector called the SLI Bridge. An example, in a two graphics card setup, the master works on the top half of the scene, the slave the bottom half. Once the slave is done, it sends its render to the master to combine into one image before sending it to the monitor.

 

This is also what used to confused me. But I think the key lies in the statement

So basically each GPU must have full set of data available in order to share the workload. Simply put, yes the memory adds up but the data storage requirements also multiply so nullifying the effect of adding the memory.

 

Dx12 can add vram iirc

 

Who said? Nvidia said, and that's all that matters.

 

Nope, that's SFR. Modern multi-GPU is AFR except for an edge case like Mantle in Civilization: Beyond Earth.

 

when exactly is that going to happen?

 

Needs developer implementation. Quite frankly given the current state of PC gaming, I wouldn't count on it AT ALL. We'd be lucky to get non-broken console ports on launch day.

 

This is correct. Each card technically has its own full set of memory, but everything gets copied to both cards to work on the data.

My suspicion for why a game would report this way is just that it was an oversight by the programmer. Ideally it would recognize the SLI config and give you a more accurate number or at least add some text afterwards to make it more clear.

 

shouldn't it just work on a hardware level to combine the memory?

 

That was me.
Windows 10 doubles the vRAM usage count in SLI, but doesn't add vRAM. Similar to how AMD CrossfireX will "add" the memory in programs like GPU-Z, but it's not actually added.
Already confirmed on Linus Tech Tips forums.
If you dual-booted Win 7 or so, you could run a benchmark you'd see say... Valley using 1000MB of vRAM and in Windows 10 it'd use 2000MB.

It's just a problem with Windows 10 interfacing with the programs. Your vRAM isn't added until I say so =D.

 

No that would be terribly slow and inefficient. Communication over PCIe is about 100x slower than how fast the top-end GPUs can access their own VRAM. If they can build multiple GPUs and their VRAM on a single silicon interposer (like what AMD did with the Fiji core and HBM in Fury), that would increase inter-GPU bandwidth by the orders of magnitude necessary for it to work.

 

didnt mean hardware hardware, i meant windows 10/dx12. at that level

 

Software level combining the performance benefit would be nothing to write home about and probably not even worth the trade-offs. DX12 isn't a panacea. BTW here is a good article: http://www.pcgamer.com/what-directx-12-means-for-gamers-and-developers/

 

So basically, we can expect SFR in memory adding practices to work only at the level of Pascal, or with current high end AMD cards. I was wondering about that.

But of course by the time Pascal is out, 8GB with HBM 2.0 should be the norm for gaming cards, so will we even need it? Seems like SFR is a pipe dream.

 

Hadn't realized it was a windows 10 issue and assumed it was probably game specific (in hindsight I shouldn't be surprised). Thanks for posting the clarification. Probably be a few more similar threads crop up as windows 10 rolls out, so very handy to know.