Saturday, January 26, 2008

Performance Wrap-up (for now)

The story on X-Plane performance is never over, but the chapter that is 9.00 pretty much is. I think we'll be RC in the next build (if all goes well). Certainly a lot of the things that are still performance "problems" will require changes larger than we can do in a late beta.

I say problems in quotes because a lot of what's been reported lately is in the form of: a huge screen res + a lot of shaders + a lot of FSAA = slow fps. That's not really a bug, that's an engine limitation. Now I want to make the engine as fast as possible, and a lot of this pixel shader stuff is new to 9.0, so if our track record for tuning stays the way it was for v8, we'll probably get some efficiency improvements later.

But unfortunately there's an underlying limitation: the new water and fog both cause the rendering engine to consume significantly more hardware resources than it would otherwise. Turn them on and you get prettier pictures at a price.

Just to post a a few general things I've found:
  • X-Plane 9 will tell you where your GPU really stands. GPUs that were very adequate for X-Plane 8 (like the GeForce 6600 GT) will turn out to have nothing left in reserve for v9, while GPUs that were bored in v8 (the GeForce 8800 GTX for example) will show what it really has.
  • Generally the cost of going from no shaders to shaders with water reflections of "none" and no volumetric fog should be very low if your screen res and FSAA don't add up to something crazy (like 16x FSAA at 2048x2048).
  • If you do have serious performance hits, try --no_fbos in the command-line; some drivers seem to have trouble with them.
  • The P180's virtual cockpit is a lot more expensive than the other ones, because it has a huge panel that is used in 3-d. We'll hopefully rebuild the cockpit at some point.
  • Turning water reflections to "complete" is very expensive. Watch the water and use the lowest setting that looks good. You don't need complete reflections if there are a lot of waves!
  • Shaders, FSAA, and screen size are all pulling from the same set of resources - be careful about cranking up all three.
  • Check your v-sync - a lot of users whose vsync clamped them at 60 fps in v8 will be clamped at 20 in v9.
  • Do your testing with texture res set low, then crank texture res later; pixel shaders also require the allocation of VRAM that can't be purged (for things like reflection images) so running out of VRAM can show up in some weird ways performance-wise.
  • The new Intel iMacs have serious performance problems with shaders on. This is due to driver limitations; given the much better performance under BootCamp, I expect the Mac performance to get better when the drivers are updated. For now I'd keep shaders off.
For now, please hold off on sending me performance reports. I just don't have time to address them. In the future I will try to solicit very specific performance data points that we need to check. Perhaps in the future we can also set up a database of fps-test results to have a more comprehensive idea of how the hardware does its job.

I expect future features to appear in v9 that further eat hardware; those features will have an off switch. You may have to pick and choose what graphics you enable; there is no free lunch here. I also expect new graphics cards to emerge that make the GeForce 8800 GTX seem quaint!

2 comments:

Kaminari said...

Can we expect an overhaul of the projected shadow engine in XP 9? I'm not really asking for complex stencil shadows, but the age-old shadow map under the plane looks terribly obsolete, especially over the reflecting water.

Actually, my main gripe with this fixed shadow texture is the fact that it's not even scaled nor casted at the proper angle, whether the sun is high or low on the horizon. Maybe a fine-tuning of the current system could provide a decent interim.

Benjamin Supnik said...

Shadows are an area of on-going research. We agree with you - we don't like the existing airplane shadow either. (I would not call it a "shadow map" because that refers to a very specific modern graphics technique, which is not what we're doing.)

cheers
Ben