Monday, May 21, 2018

Never Map Again: Persistent Memory, System Memory, and Nothing In Between

I have, over the years, written more posts on VBOs and vertex performance in OpenGL than I'd like to admit. At this point, I can't even find them all. Vertex performance is often critical in X-Plane because we draw a lot of stuff in the world; at altitude you can see a lot of little things, and it's useful to be able to just blast all of the geometry through, if we can find a high performance vertex path.

It's 2018 and we've been rebuilding our internal engine around abstractions that can work on a modern GL driver, Vulkan and Metal. When it comes to streaming geometry, here's what I have found.

Be Persistent!

First, use persistent memory if you have it. On modern GL 4.x drivers on Windows/Linux, the driver can permanently map buffers for streaming via GL_ARB_buffer_storage. This is plan A! This will be the fastest path you can find because you pay no overhead for streaming geometry - you just write the data. (It's also multi-core friendly because you can grab a region of mapped memory without having to talk to the driver at all, avoiding multi-context hell.)

That persistent memory is a win is unsurprising - you can't get any faster than not doing any work at all, and persistent memory simply removes the driver from the equation by giving you a direct memory-centric way to talk to the GPU.

Don't Be Uncool

Second, if you don't have persistent memory (e.g. you are on OS X), use system memory via client arrays, rather than trying to jam your data into a VBO with glMapBuffer or glBufferSubData.

This second result surprised me, but in every test I've run, client arrays in system memory have out-performed VBOs for small-to-medium sized batches. We were already using system memory for small-batch vertex drawing, but it's even faster for larger buffers.

Now before you go and delete all your VBO code, a few caveats:

  • We are mostly testing small-batch draw performance - this is UI, some effects code, but not million-VBO terrain chunks.
  • The largest streaming data I have tried is a 128K index buffer. That's not tiny - that's perhaps 32 VM pages, but it's not a 2 MB static mesh.
  • It wouldn't shock me if index buffers are more friendly to system memory streaming than vertex buffers - the 128K index buffer indexes a static VBO.

Why Would Client Arrays Be Fast?

I'd speculate, they're easier to optimize.

Unlike VBOs, in the case of client arrays, the driver knows everything about the data transfer at one time. Everything up until an actual draw call is just stashing pointers for later use - the app is required to make sure the pointers remain valid until the draw call happens.

When the draw call happens, the driver knows:

  • How big the data is.
  • What format the data is in.
  • Which part of the data is actually consumed by the shader.
  • Where the data is located (system memory, duh).
  • That this is a streaming case - since the API provides no mechanism for efficient reuse, the driver might as well assume no reuse.
There's not really any validation to be done - if your client pointers point to junk memory, the driver can just segfault.

Because the driver knows how big the draw call is at the time it manages the vertex data, it can select the optimal vertex transfer mode for the particular hardware and draw call size. Large draws can be scheduled via a DMA (worth it if enough data is being transferred), medium draws can be sourced right from AGP memory, and tiny draws could even be stored directly in the command buffer.

You Are Out of Order

There's one last thing we know for client arrays that we don't know for map/unmap, and I think this might be the most important one of all: in the case of client arrays, vertex transfer is strictly FIFO - within a single context (and client arrays data is not shared) submission order from the client is draw/retirement order.

That means the driver can use a simple ring buffer to allocate memory for these draw calls. That's really cheap unless the total size of the ring buffer has to grow.

By comparison, the driver can assume nothing about orphaning and renaming of VBOs. Rename/map/unmap/draw sequences show up as ad hoc calls to the driver, so the driver has to allocate new backing storage for VBOs out of a free store/heap. Even if the driver has a magazine front-end, the cost of heap allocations in the driver is going to be more expensive than bumping ring buffer pointers.

What Can We Do With This Knowledge?

Once we recognize that we're going to draw only with client arrays and persistent memory (and not with non-persistent mapped and unmapped VBOs), we can recognize a simplifying assumption: our unmap/flushing overhead is zero in every case, and we can simplify client code around this.

In a previous post, I suggested two ways around the cost of ensuring that your data is GPU-visible: persistent memory and deferring all command encoding until later.

If we're not going to have to unmap, we can just go with option 1 all of the time. If we don't have persistent coherent memory, we treat system memory as our persistent coherent memory and draw with client arrays. This means we can drop the cost of buffering up and replaying our command encoding and just render directly.

Tuesday, March 27, 2018

There Must Be Fifty Ways to Fail Your Stencil

When we first put deferred rendering into X-Plane 10.0, complete with lots of spot lights in-scene, I coded up stencil volumes for the lights in an attempt to save some shading power. The basic algorithm is:

  • Do a stencil pre-pass on all light volumes where:
    • The back of the volume failing increments. This happens when geometry is in front of the back of the light volume - this geometry might be lit!
    • The front of the volume failing decrements. This happens when geometry is in front of the front light volume and thus occludes (in screen space) anything that could have been lit.
  • Do a second pass with stencil testing for > 0. Only pixels with a positive count had geometry that were in between the two halves of the bounding volume, and thus light candidates.
This technique eliminates both fragments of occluded lights and fragments where the light shines through the air and hits nothing.

Typically the stencil modes are inc/dec with wrapping so that we aren't dependent on our volume fragments going out in any particular order - it all nets out.

We ended up not shipping this for 10.0 because it turned out the cure was worse than the disease - hitting the light geometry a second time hurt fps more than the fill savings for a product that was already outputting just silly amounts of geometry.

I made a note at the time that we could partition our lights and only stencil the ones in the first 200m from the camera - this would get all the fill heavy lights without drawing a ton of geometry.

I came back to this technique the other day, but something had changed: we'd ended up using a pile of our stencil bits for various random purposes, leaving very little behind for stencil volumes. We were down to 3 bits for our counter, and this was the result.


That big black void in between the lights in the center of the screen is where the number of overlapping non-occluded lights hitting a light-able surface hit exactly the wrap-around point in our stencil buffer - we got eight increments, wrapped to zero and the lights were stencil-tested out. The obvious way to cope with this is to use more than 3 stencil bits. :-)

I looked at whether there was something we could do in a single pass. Our default mode is to light with the back of our light volume, untested; the far clip plane is, well, far away, so we get good screen coverage.

I tried lighting with the front of the light volume, depth tested, so that cases where the light was occluded by intervening geometry would optimize out.  I used GL_ARB_depth_clamp to ensure that the front of my light volume would be drawn even if it hit the front clip plane.

It did not work! The problem is: since our view is a frustum, the side planes of the view volume cross at the camera location; thus if we are inside our light volume, the part behind us will be culled out despite depth clamp. This wasn't a problem for stencil volumes because they do the actual drawing off the back of the volume, and the front is just for optimization.

Monday, January 29, 2018

Flush Less Often

Here's a riddle:
Q: What does a two year old and OpenGL have in common? 
A: You never know when either of them is going to flush.*
In the case of a two-year old, you can wait a few years and he'll find different ways to not listen to you; unfortunately in the case of OpenGL, this is a problem of API design; therefore we have to use a fairly big hammer to fix it.

What IS the Flushing Problem?

Modern GPUs work by writing a command buffer (a list of binary encoded drawing instructions) to memory (using the CPU) and then "sending" that buffer to the GPU for execution, either by changing ownership of the shared memory or by a DMA copy.

Until that buffer of commands goes to the GPU, from the GPU's perspective, you haven't actually asked it to do anything - your command buffer is just sitting there, collecting dust, while the GPU is idle.

In modern APIs like Vulkan, Metal, and DX12, the command buffer is an object you build, and then you explicitly send it to the GPU with an API call.

With OpenGL, the command buffer is implicit - you never see it, it just gets generated as you make API calls. The command buffer is sent to the GPU ("flushed") under a few circumstances:
  1. If you ask GL to do so via glFlush.
  2. If you make a call that does an automatic flush (glFinish, glSwapBuffer, waiting on a sync with the flush bit).
  3. If the command buffer fills up due to you doing a lot of stuff.
This last case is the problematic one because it's completely unpredictable.

Why Do We Care?

Back in the day, we didn't care - you'd write commands and buffers would go out when they were full (ensuring a "goodly amount of work" gets sent to the GPU) and the last command buffer was sent when you swapped your back buffer.

But with modern OpenGL, calling the API is only a small fraction of the work we do; most of the work of drawing involves filling buffers with numbers. This is where your meshes and hopefully constant state are all coming from.

The flushing problem comes to haunt us when we want to draw a large number of small drawing batches. It's easy to end up with code like this:

// write some data to memory
glDrawElements.
// write some data to memory
glDrawElements.

Expanding this out, the code actually looks more like:

// map a buffer
// write to the buffer
// flush and unmap the buffer
glDrawElements.
// map a buffer
// write to the buffer
// flush and unmap the buffer
glDrawElements.

The problem is: even with glMapBufferRange and "unsynchronized" buffers, you still have to issue some kind of flush to your data before each drawing call.

The reason this is necessary is: glDrawElements might cause your command buffer to be sent to the GPU at any time! Therefore you have to have your data buffer completely flushed and ready to go after every drawing call.

How Do We Fix It?

You basically have two choices to make code like the above fast:

  1. If your are on a modern GL, use persistent coherent buffers. They don't need to be flushed - you can write data, call draw, and if the GL happens to send the command buffer down, your data is already visible. This is a great solution for UBOs on Windows.
  2. If you can't get persistent coherent buffers, defer all of your actual state and draw calls until every buffer has been built.

This second technique is a double-edged sword.

  • Win: it works every-where, even on the oldest OpenGL.
  • Win: as long as you're accumulating your state change, you can optimize out stupid stuff - handy when client code tends to produce crap OpenGL call-streams.
  • Lose: it does require you to marshal the entire API, so it's only good for code that sits on a fairly narrow foot-print.
For X-Plane, we actually intentionally choose not to use UBOs when persistent-coherent buffers are not also available. It turns out the cost of flushing per draw call is really bad, and our fallback path (loose uniforms) is actually surprisingly fast, because the driver guys have tuned the bejeezus out of that code path.


* My two-year old has figured out how to flush the toilet and thinks it's fascinating. What he hasn't figured out how to do is listen^H^H^H^H^Hwait until I'm done peeing. (And yes, non-parents, of coarse peeing is a group activity. Duh.)  The monologue went something like:

"Okay Ezra, wait until Daddy's done. No, not yet. It's too soon. Don't flush. Ezra?!  Sigh.  Wait, this is exactly like @#$@#$ glDrawElements!"

Saturday, January 13, 2018

Fixing Camera Shake on Single Precision GPUs

I've tried to write this post twice now, and I keep getting bogged down in the background information. In X-Plane 11.10 we fixed our long-standing problem of camera shake, caused by 32-bit floating point transforms in a very large world.

I did a literature search on this a few months ago and didn't find anything that met our requirements, namely:

  • Support GPUs without 64-bit floating point (e.g. mobile GPUs).
  • Keep our large (100 km x 100 km) mesh chunks.

I didn't find anything that met both of those requirements (the 32-bit-friendly solutions I found required major changes to how the engine deals with mesh chunks), so I want to write up what we did.

Background: Why We Jitter

X-Plane's world is large - scenery tiles are about 100 km x 100 km, so you can be up to 50 km from the origin before we "scroll" (e.g. change the relationship between the Earth and the primary rendering coordinate system so the user's aircraft is closer to the origin).  At these distances, we have about 1 cm of precision in our 32-bit coordinates, so any time we are close enough to the ground that 1 cm is larger than 1 pixel, meshes will "jump" by more than 1 pixel during camera movement due to rounding in the floating point transform stack.

It's not hard to have 1 pixel be larger than 1 cm. If you are looking at the ground on a 1920p monitor, you might have 1920 pixels covering 2 meters, for about 1 mm per pixel.  The ground is going to jitter like hell.

Engines that don't have huge offsets don't have these problems - if we were within 1 km of the origin, we'd have almost 100x more precision and the jitter might not be noticeable. Engines can solve this by having small worlds, or by scrolling the origin a lot more often.

Note that it's not good enough to just keep the main OpenGL origin near the user. If we have a large mesh (e.g. a mesh whose vertices get up into the 50 km magnitude) we're going to jitter, because at the time that we draw them our effective transform matrix is going to need an offset to bring the 50 km offset back to the camera.  (In other words, even if our main transform matrix doesn't have huge offsets that cause us to lose precision, we'll have to do a big translation to draw our big object.)

Fixing Instances With Large Offsets

The first thing we do is make our transform stack double precision on the CPU (but not the GPU). To be clear, we need double precision:
  • In the internal transform matrices we keep on the CPU as we "accumulate" rotates, translates, etc.
  • In the calculations where we modify this matrix (e.g. if we are going to transform, we have to up-res the incoming matrix, do the calculation in double, and save the results in double).
  • We do not have to send the final transforms to the GPU in double - we can truncate the final model-view, etc.
  • We can accept input transforms from client code in single or double precision.
This will fix all jitter caused by objects with small offset meshes that are positioned far from the origin.  Eg. if our code goes: push, translate (large offset), rotate (pose), draw, pop, then this fix alone gets rid of jitter on that model, and it doesn't require any changes to the engine or shader.

We do eat the cost of double precision in our CPU-side transforms - I don't have numbers yet for how much of a penalty on old mobile phones this is, but on desktop this is not a problem. (If you are beating the transform stack so badly that this matters, it's time to use hardware instancing.)

This hasn't fixed most of our jitter - large meshes and hardware instances are still jittering like crazy, but this is a necessary pre-requisite.

Fixing Large Meshes

The trick to fixing jitter on meshes with large vertex coordinates is understanding why we have precision problems.  The fundamental problem is this: transform matrices apply rotations first and translations second. Therefore in any model-view matrix that positions the world, the translations in the matrix have been mutated by the rotation basis vectors. (That's why your camera location is not just items 12,13, and 14 of your MV matrix.)

If the camera's location in the world is a very big number (necessary to get you "near" those huge-coordinate vertices so you can see them) then the precision at which they are transformed by the basis vectors is...not very good.

That's not actually the total problem. (If it was, preparing the camera transform matrix in double on the CPU would have gotten us out of jail.)

The problem is that we are counting on these calculations to cancel each other out:

vertex location * camera rotation + (camera rotation * camera location) = eye space vertex

The camera rotated location was calculated on the CPU ahead of time and baked into the translation component of your MV matrix ,but the vertex location is huge and is rotated by the camera rotation on the GPU in 32-bits.  So we have two huge offsets multiplied by very finicky rotations - we add them together and we are hoping that the result is pixel accurate, so that tiny movements of the camera are smooth.

They are not - it's the rounding error of the cancelation of these calculations that is our jitter.

The solution is to change the order of operations of our transform stack. We need to introduce a second translation step that (unlike a normal 4x4 matrix operation), happens before rotation, in world coordinates and not camera coordinates.  In other words, we want to do this:

(vertex location - offset) * camera rotation + (camera rotation * (camera location - offset)) = ...

Heres' why this can actually work: "offset" is going to be a number that brings our mesh roughly near the camera. Since it doesn't have to bring us to the camera, it can change infrequently and have very few low-place bits to get lost by rounding.  Since our vertex location and offset are not changing, this number is going to be stable across frames.

Our camera location minus this offset can be done on the CPU side in double precision, so the results of this will be both small (in magnitude) and high precision.

So now we have two small locations multiplied by the camera rotation that have to cancel out - this is what we would have had if our engine used only small meshes.

In other words, by applying a rounded, infrequently  changing static offset first, we can reduce the problem to what we would have had in a small-world engine, "just in time".

You might wonder what happens if the mesh vertex is no-where near our offset - my claim that the result will be really small is wrong. But that's okay - since the offset is near the camera, mesh vertices that don't cancel well are far from the camera and too small/far away to jitter. Jitter is a problem for close stuff.

The CPU-side math goes like this: given an affine model-view matrix in the form of R, T (where R is the 3x3 rotation and T is the translation vector), we do this:

// Calculate C, the camera's position, by reverse-
// rotating the translation
C = transpose(R) * T
// Grid-snap the camera position in world coordinates - I used 
// a 4 km grid. smaller grids mean more frequent jumps but 
// better precision.
C_snap = grid_round(C)
// Offset the matrix's translation by this snap (moved back 
// to post-rotation coordinates), to compensate for the pre-offset.
T -= R * C_snap
// Pre-offset is the opposite of the snap.
O = -C_snap

In our shader code, we transform like this:

v_eye = (v_world - O) * modelview_matrix

There's no reason why the sign has to be this way - O could have been C_snap and we could have added in the shader; I found it was easier to debug having the offset be actual locations in the world.

Fixing Hardware Instancing

There's one more case to fix. If your engine has hardware instancing, you may have code that takes the (small) model mesh vertices and applies an instancing transform first, then the main transform. In this case, the large vertex is the result of the instancing matrix, not the mesh itself.

This case is easily solved - we simply subtract our camera offset from the translation part of the hardware instance. This ensures that the instance, when transformed into our world, will be near the camera - no jitter.

One last note: on some drivers I found the driver was very finicky about order of operations - if the calculation is not done by applying the offset before the transform, the de-jitter totally fails. The precise and invariant qualifiers didn't seem to help, only getting the code "just right" did.

Wednesday, June 07, 2017

How to Reset Steam VR When It Can't Talk to the Rift

Periodically in the coarse of writing an OpenVR app, I find that SteamVR can't talk to my HMD. One of the 500 processes that collaborate to make VR work has kicked the bucket. Here's the formula to fix it.

First, kill the process tree based on OVRServer_x64.  All the Oculus stuff should die and then immediately respawn. Minimize their portal thingie.

Kill every vrXXX process (vrserver, vrmonitor,vrcompositor, vrdashboard).  SteamVR should not look like it's running and will not auto-relaunch.

Now you're good - relaunch your game and SteamVR should restart and be able to communicate with the headset.

Friday, June 02, 2017

"Quick Recalibrate" Fixes the SteamVR Floor for the Oculus Rift

I have the same problem that a lot of users report on the web: while my Oculus Rift knows where my floor is when I am in the Oculus home room, SteamVR gets confused and moves the floor up a foot or two, which makes me 3 feet tall when using a "standing" game. You can tell this problem is caused by SteamVR because even the SteamVR loading screens and settings are wrong, not just games.

Turns out there is a quick fix! Go to SteamVR settings, and in the developer section, you can use "Quick Calibrate."  Put your HMD on the floor in the middle of your play area, click once, and everything is fixed.

The only down-side is: you need a play area where your sensors can see the floor.  For me, this means putting the center of the play area unnecessarily far back, since the Rift sensors are on my desk.

Now I just need to find a way to fix the condensation inside the Rift.

Saturday, March 18, 2017

Why Your C++ Should Be Simple

I have a lot of strong (meaning "perhaps stupid") opinions about C++ and how to use it (or not use it), and I've been meaning to write posts on the thinking behind these stupid^H^H^H^H^H^Hstrong opinions, particularly where they go against the "conventional wisdom" of the modern C++ community.

This one is sort of an over-arching goal:
Write C++ that is less sophisticated than you are capable of writing.
This idea is a rip off^H^H^H^H^H^Hriff on a Brian Kernighan quote that I think is perhaps the best advice about  programming you'll ever hear:
Everyone knows that debugging is twice as hard as writing a program in the first place. So if you're as clever as you can be when you write it, how will you ever debug it? 
-- Brian Kernighan, the Elements of Programming Style
First, that's not wrong, that's dead on. So for debugging alone, don't write the most complicated C++ you can. C++ is an extremely complex language in its spec and if you use it to its full capability, you can write something that is completely un-debuggable.

Limited Brain Bandwidth

Here's my take on the idea. You're welcome to dismiss this as the mad ramblings of a father who is over 40 and literally cannot remember to finish steeping my tea because it's been an entire four minutes since I left it in the kitchen.

There's a limit to your mental bandwidth. That mental concentration, focus, short term memory, and whatever else the squishy gray GPU between our ears does can be spent on a few different things:

  • Figuring out how to write code.
  • Figuring out how to make code faster.
  • Figuring out how to solve non-trivial algorthm problems.
  • Figuring out how to solve business problems and design the architecture of a large production system.
  • Finding and fixing bugs.
  • Multi-tasking.
  • Looking at cats on the internet.
  • Swearing at your customers.
  • Trying to maintain focus while your 2 year old repeatedly smashes your keyboard with your mouse.
  • Figuring out the fastest path through a series of coding tasks.
I'm sure there's more, but you see where I'm going with this. You have a lot of things you can use your brain for, and some are more useful for getting your job done than others. You have to decide where to spend your brain budget, and you can't spend it on everything. (If you feel like you're not tight on brain budget, it means you're not working on a problem that's hard enough yet! You can always make the problem harder by shortening the time frame. Shipping things sooner is pretty much always a win.)

My view is that "being clever with C++" isn't a fantastic investment. Using the most bleeding edge C++ doesn't have enough benefit in other areas (code is faster, code is more maintainable, code is easier to debug, code is easier to understand) to justify burning brain power on it. In some cases, it actually moves you in the wrong direction.

Just because C++ lets you do it doesn't mean you have to or that it's even a good idea. Here are some examples where I consider a C++ feature to add complexity and not provide returns:

  • Clever operator overloading. Your coworkers will thank you if you give your functions good names and don't make operator+ turn on the coffee maker. You could skip operator overloading entirely and your code will be fine. (I'm okay with a small amount of overloading for a few obvious cases: dereference on smart pointers and math on math types).
  • Template meta-programming. I make myself watch talks on this from cppcon and what I see is the smartest C++ programmers in the world spending huge amounts of brain power to accomplish really trivial tasks (writing a program to make a program) in a way that is almost completely impossible to understand. You don't have to use the compiler to do your meta-program generation.
  • Heavy Duty Templating. This is a matter of degree - simple use of templates is fantastic and I use them in my code all the time. But at some point, as you add layers, you go past an inflection point and the cure starts to hurt more than the disease. This one is a little bit like smut: I don't have a great definition for when you've jumped the shark with your templates, but I know it when I see it. A good rule of thumb: if templates are making it harder to debug, don't add more. (Debuggers are easily good enough to debug the STL, generic algorithms, traits...you don't have to drop that stuff. It's not 1998 anymore!)
  • Overly Complicated Class Hierarchies. This is also a matter of degree, but at some point, it's okay for your class hierarchy to be less pure, clean and perfect if it's simpler and creates less chaos in the rest of the program. For example, our UI framework has one view type - parents, children, leaf nodes, roots, none of these are specialized by type. I've used a lot of other frameworks, and my finding is that clever "factoring out" of aspects of a view hierarchy does nothing to make the code better, but it creates issues you have to work around. Just keep it simple.
I could go on, but you get the idea...I'm one of those old bastards who thinks it's okay to just use C++ as "a nice version of C". Because it is! In my time here in the matrix, I have found that I have written past code that was too complex, language wise, and too simple. Here's the difference:

  • For the code that was too simple (I wrote a C struct when I needed a class), the upgrade is simple, easy to write, easy to understand, doesn't produce a lot of bugs, and the compiler helps me.
  • For code that was too complex (I wrote something worthy of Boost when  I should have made POD), ripping out that complexity is time consuming and goes slowly.
So why not err on the side of simplicity? Here are some other things you could do with your brain instead of writing C++ for the C++ elite:

  • Watch John Oliver on Youtube.
  • Put debugging facilities into your sub-system for more efficient debugging.
  • Put performance analysis data collection into your sub-system so you can tune it.
  • Document your sub-system so your coworkers don't poison your coffee.
  • Get the code to high quality faster and go on to the next thing!
If you are a professional software engineer, you are paid to solve people's business problems, and you use code to do that. Every Gang of Four pattern you use that doesn't do that is wasted brain cells.