In my previous post I described some of our findings for X-Plane 10 with respect to GPUs. This begs the question we get asked all the time: what about SLI/Crossfire?
For those of you not familiar with SLI and CrossFire, they are technologies (from NVidia and AMD respectively) that use two graphics cards to share the load of drawing a 3-d scene. The idea is to double the shading power of your graphics system by having each card render every alternate frame.* You can get SLI/CrossFire with two GPUs and an appropriate motherboard, or by buying one of the monstrous “x2” GPUs.
Now to try to answer some often-asked questions:
Can X-Plane take advantage of Crossfire/SLI?
No. Neither X-Plane 9, nor 10.0, will be capable of running with Crossfire or SLI. From what I can tell, there are a few sites in the code that need to be reworked a bit to be ready for these technologies. If you have an SLI/Crossfire setup, I would expect you to get the same framerate with only one of the GPUs.
Will X-Plane 10 ever take advantage of Crossfire/SLI?
Maybe someday. If we can get our code clean enough to work with these technologies, I will post an update on this blog. But I can’t promise anything – for all I know we may someday hit some horrible show-stopping problem.
Would SLI/Crossfire be useful for X-Plane?
Not for X-plane 9. X-Plane 9 can’t even max out a single GPU from the previous generation’s top end cards, so there’s really no need for two of them.
For now there are resolutions where two GPUs would be necessary to get better frame-rate. If we can run 2560 x 1440 at 20-30 fps on one card, it would in theory be nice to run at 40-60 fps on two.
But the next generation of GPUs is on the way, so it may be that the next generation of cards will be fast enough on their own.
Should I Buy an SLI/Crossfire setup?
Not for X-Plane – we can’t take advantage of it right now.
Here’s another way to look at it.
- If you wanted 2400 GFLOPs on your GPU in August of 2008, you could buy the Radeon HD 4870 x2, which is basically two 4870s jammed together on a single card via Crossfire. This would set you back $549.
- 13 months later, in September of 2009, AMD released the 5870, which could put out 2720 GFLOPs as a single GPU for $400.
The performance curve for GPU power is really quite steep, and a dual-GPU system typically costs at least twice as much as two of the single-GPU form. That’s a lot of money to pay for fill rate that will be available in about 18 months in single-card form.
So the short version is: we don’t support SLI/Crossfire yet. Someday we may work with these technologies, but even if/when we do, they’ll only make sense if you really like high resolutions and framerates and money isn’t an object.
* SLI can operate in “split-frame” mode where each video card takes half of a frame, but this is not so good because a full frame of geometry must be sent to each card for each frame, effectively doubling PCIe bus use. In alternate-frame mode, each card only sees every other frame, and thus only needs half the bandwidth – both cards together use the same bandwidth as one card would have.