Category: News

X-Plane 10 and GPU Power

As X-Plane 10 gets closer to shipping, we’re getting a better idea of what its performance characteristics are going to be.  This will be the first of several posts on hardware and X-Plane 10.  Spoiler alert: the buying advice I can give is at best fairly vague and obvious (don’t buy an old piece of junk DirectX 7.1 graphics card for $29) – once the sim ships and people start posting real performance comparisons, I suspect community data will be a much richer source of information for planning a new system (or a system upgrade). Please don’t ask whether your particular graphics card will work well with X-Plane 10.

Last week I had a chance to bring X-Plane 10 to ATI and run the sim on a few different modern GPUs.  It’s the Mac team that is nearby, but ATI’s mac drivers are of a quality such that platform isn’t really an issue.

We ran X-Plane on a current-generation Mac Pro with a single Radeon HD 5870, driving a 2560 x 1440 display.  The 5870 is a very solid card – to do better you have to spend a lot of money.

Here’s what we found.  Bear in mind that the art asset set is still a partially completed one, and the shaders are still being debugged and optimized, so anything could change in both directions.

Geometry: How Much Stuff Can You See?

First: geometry and 3-d.

• I could turn up almost everything that creates geometry.  To keep fps smooth I had to turn down something one notch – either world LOD, visibility, or autogen density – but only one notch.  This is a big step forward from X-Plane 9.  (But also remember this is a current generation CPU.)
• Shadows took a big bite out of the geometry budget – basically the sim has to draw geometry again to build the shadows, amplifying the cost of your objects and trees.  Current generation hardware simply doesn’t have enough kick to max shadows out and max all of the 3-d out.  Not yet.*
• PCIe 16x matters – we saw a significant reduction in performance on the newer 8x PCIe Thunderbolt Mac laptops.  You definitely want PCIe 16x!

Geometry in X-Plane 10 is a little bit different from X-Plane 9 – it can be bottlenecked not only by your CPU, but also by PCIe bus bandwidth.  Because CPU use is more efficient in version 10, we sometimes get stuck when there isn’t enough bandwidth to transfer geometry from the CPU to GPU.**

(Please note that the fast path for geometry in X-Plane 10 requires OBJs to be stripped of most attributes; if you load a version 9 DSF and put objects on insane, you’ll get high CPU use and low fps because the objects are going to hit the slow code path.  The autogen in X-Plane 10 is being completely redone, so it should all hit the fast path.)

Hardware Advice: Make sure you have your video card in a PCIe x16 slot!

The new Thunderbolt Mac laptops have only 8x PCIe to their discrete GPUs; this may turn out to be the limiting factor for X-Plane.

Pixels: How Much Do Effects Cost?

How about shader effects?  Some of the new effects in X-Plane 10 (atmospheric scattering, fade-out level of detail, FXAA, linear lighting, deferred rendering) use only pixel shading power on the GPU.  We turned on pretty much every effect that isn’t CPU/bus based (that is, the GPU pixel effects) and found that we could run with all of them at 20-30 fps.  Heck, we even turned on a few things that aren’t going to be ready for 10.0 but may come out during the version run.

This is pretty different from X-Plane 9.  With X-Plane 9, you can turn on all of the GPU effects (there is really only one, “per pixel lighting”, crank 16x FSAA, run at 2560 x 1024 and get 27 fps on my older Radeon 4870.  In other words, X-Plane can’t even keep a previous generation GPU busy at very high settings.

(To compare, the HD 5870 has double the shader cores and a 13% faster clock, so it is at least double the GPU of the 4870 for fill and shading.  In other words, in the test system we used that machine would have run X-plane 9 at perhaps 45 fps when completely maxed out on a huge display.)

In other words, X-Plane 9 couldn’t max out a GPU; X-Plane 10 can.

If you’ve been using a “medium” tier car (e.g. a GTX 560 instead of a GTX 580, or a 5650 instead of a 5870), this is going to mean less “stuff” in X-Plane 10: less eye candy, or less resolution, or less fps.  With X-Plane 9, a top-end video card would be bored; with X-Plane 10 it might have something to do.  I think this may surprise some users who have been buying cheaper video cards and “getting away with it” for a while now.

Hardware Advice: if you want the highest quality visuals at a high resolution, get a full powered video card!

I still concur with Austin’s advice to the news list to buy the second most expensive video card.  Looking up the price and stats of the Radeon 6970 vs 6950, you pay a 40% premium to get about 19% more fill-rate.  More importantly, the HD 6990 is actually just two video cards jammed together using internal CrossFire, at over double the price of one card.  But since X-Plane does not yet leverage CrossFire, you’re paying a lot of money and you won’t see a fill-rate improvement.

The one case where you might want to go really crazy is if you want to drive an insane number of pixels, e.g. multiple huge monitors.  Your framerate is going to be inversely proportional to the number of pixels on screen, so if you are getting 30 fps with one 2560 x 1440 monitor, you’re going to get 15 fps if you try to run at 5120 x 1440 on two monitors.

Epilogue: Are We Winning?

Something to think about: is it good that X-Plane uses more GPU and less CPU?  I think the answer is “yes” – and these performance results are the result of a steady process of modernizing our rendering engine.

You can max out X-plane 9 with a cheaper video card; not so with X-Plane 10.  But this isn’t because X-Plane 10 is more of a pig; it is because X-Plane 10 provides new options (options you can turn off) that X-Plane 9 did not have.  With X-Plane 9, it was as if we had decided for you to turn off advanced rendering settings, leaving your GPU idle.

I believe that it must be the goal of the X-Plane 10 rendering engine to scale, meaning X-Plane should be able to max out a very powerful system (and provide increased visual quality in doing so), but also be able to run on a less powerful system by backing off on rendering quality.

The Radeon HD 2600 has between 144 and 192 GFLOPS of computing power; the 5870 has 2720 GFLOPS – that’s at least 14 times more computing power between an older iMac (but still one that can run HDR mode if you’re a little crazy) and the current maxed out Mac Pro.  (If you have the HD 2400 in your iMac, that gap widens to 68x!)  It is in the face of that performance gap between the high and low end system that we are trying to make X-Plane as scalable as possible.

* PCIe 3.0 will be out one of these days – I don’t know when, and I haven’t had a chance to run X-Plane on such beastly hardware, but it is my hope that we’ll get additional shadowing performance via the increased geometry budget.

** Why not keep all of the geometry in VRAM?  That’s a long sad story about the OpenGL API that we’ll have to save for another day.

When is X-Plane 10 coming?!?!?!

If I had a dollar for every time I saw this question asked on Facebook, forums etc I’d be richer than Warren Buffett. I have to say though, I can’t really blame people for asking because our track record for releasing news has not been stellar but that’s changing as we improve our online front. Stay tuned for more news on that!

You have to understand that it is not as if we’re keeping the release date a secret, it’s that we don’t have a hard date set at the moment. I will reiterate that we’re all working really really hard. This is not a 9-5 job where you punch in and punch out and go home and forget about work. We’re all pretty much chained to our computers most of the time because like you, we REALLY want X-Plane 10 to go live as well. It’s going to be a great step forward for the flight simulator community.

So when is it coming out? When it’s ready and not a moment sooner or later! Trust me, you’ll be the first to know when we have updates to share.