Saturday, May 15, 2010

Where Has All My VRAM Gone?

In the past I have suggested that there is a limit to the value of additional VRAM when buying a graphics card. When I posted this, Nicholas from the org emailed me to point out that with third party add-ons VRAM was a lot more important.

At the time I wasn't really convinced, but I've finally come around; it takes a while for the trend to get back to me. (I just don't have time to look at everyone's add-ons the way I used to 4 or 5 years ago.) It seems clear that airplane authors (and to some extent scenery authors) are using VRAM pretty aggressively. If you want to use third party add-ons and you care about texture res and texture sharpness, spring for some VRAM. It doesn't cost as much as it used to, and authors are starting to use it.

What Lives in VRAM

Bear in mind that in any discussion of how your video card operates, anything I post is informed speculation. The driver provides an abstraction (OpenGL) of what the hardware does, and a lot of the bookkeeping isn't visible to X-Plane at all. So what I am describing is typical of past video drivers that we have had insight to in the past, but it's not universal, and it's not at all guaranteed. (X-plane can't demand any of this behavior of the video driver.)

In order of how "stuck" in VRAM things are we have:
  • Video memory used for on-screen rendering. Depending on rendering settings you can lose anywhere from 12 to 24 MB of VRAM per million pixels on screen. So if you're running at 1920 x 1200, you might be using 50 MB of VRAM just for the screen. If you use FSAA, you're going to chew up VRAM even more aggressively. (Costs vary depending on the scheme; you might lose 4-16 MB x the FSAA level per million pixels on screen, depending on your GPU and driver.)
  • Off-screen rendering for things like the water reflections, the panels, the cloud shadows, the airplane shadow, etc. These don't have to be in VRAM all of the time, but they have to be in VRAM almost all of the time. Because they are created by the GPU, the driver tries hard not to move these out of VRAM. You might lose 6 to 16 MB of VRAM for these, depending on the airplane you use and settings. (Given 4 1024x1024 panel regions, the panel will chew up 32 MB!)
  • Textures end up in VRAM, but only when they are used. The key here is "working set". Only texures that are drawn need to be in VRAM, so over time stuff that isn't on screen will be removed from VRAM. This is why when you see "600 MB of texture memory" in the rendering settings, there is no need to panic. The working set is usually much smaller.
  • OBJ geometry actually lives in VRAM too, sometimes. Again this is a working set issue; objects that aren't drawn don't get cached there.
  • Textures from airplanes and scenery packages that are not loaded don't ever end up VRAM or even system memory; we only load what we need. Paged orthophotos have their resolution reduced while you fly, which makes their VRAM footprint quite small, even when drawn.
There are a few things authors and users are doing now that use up a lot of VRAM.
  • Monitors have gotten bigger; the VRAM used for the screen itself can never be swapped out, so the advent of the 1920x1200 LCD has taken its toll.

  • Since the panel texture is drawn off-screen, the panel texture is in an expensive category of VRAM use. Authors can limit the cost of this by using a single 1024x1024 panel region texture, if possible.

  • There is a hidden cost here: we pack together instrument textures into "atlases" to help with performance. The problem is that we pack for fit. Some of your instruments may be hidden but loaded into VRAM anyway because they sit in the same atlas texture as other instruments that are drawn. Thus you may be paying for the VRAM used by your entire panel even if a lot of it is hidden.

  • Because a lot of this VRAM is going to airplanes, reducing texture resolution doesn't have as much impact as it used to; X-Plane tries to keep the user's plane's resolution as high as possible since it is viewed up close. The panel cannot have its resolution reduced at all.

But It Works Sometimes

I think what drives users crazy about VRAM exhaustion is that X-Plane will sometimes run smoothly, and then fail later. And sometimes really strange things, like moving X-Plane to the background, then the foreground, or changing liveries or rendering settings in a trivial way, will change performance.

I have discussed this a bit in past posts. But the key here is "working set":
  • In any one frame, we can access everything that is permanently in VRAM, plus as much data as we can put through the PCIe bus from the CPU to the GPU.
  • We only need to access data in the working set (what is on screen).
  • Some of VRAM is permanently used (e.g. memory for the screen itself).
  • We lose PCIe bandwidth to both drawing from main memory (terrain is in your system memory and must go over the PCIe bus per frame) and from bandwidth spent juggling textures.
So the actual maximum throughput will have a lot to do with whether the video driver makes good decision about what lives in VRAM and what does not.

But how does the video card know what should be in VRAM? The answer is that it has to guess. It looks at frames going by and tries to use heuristics (that is computer-science geek speak for "carefully formulated wild guesses") to decide what goes in VRAM and what does not. When the heuristics happen to make good decision, your video card kicks ass. And when it does not, your framerate tanks.

The only way to guarantee good framerate is to use so little VRAM that everything that needs to be in VRAM can be in VRAM, without depending on the video driver to make lucky guesses with its juggling.

And this helps us understand why strange things like livery reloads and backgrounding the sim can affect framerate (for better or worse). These operations seriously reshuffle VRAM - either by deleting textures and loading new ones, or by forcing everything out of VRAM so the video driver must try to repack video RAM all over again.

Unfortunately as a user this means that there's not much you can do about this as a user. The main things would be: reduce screen size or FSAA or texture resolution, use fewer add-ons, or get more VRAM. Those peak bursts of framerate you see, they're not going to be sustainable .


Anonymous said...

Just want to thank you Ben for these extremely informative posts on performance, the obj format etc. They are of great value to us planemakers.

I'd also like to second Nicolas' remark on VRAM. For my latest creation, I'm using 14 2048x2048 texture maps. Might sound silly but that's what you need if you want to create a really crisp exterior and 3D cockpit. In general, I like to think that I optimize my work pretty well in terms of ATTR-use and the kind of things you've mentioned in earlier posts, but when it comes to texture clarity, there's not much you can do other than hope that the users have plenty of VRAM.


Jack Skieczius said...

What is your opinion on DDS files and the way they help loading times and how this effects VRAM?

On my plane i am using 8 2048 textures(tho i can go lower on a few parts) and i find DDS to load faster, but is this what I as a new developer should be using for all planes in the future or will PNG files still be the dominate texture file we will see.

Benjamin Supnik said...

DDS is good.

If the user has tex compression on, DDS and PNG VRAM use are the same, DDS just looks better.