Going to get a new computer soon - SLI/Crossfire questions

SaltyNuts

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I never really paid much attention to SLI/Crossfire as I never really wanted ultra high-end and the money it cost to get there, especially when the idea first came out and I do not think was effective as it is today. But I now want a pretty ultra high-end system - something that can play games coming out for a good while at 4k resolutions with all the goodies turned on.

I had always thought that to use SLI/Crossfire you had to have two identical cards clocked the same. But I could have sworn I saw an article the other day - I think on Anandtech but I'm seeing it off hand - that actually did SLI/Crossfire with different cards - and it works with one card being Nvidia and the other being from AMD?!?! Sorry, this might be old hat, but is that right?

And if so, has any of the sites or anyone on here done some testing/thinking about what cards might be best to mix and match to get the performance needed for 4k gaming but saving some money? For example, I saw where two 970s generally outperformed a 980TI or Titan or whatever, but the total cost comes up to ~$600, so you are not saving that much money over the ~$650 of the TI. But what about two 960s? 950s? R9 380/370/etc? Mixing one AMD and one Nvidia?

Also, does a really powerful card work good with a much weaker one? A 980 regular with a 960 for example?

Finally, if I have two SLI/Crossfire cards with 4gb of memory each, is that the functional equivalent of having 8gb or memory or still only 4 since different cards used for different frames or what not?

Seems so many options here! Many thanks for any input, and sorry for the possibly grossly out of date questions.

Thanks!
 

bystander36

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Apr 1, 2013
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With SLI, for the most part, you need the same card. AMD gives slightly more wiggle room in that you can use cards based on the same GPU arch, even if one is a cut down version. That means the 7950 could pair with a 7970 and likely means a 290 can pair with a 290x.

That article you saw was a test game using DX12, where the dev was testing the ability to pair an AMD card with an Nvidia card. This does not exist in any released games, and will likely be very rare in the next few years. Do not buy based on this technique.

Generally speaking, a 980ti is going to give a better experience to 970 sli. You have more VRAM, less latency, and more consistent frame times. The general except wisdom is to save SLI or CF for when you want/need performance beyond what a single card can give. That means it makes most since with 980ti's, Fury and Fury X.

When dealing with SLI and CF, at least for now, two 4Gb cards behave like 4Gb single cards. You do not gain any benefit from stacking their VRAM. There has been talk from dev's that with DX12, they may be able to share the VRAM between the cards, but no game currently does this, and no test games exist either. It'll likely only work in limited amounts when/if it does come true.

If you are gaming at 4K, you may want to be prepared to lower some settings, regardless of what GPU's you get. If you get a single 980ti, expect to play between medium and high settings, and with 2, you may be able to play with high and Ultra, but with a few settings turned down. You won't likely be able to play at maxed out settings, with AA turned up on the more demanding games.
 

thesmokingman

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When dealing with SLI and CF, at least for now, two 4Gb cards behave like 4Gb single cards. You do not gain any benefit from stacking their VRAM. There has been talk from dev's that with DX12, they may be able to share the VRAM between the cards, but no game currently does this, and no test games exist either. It'll likely only work in limited amounts when/if it does come true.


Good post all around, except a correction for this. Fireaxis' Civ Beyond Earth has a special mode when used with AMD cards because of it's support for Mantle. Using Civ: BE with Mantle and crossfire allows the game to use SFR for crossfire. This IS that mode that doesn't exist or does it? Joking aside, it is the only example of SFR thru the use of a close to metal API. It should become more prevalent for games similar to it under DX12, for ex. games where max fps is not important, ie. non-twitch games.
 

SaltyNuts

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Bystander, thank you so much. Very informative and helpful, I now understand! So I think I'm going to bite the bullet and grab a 980TI, then maybe another one when next gen comes out and they drop in price. Cheers!
 

bystander36

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Good post all around, except a correction for this. Fireaxis' Civ Beyond Earth has a special mode when used with AMD cards because of it's support for Mantle. Using Civ: BE with Mantle and crossfire allows the game to use SFR for crossfire. This IS that mode that doesn't exist or does it? Joking aside, it is the only example of SFR thru the use of a close to metal API. It should become more prevalent for games similar to it under DX12, for ex. games where max fps is not important, ie. non-twitch games.

Just because a game uses SFR does not mean they share VRAM data between cards. They made no claim that the VRAM stacks in this game. The reality is, if they can share the VRAM between cards, they likely will still want a lot of redundant data as well, as it is a lot faster to have that data on the card.
 

thesmokingman

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Just because a game uses SFR does not mean they share VRAM data between cards. They made no claim that the VRAM stacks in this game. The reality is, if they can share the VRAM between cards, they likely will still want a lot of redundant data as well, as it is a lot faster to have that data on the card.


It's hilarious, you fundamentally don't understand how SFR works to make a post like that.
 

bystander36

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It's hilarious, you fundamentally don't understand how SFR works to make a post like that.

SFR splits a frame up so 2 GPU's render it. While they would share some information, all the data used to create and render the frame still needs to exist in both GPU's VRAM. Textures and Geometry has to exist on both cards at all times, that data can exist on any part of a frame at any given time. While they claim they can share some resources, you can't expect VRAM stacking by any means.
 
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thesmokingman

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Stop making [things] up.

Stop posting obscenities in this forum.
-- stahlhart
 
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Flapdrol1337

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It's possible to divide the work so you have effectively more vram.

But it's probably easier to have everything on both. If I were a dev I wouldn't spend too much effort on multigpu efficiency, since that's only a fraction of the userbase.
 

thesmokingman

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It's possible to divide the work so you have effectively more vram.

But it's probably easier to have everything on both. If I were a dev I wouldn't spend too much effort on multigpu efficiency, since that's only a fraction of the userbase.


What the hell is this, open mike night? Replicating data on both would be AFR, it would defeat the point of SFR. Why you no google what the devs say?

With a traditional graphics API, multi-GPU (MGPU) arrays like AMD CrossFire are typically utilized with a rendering method called "alternate-frame rendering" (AFR). AFR renders odd frames on the first GPU, and even frames on the second GPU. Parallelizing a game’s workload across two GPUs working in tandem has obvious performance benefits.
As AFR requires frames to be rendered in advance, this approach can occasionally suffer from some issues:

  • Large queue depths can reduce the responsiveness of the user’s mouse input
  • The game’s design might not accommodate a queue sufficient for good MGPU scaling
  • Predicted frames in the queue may not be useful to the current state of the user’s movement or camera
Thankfully, AFR is not the only approach to multi-GPU. Mantle empowers game developers with full control of a multi-GPU array and the ability to create or implement unique MGPU solutions that fit the needs of the game engine. In Civilization: Beyond Earth, Firaxis designed a "split-frame rendering" (SFR) subsystem. SFR divides each frame of a scene into proportional sections, and assigns a rendering slice to each GPU in AMD CrossFire configuration. The "master" GPU quickly receives the work of each GPU and composites the final scene for the user to see on his or her monitor.

Warning issued for inflammatory language.
-- stahlhart
 
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bystander36

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What the hell is this, open mike night? Replicating data on both would be AFR, it would defeat the point of SFR. Why you no google what the devs say?

Nothing from that says they share their resources. They split up the frame, so each GPU renders half (more than half many/most methods, as they need to render past their boundaries or you'll get a sort of line between them), but all the assets needed to create the half frames need to exist on both GPU's. Or at least, that is what typically is done. What some dev's have hinted at is that they may be able to share those resources. That is what I'm talking about.

SFR splits a frame up by default, but that doesn't mean they share the assets, such as textures and geometry. And to be clear, I'm talking about sharing in that the data only exists on one GPU and both GPU's have access.
 
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Flapdrol1337

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May 21, 2014
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What the hell is this, open mike night? Replicating data on both would be AFR, it would defeat the point of SFR. Why you no google what the devs say?

Because I am blessed with a logical mind.

AFR requires everything to be in memory on both gpu's, because the gpu's have to render full frames.

SFR only means both gpu's are working on the same frame. If you had al the even image lines on 1 gpu and all the odd lines on the other and interleave them the gpu's still need all the data for full frames, but it would also be SFR.
With more advanced SFR methods you wouldn't need everything on both gpu's, render some objects on 1 and other objects on the other.
 

Headfoot

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AFR is one method. You could do AFR and share data between cards in DX12 if you developed a technique that benefitted from it. I'm fairly convinced we will actually see something like this within a couple years.
You can also do "SFR" that doesn't share data, in that one GPU renders the frame and a second GPU applies post processing. The only data that needs to be shared is the final frame image. These things aren't set in stone. It's like Anti Aliasing. There are many methods with different advantages and disadvantages. Not sharing data doesn't automatically mean its alternate frame rendering in the brave new world of DX12. Neither does sharing data mean its so-called "SFR."

Needless to say OP, no current game lets you mix GPUs beyond Crossfire and SLI. Ashes of the Singularity will probably add it at some point in time to the full release here (Anandtech review was on a special press version of the game), but then its only one game out of thousands. Stick to CF and SLI, or even better, single GPU for the next year until things shake out more.
 
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