Saturday, May 31, 2008

Probability and Certainty

I've been reading Fooled by Randomness (highly recommended - it will either change your life or you won't finish it because Taleb's writing style annoys you) - and it has me thinking about the nature of certainty in software development.  Consider two approaches to uncertainty and how they are completely at odds with each other.

No Weird Behavior

The "No Weird Behavior" approach goes like this: you never want a harmless behavior inside your code that you don't understand.  The reason is that if you don't understand the behavior, you don't really know that it's harmless!

In fact this isn't just a theoretical problem - I only truly started to believe in "No Weird Behavior" after fixing several very hard to find, hard to reproduce crash bugs, only to discover (once the analysis was done) that the broken code also produced a very frequent, less harmful behavior.  Had I taken the time to investigate the "weird behavior" first (despite it not being of high importance) it would have led me to a ticking time bomb.

No Weird Behavior implies that a bug isn't fixed until you know why it's fixed, and that randomly changing code until the behavior goes away is absolutely unacceptable.  This shouldn't be surprising; if a bridge was swaying would you accept an engineer who fixed it by randomly tightening and loosening screws until it stopped?

Wait And See

I get a lot of email with bug reports, questions, and reports of strange sim behavior.  These emails have some unfortunate statistical properties:
  • A good number of them are resolved by the user who emailed within a day or two.
  • A certain percentage are simply never resolved.  (Often I email a question that would point to a clear diagnosis and the user never writes back.  I can only speculate that the user answered the question, found the problem, fixed it, and promptly forgot about our emails.)
  • A certain percentage are solved by the user, who tells me what the problem was, and it was something I would never, ever be able to help them with.  (Things like "I have this program called WickedMP3Player - it turns out if I set its visualizer setting to 'Rainbow' then X-Plane stops crashing" when I've never even heard of the WickedMP3Player program to begin with.)
  • There is a high correlation between bug reports reported by a very small number of users and a resolution from the above categories, or a resolution by the user changing his or her local configuration.
So playing the odds, the right thing to do when presented by a third party with weird behavior is to wait and see who else reports it; the frequency of reports gives us information about the likely resolution.

(And for those of you who think our tech support are being lame when they ask if you've updated your drivers, they are playing the odds to the hilt - they ask you about drivers first because updating drivers fixes an absurdly huge number of the tech support calls we get.)

Getting It Wrong

So we have motivation to investigate everything (no weird behavior), motivation to ignore everything (wait and see) and a rule of thumb that the frequency of reports can help us pick which strategy is best.  Of course, sometimes the rule of thumb is completely wrong.

One user reported a crash using the current web updater (version 2.04) - I had not seen this crash anywhere and it was inside the operating system UI code, so I assumed it was a local problem, e.g. some kind of extension or add-on that caused the OS problems.

The problem, it turns out, is simply low frequency - as the incorrect code made it into 902b1, I got a few reports from more users and realized that this wasn't a local problem, it was weird behavior!  (The bug will be fixed in the 205 installer and 902b2, both of which will be out in June.)

Consider this: if you make a measurement of a known quantity with a dubious measuring device, you learn more about the measuring device than the object you are measuring.  (If you have a ruler and you don't know the units, you could determine them by measuring yourself.)

In a number of cases, we have seen serious "should-happen-all-the-time" crash bugs that get reported by very few users.  (Once we know the actual root cause of the bug, we can deduce logically whether the bug should happen to all users with the configuration or be intermittent.) We can then look back at the actual number of reports to make a judgement call on how much testing is really happening.

For example, we can make some guesses about how quickly a new Macintosh have saturated the X-Plane user base when there is a hardware-specific serious bug in just that machine.

We had this with the iMacs (where the runway lights were broken) and we could watch the machines sell - slowly at first, but then quite quickly.  (BTW, I think that 10.5.3 fixes this and anti-aliasing problems.)  We can even see who runs with anti-aliasing when there is a setting-specific bug (a lot of you do)!

In the end, I think the right approach to balancing "no weird behavior" and "wait and see" is to remember a truth about uncertainty that is very hard for humans to grasp:

The most likely outcome of an uncertain situation is not guaranteed to happen - in fact a lot of the time it won't.*

So we can play the rule of thumb and wait and see, but we always have to keep one eye toward the improbable, which is still possible!

* Blatant blog cross-promotion...the point of my big rant here was essentially the's easy to expect the most likely outcome, but the unlikely outcome will happen some of the time. 

Wednesday, May 28, 2008

More Scenery Features

Since X-Plane 9 went final I've been going in about 5 different directions with the code, and as a result my posts have diverged pretty far from my charter within the company, namely scenery. Here's some of what I'm looking at on the scenery front:

Texture Paging

Texture paging is a new feature where X-Plane can change the resolution of orthophotos while you fly. The big limitation of orthophoto-scenery right now is that X-Plane loads every orthophoto at the maximum user-defined resolution when the underlying DSF/ENV is loaded. This can cause X-Plane to run out of virtual address space and crash. With texture paging, only the nearby textures are loaded at high resolution.

Far away textures may be low res, but you'll never notice because they are far away (and thus you're seeing them over only a few pixels on screen anyway.

The cost of this technique is that textures are being loaded over and over and over. But this cost is made less painful by two features:
  • DDS texture loads are very fast and cheap, especially at very low resolutions.*
  • With the next major patch, texture loading will be entirely on a second core (if you have one) and can even span multiple cores.
This feature will require modification of scenery packs in that you'll have to add some "paging" information to your .ter files; I will try to get a preliminary spec posted soon. Because you only have to modify text files, it should be possible to create "paging patches" to existing DSF orthophoto sceneries that are fairly small.

I do not know if paging will be available for .pol files. My guess is yes, but I reiterate that using .pol files for large areas of orthophotos is a bad idea! Make a new mesh!

Improved Multi-Core Performance

This is going to be an on-going process through the entire v9 run and beyond, but basically with every change to our rendering engine (and we make some serious modifications with almost every patch to keep it modern and competitive) we try to move toward using the new hardware (meaning machines with 2-4 cores) more efficiently. Some of this will be in 920, although my guess is we'll need at least one more patch after 920 to really see the improvement.


It's going to be a little bit before I can put more time into the various scenery tools. My top priority is to keep up with user feedback and work on MeshTool. Hopefully we'll also declare WED final soon. But for now, since I am working on cockpit and airplane modeling features, my next set of work will probably be for the airplane authors.


I do want to put additional shader work into v9. I realize I am at the edge of provoking a bunch of rants in the comments section, so let me save you some time:

"Hey Ben, stop working on eye candy and create some more tools. I don't want a shader feature that makes my 1.2 ghz G4 with a GeForce 2 MX run any slower. You should finish the tools so they do exactly what I want!"

Okay, I admit, that was totally unfair...there is a lot of truth in the complaints about shaders vs. tools.
  • I really do try to keep an eye on system requirements, particularly once we shipped. I'm going to try to prioritize shader features that improve existing rendering techniques, rather than introduce new rendering techniques, so we don't seriously lower fps during the version run. But also bear in mind that shaders can be turned off, and there are users who have GeForce 9s and such.
  • Tools are very important, hence the effort to get MeshTool out. But tools without engine work aren't very useful either; most of the engine work we do is needed to keep performance up so that new scenery (made with those new tools) don't bury the sim. For example, MeshTool without texture paging would be a could easily make a MeshTool scenery that you can't fly.
So in planning what goes out when, I look for clumps of features and tools that can go together to make some sense: WED to use apt.dat 850, texture paging to go with MeshTool. It wouldn't make sense to defer texture paging to make the next tool while MeshTool is waiting for engine enhancements.

* A DDS already has all of the smaller-version pre-compressed textures in the file. So loading a DDS at low res involves loading a small amount of data from disk to memory and sending it to the graphics card.

By comparison, a PNG file only contains the maximum size, so to load a PNG at low res, we load the largest size, shrink it on the fly, then apply DDS compression.

Friday, May 23, 2008

Drivers and Builds To Try

For those who posted comments, sorry it took so long to moderate them - for some reason my spam filter decided that notifications of comments are, well, spam, so I just found them now. I should have known people would have jumped into a Vista-bashing thread. :-)

There is an X-Plane 9.02 beta 1 posted - like 901 we've been pretty quiet about this, but you can get it by enabling "get betas" and running the X-Plane updater. Please give it a try. Like 901 it is a small change for the purpose of localization, but it actually has an interesting feature pair:
  • True-type fonts and
  • Unicode-aware.
This is part of some rework we did to provide better language support. should be able to run X-Plane no matter what weird characters* are in your folder names, name your airplanes funny things, and see diacritical marks. 902 uses a font that provides all of the Latin and Greek/Cyrillic code pages.

Also I have heard reports of improvements based on drivers:
  • nVidia has 175.16 drivers out and they apparently address "stuttering" issues. The stuttering issue has been on my list to investigate because it happens under Windows but not Linux. If you have stuttering performance on high-end NV hardware, particularly with forests and Windows, please try 175.16 and let me know how it goes.
  • ATI has released Catalyst 8-5. Catalyst 8-3 and 8-4 were causing "incomplete framebuffer" errors for some users, but I was unable to reproduce it (after spending a good day trying to jam Windows XP onto an iMac already crammed with Windows and Linux....yet another episode of a Tale of Three Operating Sytstems). Anyway, at least one user reported the issue as fixed in Cat 8-5, so if you are having problems, please try the new driver set.
As always, bugs in the X-Plane beta should go to our bug report form, on the X-Plane contacts page.

* You might accuse me of being American-centric in decrying diacritical and greek letters weird - but the truth is I am computer-centric...anything that is not in the original ASCII set is weird. :-)

Friday, May 16, 2008

Commodification and Operating Systems

I'll warn you in advance: this is going to start off topic and go way off topic. "Catching up" with the changes to Mac OS, Windows and Linux has me thinking about the nature of technology. I feel a little bit guilty about this post because it's going to turn into a rant about Vista, and ranting about Vista is like shooting fish in a barrel. On the other hand, having used Vista, well, I have a lot of rant to give.

One of the most important things to understand about technology (and computers are no exception) is that changes in the scale of the technology change the very nature of the technology. That is, as you make computers faster and cheaper, at some point the sum of all of those small improvements changes the fundamental nature of the beast. We've seen this as the computer transformed from main frame to desktop (which is really just a change in cost and size), finding an entirely new audience, and now again as the computer changes from what we know of now as a computer to cell phones, MP3 players, and other small, mobile devices.

"Commodification" is what happens when, as things get better, cheaper, faster, etc., consumers stop caring about the marginal improvement. Back in the days of Windows 95 and 386's, there were ways you could improve the operating system and hardware in substantial ways; a doubling in processor speed and a rewrite of the operating system got you protected memory, which meant less data loss.

A few years ago we reached the point where desktop hardware became commodified. For the average user, 1.8 vs 2.2 ghz makes no difference at all. It's a question of how quickly your computer can wait for keystrokes and data from the internet. (Answer: even a lowly Celleron is light-years faster than the I/O devices it typically has to talk to. Even if you're the last kid in your class at Harvard, you're going to be bored discussing politics with a bunch of four-year-olds.) At that point things became very difficult for major vendors like IBM (sold out), HP and Compaq (merged), Gateway (bought out of it's misery), etc. The price of a desktop plummeted from over $1000 to less than $400.

I believe we've reached the point where operating systems have become a commodity as well;
  • Every major operating system has all of the features of a "real" operating system - that is, protected memory, virtual memory, plug & play driver support, etc.
  • The performance for normal applications is just about the same; there are some specific variations that matter in the server market, but for all practical purposes the operating system is not in the way, and the machine is much faster than users need anyway.
  • Every major operating system has a similarly designed GUI experience that, once you get used to the quirks of where the close box is, is just about the same, more or less. (Mac users - keep your pants on. :-)
And this is why life is not so good for Microsoft. In a non-commodified market, you can charge a premium for incremental improvements over the competition. That's a game Microsoft can play - they have a lot of capital to invest in changing their operating system, as long as they are rewarded with a lot of cash for doing it. (And normally they are - about six billion dollars for a major OS revision, I'm told.)

The problem is that operating systems are now a commodity. Simply put, users don't need a new operating system. There are no big ticket features missing from OS X 10.4, Windows XP, or Linux 2.6. This makes Microsoft's business model fundamentally vulnerable to Linux for the first time. If the name of the game is:
  • Keep costs down, as low as possible.
  • Incrementally improve quality very slowly without ever causing the pain of a major OS upgrade.
That's a game Linux, with their army of distributed bug fixers and free source code, is going to win.

When I looked at Windows XP and Ubuntu 6.06 I was afraid that Linux wouldn't make traction into the desktop market. I blamed the adoption of X11, the KDE/GNOME schism, and the Linux communities' being made up of Shell nerds for the tolerable desktop experience.

But look where we are now: Vista is a vehicle for bloat. Combine "we make money by shipping major features" and "there are no more major features to ship" and you get attempt to change a lot of things when you should have left things alone.*

By comparison, Ubuntu pretty much just works - you put the live CD on your machine, it asks you some questions and knows about more hardware, has less bugs, more drivers, and a better user experience. In a commodified operating-system space, the only thing to do is try to avoid a bad user experience - if you can't offer a really juicy carrot to users, try to avoid hitting them with a stick.

And it is in this environment that the Mac is actually gaining market share. Apple's business model has always been at odds with the industry. Complete vertical integration meant higher costs, lack of market share, and out-of-date technology - back when having more for less meant something, that was a real weakness, and explains why the Mac never dominated in market share.

But what a difference a decade makes! Hardware is now commodified (and Apple is integrated at the system-building level, leveraging cheap third party parts like they always should have). Operating systems are commodified. But on the one frontier left, quality of user experience, Apple's vertical integration gives it an immense advantage.

The question is: why does an operating system "just work"?
  • Vista: it doesn't. There are too many systems and not enough testers and engineers trying to solve the problem.
  • Linux: massive distributed engineering. For any given hardware system, eventually a Linux nerd will integrate it. Anyone can solve the problem of poor user experience.
  • Apple: they have it easy. With only half a dozen machines in production (and maybe another two dozen legacy configurations to support) they have a much smaller configuration space to worry about than anyone else.
I don't kow what Microsoft's future is, but it can't be very good. At some point they are going to have to transition from a "major revision for cash" to an "incremental tuning" approach to operating systems. As long as they have market share, they still get the "Windows tax" - that is, their OEM pricing from major vendors on every new computer that is built. It's going to be harder and harder to convince the entire world to make a major jump (see how well XP to Vista went). In this situation, they'd be better off with a more solid operating system. It's unfortunate that they're going to have to try to sustain market share with Vista.

Their best-case scenario is that they eventually get Vista back to an XP-quality experience, in which case all they've done is spend a huge amount of R&D money and pissed off a lot of customers to maintain the status quo.

* I have mixed opinions on Vista's video-driver-model change. But that's a different post.

Wednesday, May 14, 2008

How Much Can I Chew?

Austin posted another State-of-the-Union yesterday, and he was good about only mentioning things that are fairly close to completion (e.g. the panel texture optimizations) as well as things that are general (e.g. threading optimizations). After a long beta, when 9.0 went final I sort of went crazy and started a whole series of new projects at the same time; here are some things that I have in progress:
  • Major rewrites to the texture management code for both better orthophoto handling and better threaded performance.
  • Moving the sim to unicode and true-type fonts, with a new system for translating the application's strings. (I'll post more on this soon - thanks to those who have already volunteered to work on translations!)
  • Working on new shader technology to take better advantage of DirectX-10-class hardware.
  • A big pile of airplane features to complement what was added to 9.0.
That's a bit much for the next patch, so it's likely that only some of these things will actually make it into the next patch, and I'm not sure what you'll see. A lot of sim work goes in as a series of small independent pieces; only the last parts of a feature are user-visible.

For example, the first part of texture work was simply rearranging how the code was structured to make new things possible. Change to the user experience: none. The second part changed threaded handling of textures, which at least shows up as performance improvements. But both the stage for new features later.

So even if a lot of the above doesn't make it into the next major patch, a lot of ground work is going in, setting us up for features later.

Monday, May 12, 2008

Multi-Core Texture Loading

In a previous post I discussed the basic ideas behind using multiple threads in an application to get better performance out of a multi-core machine.

Now before I begin, I need to disclaim some things, because I get very nervous posting anything involving hardware. This blog is me running my mouth, not buying advice; if you are the kind of person who would be grumpy if you bought a $3000 PC and found that it wouldn't let you do X with X-Plane (where X includes run at a certain rendering setting, framerate, or make your laundry) my advice is very simple: don't spend $3000. So...
  • I do not advocate buying the biggest fastest system you can get; you pay a huge premium to be at the top of the hardware curve, particular for game-oriented technologies like fast-clock CPUs and high-end GPUs.
  • I do not advocate buying the Mac Pro with your own money; it's too expensive. I have one because my work pays for it.
  • 8 cores are not necessary to enjoy X-Plane. See above about paying a lot of money for that last bit of performance. that I have enough crud posted to be able to say "I told you so"...

My goal in reworking the threading system inside X-Plane for 920 (or whatever the next major patch is called) is, among other things, to get X-Plane's work to span across as many cores as you have, rather than across as many tasks are going on. (See my previous post for more on this.)

Today I got just one bit of the code doing this: the texture loader. The texture loader' job is to load textures from the hard drive to the video card (using the CPU, via main memory) while you fly. In X-Plane 901 it will use up to one core to do this, that core also being shared with building forests and airports.

With the new code, it will load as many textures at a time as it can, using as many cores as you have. I tested this on RealScenery's Seatle-Tacoma custom scenery package - the package is an ENV with about 1.5 GB of custom PNGs, covering about half of the ENV tile with non-repeating orthophotos.

On my Mac Pro, 901 will switch to KSEA from LOWI in about one minute - the vast majority of the time is spent loading about 500 PNG files. The CPU monitor shows one core maxed out. With the new code, the load takes fourteen seconds, with all eight cores maxed out.

(This also means that the time from when the scenery shifts to when the new scenery has its textures loaded would be about fourteen seconds, rather than a minute, which means very fast flight is unlikely to get to the new area before the textures are loaded and see a big sea of gray.)

Things to note:
  • Even if we don't expect everyone to have eight cores, knowing that the code can run on a lot of cores proves the design - the more the code can "spread out" over a lot of cores, the more likely the sim will use all hardware available.
  • Even if you only have two or four cores, there's a win here.
  • Texture load time is only a factor for certain types of scenery; we'll need to keep doing this type of work in a number of cases.
This change is the first case where X-Plane will actually spread out to eight cores for a noticeable performance gain. Of course the long-term trend will be more efficient use of multi-core hardware in more cases.

Saturday, May 03, 2008

Why Is the v9 Global Scenery Split In Half?

The global scenery comes in two packages in version 9: -global terrain- and -global overlays-. The global terrain package contains the base meshes (with beaches); the global overlays contain roads, forests and objects.

Why is this scenery split in half? The answer is unfortunately not "so you can replace the base mesh but keep the overlay 3-d stuff." That would have been clever, but I must admit I didn't think of it at the time; MeshTool didn't exist and people just weren't making base meshes.

My actual goal was to make it cheaper to replace a significant number of overlays. I don't know if we'll ever do this, but one of the obstacles to patching global scenery is the file size; we can only hope to replace a fraction of the files during a version run before the web update size gets too large. But most of that size is in the base mesh. With the base mesh and overlay split, we could potentially replace more overlays.

(Note: we did not actually issue any DSF replacements during the v8 run, and I don't know if we will or will not during the v9 run. The only thing I am sure of is that if we provide replacement v9 DSF tiles, they'll be a free download, like all v9 patches...if you buy v9, you get everything.)

The fundamental problem with replacing the base mesh but not the overlays is that the scenery system provides no good way to do this. The Global Scenery folder is always scanned after the Custom Scenery folder* so you'd have to install custom scenery into the global scenery folder with the right file name to get access to the overlay content.

I'm not sure what to do about this yet; the trend in scenery development is for authors to want more control to replace individual parts of the system; the overlay system provided part of that.

* Users with v9 beta DVDs will have the two global scenery folders in the Custom Scenery folder. But -- the sim detects this and simply treats them as if they were in the Global Scenery folder, ignoring alphabetic ordering.

901 - Stealth Release

901 is out now...basically it's just 900 + French and German localizations. Also a few minor tweaks:
  • We added a few datarefs and commands by request - check DataRefs.txt.
  • The plugin font size is back to 6 pixels, like v8. It looks a bit ugly now, but we'll put a new font in place soon. The important thing is: 6-pixel fonts for v8 and v9, so one plugin can operate everywhere.
  • A few plugin bug fixes - see the SDK known bugs page.
  • Linux won't complain if your disk is mounted as iso9660. Since the v9 final disks are all mastered as ISOs and not UDF, asking you to mount them as UDF is asking the impossible.
  • If you have a demo install, it will add the word "Demo" to its name in the startup screen. This is to help users with more than one install identify which one they are running.
If this all seems minor and tweaky, well, it is. The main point of 901 was to get the localizations in. In the future we'll have a feature patch that brings new airplane features in and other cool stuff.

901 is freely available using the X-Plane web updater...the download size is only about 14 MB.

Finally, I've posted all new DVD installer/updater/web demo installer apps - version 2.04 localizes to French and German too.

A Tale of Three Operating Systems, Part II (Why You Need Bootcamp)

A while ago I put three operating systems on my laptop. With the Mac Pro I've done the same thing - it's a huge win to be able to cover such a wide swath of OS/GPU/CPU combinations with fewer machines. Last time it was OS X 10.4, Windows XP SP2, and Ubuntu 6.06. This time I repeated the process with OS X 10.5.2, Windows Vista RTM, and Ubuntu 8.04. Random observations:
  • Linux really just keeps getting stronger. I've always been a bit skeptical about Linux as a desktop environment, particularly as a Windows/Mac developer (that is to say, I'm spoiled by free high quality IDEs ad debuggers that require no setup to use the platform SDK, comprehensive platform documentation in one location, etc.). But Linux installation is becoming more plug & play and trouble-free each time I make myself a live CD.
  • Windows Vista is a train wreck. I feel a little bit lame blogging this, as taking pot-shots at Vista is sort of like shooting fish in a barrel. But the contrast between Ubuntu, which has become easier to use over a year and a half, and Windows, which has not, is stark.
  • There are some positive things to say about Vista. The partition-aware installer is a real convenience for multi-booters. And once you figure out where everything has been moved to and go back to "classic" views, the OS is tolerable. But you'll still find plenty of things that will make you want to tear your hair out. My recommendation: stick with XP. (Duh.)
Now on to the performance numbers. These numbers are the Xp900 time demo fps tests 1, 2 and 3. Each set of 3 numbers is from the three phases.
      1           2               3
MAC 49/ 60/ 62 38/ 43/ 44 21/ 20/ 21
WIN 121/128/133 114/115/119 77/ 75/ 82
LIN 143/144/157 130/123/132 92/104/113
That's not a typo. Linux is beating out Vista, but both are absolutely killing OS X. What's going on here? I don't know. But there appears to be something that isn't well optimized in the GeForce 8 drivers on OS X.

I suspect Apple will close this gap eventually; don't bother asking me for status information on this because if they ever tell me what's going on, I'll be bound by NDA not to tell you.

For now my recommendation is: consider dual-booting into Linux - it's pretty easy to install Ubuntu and you'll get great X-Plane performance. With good drivers, the Mac Pro and 8800 are just monstrous.