I’ll try to get it fixed soon….it’s a bit frustrating, because it’s the second time in the last week that I made a change to X-Plane to try to improve performance, tested it on my hardware, then discovered in-field that it helps some machines but screws up a lot of others.
The fog was a screw-up though…it’s broken on the 9600XT and I have one of those.
(I’m not entirely sure what the minimum graphics card for volumetric fog will turn out to be…right now we let you use it no matter how slow it makes the computer, but generally it’s been sort of a performance problem…it’s just a very expensive algorithm that needs some kind of restructuring.)
X-Plane 8 didn’t care much whether you used ATTR_cockpit in scenery objects or other strange places. It would simply show the cockpit panel texture, and if it hadn’t been updated, you might see an old one, and if it had never been used, maybe you’d see the random (but colorful) contents of memory. Similarly if you could get close enough to another airplane to look in the window, you’d see your own panel, since there is only one panel texture (for the user’s airplane) in the entire scenery system.
This is a bigger problem in X-Plane 9.
- Because the panel texture can be expensive for big panels, we are a lot more aggressive about not setting up the panel texture if we can avoid it. This means that sometimes the texture doesn’t exist at all. This is why in beta 14 you get an error if you do a formation flight having only been in “w” (forward 2-d) view…the panel texture doesn’t yet exist, but the exterior view of the Cessna tow plane uses it.
- With panel regions there can be up to four panel textures, so you can see the potential for anarchy.
- Panel textures aren’t even the same size any more, causing the wrong-panel-in-AI-plane problem to look even weirder than before.
So in beta 15, the panel texture is replaced with a dummy white texture for:
- Any cockpit object for an AI plane.
- Any scenery objects that are illegally using the panel texture.
This prevents crashes and other nasty stuff. If you want to make the panel be visible in your AI plane, consider using LOD to make a non-panel-texture “fake” cockpit image (at a very small res) at farther LODs. My guess is that in normal usage of the sim you’d really have to do something dangerous to get close enough to see the hack.
We did discuss live panels for all planes (for all of about 3 seconds), but the live panel texture in 3-d is so expensive that it’d be prohibitive to most users for even one AI airplane, let alone 20!
We’ve been preaching “one big texture, not lots of little textures” for a while now, and generally speaking, packing a lot of art into one big texture makes life eaiser for X-Plane, because it can draw more triangles at once before it has to tell the card to change what it’s doing. Inside the company we call this the “crayon rule“.
Now the total set of geometry and textures that X-Plane needs to use for one frame is the “working set” – you can think of it as the crayons that you keep out of the box because you need them all the time. And as I said before, if the working set becomes too big, your framerate dies.
Now with large panels we’re seeing a new phenomenon, one of the first cases where the crayon rule might not be true. The reason is due to working set.
When you make an airplane with a large panel in version 9, you can either use ATTR_cockpit, which lets you use the entire panel as a texture, or you can use ATTR_cockpit_region, which will let you use several parts of the panel. Each ATTR_cockpit_region is a texture change, so that’s more crayons. And yet ATTR_cockpit_region is usually faster.
The reason is two-fold:
- You can often use cockpit regions that don’t cover the entire cockpit texture. Large panels are rounded up to 2048 if the are larger than 1024 in any dimension, so the “wasted space” in a 1600×1600 panel is actually quite huge. If you can get away with some smaller regions, your total panel texture area is smaller because there isn’t wasted space due to this rounding, and you can also skip things like Windows. Prepping the panel texure takes time, and it’s done once for lit and once for non-it elements, so it adds up!
- It turns out there are two categories of textures that contribute to the working set: static texures and dynamic ones, and their impact on VRAM is very different. Dynamic textures are much more expensive. The panel texture is dynamic and it’s uncompressed, so it really costs a fortune. (32 MB of VRAM for 1600×1600. That’s not a lot for a static texture but for a dynamic one that’ll kill you.)
Here’s the details on dynamic vs static textures: the OpenGL driver keeps a backup copy of a texture in main memory, so that if it has to purge VRAM (to make room for more stuff) it still has the texture. As it “swaps” textures, the process is to simply send textures as needed from main memory to VRAM. No big deal.
But with a dynamic texture, the texture has been modified in VRAM! So the copy in system memory is old and stale. The graphics card thus must send the texure back to main memory, consuming twice as much bus bandwidth as normal. (To free 16 MB of VRAM and refill it takes 32 MB of transfer, 16 MB to copy the old texture back to system RAM and another 16 to send the new textures to VRAM.) On non-PCIe cards, this back-transfer might be at 1/8th the speed of the transfer to the card, so this is even worse on AGP cards.
Thus the driver does its best to not throw out dynamic textures. And this is why the panel texture is so expensive. That P180 will cause X-Plane to make two 16-MB dynamic texures, and those textures will cause 32 MB of VRAM to basically be off the table. That’s less space for the other textures to swap in and out of. This kind of “permanent allocation” makes the VRAM budget tighter for all other drawing operations.
Given the right combination of large panels, large res, pixel shader effects (which make more dynamic textures), clouds, and FSAA, you can easily get even a 256 MB card to a state where the free space into which static textures are shuffled becomes horribly small, and the framerate just dies.
So the moral of the story is: yes, it can be worth 4 crayons (using panel regions) to avoid the huge cost of dynamic textures from large panels.
As to static textures (regular DDS files) that are 2048×2048 – the jury is still out but my guess is they don’t represent a huge performance problem. As one user pointed out to me, they’re only 2 MB when compressed (maybe more with alpha) so they’re not insanely huge, and they can be swapped out.
Lori and I are about to leave for a New Years Eve ski trip, but before I shut down the laptop for the last time in 2007 I wanted to say: Happy New Years to everyone in the X-Plane community. I had a lot of fun working on X-Plane in 07 and hopefully the sim brought you enjoyment too. I think 2008 is going to be very exciting – version 9 plants the seeds for a lot of interesting new possibilities during the version run.
When I get back I’ll post a bit on panel regions, for which I have the first performance numbers, as well as some of the strange effects FSAA has on the sim. We should have a progress report on Linux soon too.
See you next year!
I found the root cause of another NVidia specific bug, and once again it’s my own stupid code. If you Google for driver bugs, you’ll find plenty of grumpy developers ranting about how card X does this wrong thing and card Y does that wrong thing…I figure it’s only fair to follow up and say “yep, that one was mine.”
Like the previous nVidia-only crash, this was a case where X-Plane was always doing something wrong, but only some drivers had problems with the behavior. So the crash was NVidia-specific, but X-Plane caused.
I believe that this bug was manfiesting itself either as a message that “scenery shift took more than 30 seconds” or some kind of crash. One of the problems was that the diagnostics for this particular bit of code were really bad. So we’ve improved things a bunch…
- There is more careful error checking during scenery shift, and those error messages are reported.
- If the sim does crash, some new code will output a crash log on Windows that helps us isolate what actually happened.
Beta 12 will be out soon with the fix that caused problems on NV hardware as well as the improved diagnostics. So you may find that the sim just works better, but if it does still crash or report errors, please tell us – now we’ll have log files that will let us diagnose the problem a lot faster!
At this point my in-box has approximately 180 emails from the month of December regarding X-Plane 9. So while I appreciate all of the feedback we’ve gotten (bug reports, performance, etc.) it’s going to take a while to reply. If you haven’t heard from me, don’t panic! I hope to answer a whole pile of emails next week.
In the meantime, I’ve been working on improved crash logging on Windows. Right now when we have a crash on Windows, all we know is that (1) X-Plane crashed and (2) what DLL we crashed in (which is always us or the video driver – not useful).
Coming soon, X-Plane will catch the fatal crash, examine memory to see what was going on, examine its own EXE to figure out the names of the functions going on, and output it all to a crash log that users can send us to get a much clearer picture of what’s going on. This information is called a “backtrace” – we’ve had it for the Mac or a while (OS X provides back-traces automatically with a crash) and it’s really useful.*
So my top priority is all of the users who are seeing problems during scenery load, and a new build with a back-trace should help to reveal what’s really going on.
I’m also working on putting additional timing and performance information into the sim so we can learn more about why some users have poor performance. So far here’s what I’m seeing:
- 8800 users seem to have great performance. If you have this card and don’t have good fps, adjust your x-plane settings and NV control panel settings – this card can bring it.
- 8600 users sometimes have performance problems – not sure why.
- Older nvidia GPUs (7600, 6800) sometimes have performance problems with the new eye-candy features – I am investigating.
- The pixel shaders seem to slow down the new HD2x00 Radeons a lot more than expected…I still need to investigate this. This is the most surprising datapoint – the X1600 does very well, so I would expect newer GPUs to at least have that level of performance. I think this is something we might be able to address.
However not all of the reports are consistent, so I think it’s too soon to make some calls on recommended hardware. The only thing that’s clear is that most 8800 useres who we do careful perf experiments with end up with huge framerates.
* Microsoft provides some back-trace facilities, but since we don’t use their compiler tools, we had to roll our own.
To those who have sent performance info: thank you, but you probably won’t hear for me for a week. I’m up to my eyeballs in reports and it’s going to take a while to get through them.
I finally found the code that allows X-Plane to turn off V-Sync. This should help nVidia users who are having framerate problems.
The basic idea is this:
- X-Plane tells the graphics card to draw a lot of stuff.
- The video card accumulates this “todo” list and works on it while X-Plane runs.
- X-Plane indicates that the entire frame to be seen is done and tells the card to show the results.
- Eventually some time later the card finishes the todo list and then shows it to the user.
V-Sync relates to the question of when this last step happens. When V-Sync (vertical sync) is off, the card shows the results as soon as it is done drawing.
But when V-Sync is on, when the card finishes drawing the world, it then waits until the monitor is done drawing its frame, and then shows the results. Without V-Sync we can have a situation where the top half of the monitor is showing a new frame and the bottom half is showing an old frame. (This is called “tearing”.)
So normally V-Sync is good because it prevents tearing. But the problem with V-Sync is that a frame can only be shown when the monitor refreshes. The video card has to wait until this happens, and this slows our framerate down.
In particular, most users have their monitors set to 60 hz. If X-Plane can only produce frames at 50 hz, the video card will have to further slow the framerate down to30 hz (one x-plane frame for every two monitor refreshes). If X-Plane falls below 30 hz, we end up with 20 hz (one X-Plane frame for three monitor refreshes), and if X-Plane goes below 20 hz, we would clamp at 15.
So when monitor refresh is on, there can be large framerate hits for small losses of performance in the sim, and a real risk of getting locked around 20 fps.
(The minimum framerate in X-Plane is intentionally set to 19 so that we won’t fog up if the video card clamps us at 20 fps.)
So when beta 11 comes out, you may get some framerate back if you haven’t already hacked your graphics card’s control panel settings. If you still want v-sync, you can always set it this way in the control panel. But most users I’ve talked to are happy to have it off.
In an only vaguely related note, one of the reasons to have high frame rate is to have a smooth flight model. But Austin has now put a new setting in the operations-and-warnings dialog box: you can pick how many times per graphics frame the physics run. The normal ratio is 1:1, but for fighter and acrobatic pilots, you might find that you can get a nice feel at lower fps (20-30) by setting a higher ratio.
Random thoughts:
Update your drivers! Version 9 uses driver features that version 8 does not. Just because version 864 works doens’t mean that your drivers are up to date and bug free! First thing to try when weird things happen in version 9 is a driver update.
If you have 60 fps and a rendering setting cuts it to 30, you probably have vsync on – that is, your graphics card can only run at an even divisor of your refresh rate. The next hit will be 20, then fog. Change your monitor refresh rate to 75 or 80 hz. If the framerates all change (to 80, 40, 20, etc.) it must be v-sync. Turn v-sync off for better framerates under heavy load. Nvidia users, you need to turn v-sync to off, not application controlled.
X-Plane 900b7should be able to put sustained load on three cores – if you’re recording a QuickTime movie, one core draws the world, one compresses QuickTime frames, and one rebuilds 3-d as you fly. So…I guess we’ve already hit a point where a quad-core machine has some benefit over a dual-core machine. (I think we’ll start to see more central features use more cores during the v9 run.)
The new forests rebuild 3-d very frequently – dual core users who run on “tree hugger” should see utilization of 75% or higher on both cores, depending on video card power. (If your CPU usage isn’t 100% then probably your video card is holding you back – turn down FSAA.)
I think we’re reaching the point where I can start looking at version 9 performance in detail. Over the past few betas we’ve fixed some very general and global performance problems and also some crash bugs. Now we’re reaching the point where some users have fast performance and others don’t, and it doesn’t quite match up to the hardware the way we’d expect.
The biggest single complaint I’m hearing is “pixel shaders slow may machine down”. This is expected, but the level of slow-down we’re hearing about (2x, 3x, 4x fps loss) is a lot higher than we’d expect.
Here’s a very basic test case you can try:
- Start with shaders off, no FSAA, 4x anisotropic filtering, and a texture res that doesn’t push your card too far (“very high” for 256 MB cards). Tune objects to get between 30-50 fps all the time. Keep forests to “none” for now – they use the second core, so better not to bring in another variable for testing. Measure fps while on the ground somewhere, so that the camera isn’t moving around. Pick a view that has no panel at all, like shift-w (forward-no-HUD view). Use the default 747.
- Now turn shaders on. Turn water reflection to none, turn volumetric fog off. What’s the new fps (keeping the same forward-no-HUd stationary 747)?
- Now turn volumetric fog on (water is still “none”). What are the fps now?
- Finally turn water reflections to “default”. What are these fps?
I’d like to know these four fps numbers because they show the incremental degradation to performance as settings are slowly brought in.
Now if these numbers come out okay, but some real configuration that you tried was terrible, tell me, what’s the delta? (For example, did you have FSAA cranked up when you were testing? What was the texture res?) That will tell me the delta between controlled situations and real-world usage cases.
It will probably take me up to a week to get back to performance reports. Best venue right now is probably: send me an email with attachments for your log.txt file. Please don’t post performance to the blog comments section, I think the thread will get unreadable.
It’s too soon to evaluate whether you need a new video card. I’m hearing reports of GeForce 8800 users with low performance – and yet I have good performance on an X1600. This implies that there’s a driver call we make that isn’t fast in some cases that we need to replace with something better. Let’s get that worked out before you go out shopping!
I woke up this morning to one of those funny coincidences that defines the experience of working on X-Plane: two users had emailed me. One was asking whether we’d be extending water reflection technology to airplane fuselages (like some other programs have) and the other made the case that such an extension was not necessary. The two emails arrived in sequence! (Perhaps there was a forum debate on the subject somewhere.)
First, I can tell you that if we ever have reflective airplanes, it won’t be that soon. I have a number of features for version 9 that are in progress and need to be finished off before I can start anything new.
Reflective airplanes are on my “investigation list”…that is, a feature where we want to do the initial research to see if it could be implemented in a way that makes sense (for X-Plane this means: would it look good and not kill fps too badly for at least some segment of our users).
I believe the X-Plane 9 version run will start to contain more features for serious 3-d modeling of airplanes. Version 9 already features 3-d lighting in the 3-d cockpit, key frames for animation, and a ton of new datarefs to drive that animation. We’re going in the direction of being able to model the plane in absurd detail.
We’re also looking at the lighting model in X-Plane. We’ve only started this work for version 9, but consider pixel-shader-based water. Even in the “no reflections” case, the pixel-shader based water is a reflection of the real sky (as rendered) with a procedural texture to create waves. When you compare this to the version 8 water, you can see how having really close alignment of the coloring scheme for all parts of the sim creates more of a sense of realism.
So reflective airplanes are at least on my list of things to try. I have seen users do wonderful amazing “metal” textures on airplanes, but the one thing that I think holds them back is that all metal airplanes have some kind of tinting assumption to them based on the reflection used…typically these are “blue-based” (meaning they look right on a sunny day) or “gray based” (meaning they look right on a cloudy day). But if you put the plane in the other environment the texture looks a lot less convincing. Reflective textures would let authors really use the real sky color on the plane, for consistent lighting (especially when the plane’s orientation changes and the blue side isn’t up anymore).
On the other hand, reflections are expensive. Planes reflect light from all sides, so we would need to take reflections from all angles (the water always reflects up, which is a huge savings). For low-quality settings for water, we drop the terrain, and since the terrain only reflects at the water’s edge, this is a pretty tolerable omission. An airplane reflection with “sky on the bottom” would look absurd. (Similarly, the water tends to only reflect things that aren’t on camera, so the total rendering load of water + the world tends to be static. The plane would pick up a lot of 3-d objects even in orientations where they don’t do much good, so plane reflections would become expensive.) And the plane reflection isn’t usable for any other plane…do we build them for all planes or just the user’s plane?
Certainly right now it’s still too soon to tell. Not only have I not done the research into this feature, but we still don’t have comprehensive performance data on the water across lots of hardware. A number of users are reporting huge framerate loss on the lowest water settings. This implies that our “render-to-texture” code is slow on some hardware but not others. (The fps loss on my laptop with the lowest water setting is less than 4%.) Render-to-texture is new to v9 and used heavily, so we need to understand how it scales for all users before we go further.
Finally, there is a whole area of 3-d techniques that X-Plane does not yet use that could make sense for airplane modeling: artist controlled fake lighting.
For example, imagine if the airplane contained a single “reflection” texture – this texture would contain a fake ground texture and alpha transparency where the sky color goes. X-Plane could then fill in the sky color (where there is transparency) only when the weather conditions change, and then apply the texture keeping the plane’s orientation in mind. Such a proposal would give the plausibility of reflections (correct coloring on all parts of the plane across lighting, orientation and weather conditions) for a fraction of the cost of “real” reflections. I’m not saying this is the best idea, just that there’s a lot of intermediate ground between “full reflections” and “make a static texture”.
