Beyond porting

Beyond Porting

How Modern OpenGL can
Radically Reduce Driver Overhead

Who are we?
Cass Everitt, NVIDIA Corporation
John McDonald, NVIDIA Corporation

What will we cover?
Dynamic Buffer Generation
Efficient Texture Management
Increasing Draw Call Count

Dynamic Buffer Generation
Problem
Our goal is to generate dynamic geometry directly in place.
It will be used one time, and will be completely regenerated next frame.
Particle systems are the most common example
Vegetation / foliage also common

Typical Solution
void UpdateParticleData(uint _dstBuf) {
BindBuffer(ARRAY_BUFFER, _dstBuf);
access = MAP_UNSYNCHRONIZED | MAP_WRITE_BIT;
for particle in allParticles {
dataSize = GetParticleSize(particle);
void* dst = MapBuffer(ARRAY_BUFFER, offset, dataSize, access);
(*(Particle*)dst) = *particle;
UnmapBuffer(ARRAY_BUFFER);
offset += dataSize;
}
};
// Now render with everything.

The horror
This is so slow.
offset += dataSize;
}
};

Driver interlude
First, a quick interlude on modern GL drivers
In the application (client) thread, the driver is very thin.
It simply packages work to hand off to the server thread.

The server thread does the real processing
It turns command sequences into push buffer fragments.

Healthy Driver Interaction Visualized

Application
Driver (Client)
Driver (Server)
GPU
Thread separator
Component separator

State Change
Action Method (draw, clear, etc)
Present

MAP_UNSYNCHRONIZED
Avoids an application-GPU sync point (a CPU-GPU sync point)
But causes the Client and Server threads to serialize
This forces all pending work in the server thread to complete
It’s quite expensive (almost always needs to be avoided)

Client-Server Stall of Sadness

Application
Driver (Client)
Driver (Server)
GPU
Thread separator
Component separator

State Change
Action Method (draw, clear, etc)
Present

It’s okay
Q: What’s better than mapping in an unsynchronized manner?
A: Keeping around a pointer to GPU-visible memory forever.
Introducing: ARB_buffer_storage

ARB_buffer_storage
Conceptually similar to ARB_texture_storage (but for buffers)
Creates an immutable pointer to storage for a buffer
The pointer is immutable, the contents are not.
So BufferData cannot be called—BufferSubData is still okay.

Allows for extra information at create time.
For our usage, we care about the PERSISTENT and COHERENT
bits.
PERSISTENT: Allow this buffer to be mapped while the GPU is using it.
COHERENT: Client writes to this buffer should be immediately visible to
the GPU.
http://www.opengl.org/registry/specs/ARB/buffer_storage.txt

ARB_buffer_storage cont’d
Also affects the mapping behavior (pass persistent and coherent
bits to MapBufferRange)
Persistently mapped buffers are good for:
Dynamic VB / IB data
Highly dynamic (~per draw call) uniform data
Multi_draw_indirect command buffers (more on this later)

Not a good fit for:
Static geometry buffers
Long lived uniform data (still should use BufferData or BufferSubData for
this)

Armed with persistently mapped buffers
// At the beginning of time
flags = MAP_WRITE_BIT | MAP_PERSISTENT_BIT | MAP_COHERENT_BIT;
BufferStorage(ARRAY_BUFFER, allParticleSize, NULL, flags);
mParticleDst = MapBufferRange(ARRAY_BUFFER, 0, allParticleSize,
flags);
mOffset = 0;

// allParticleSize should be ~3x one frame’s worth of particles
// to avoid stalling.

Update Loop (old and busted)
offset += dataSize;
}
};

Update Loop (new hotness)
void UpdateParticleData() {

mParticleDst[mOffset] = *particle;
mOffset += dataSize; // Wrapping not shown
}
};

Performance results
160,000 point sprites
Specified in groups of 6 vertices (one particle at a time)
Synthetic (naturally)
Method

FPS

Particles / S

Map(UNSYNCHRONIZED)

1.369

219,040

BufferSubData

17.65

2,824,000

D3D11 Map(NO_OVERWRITE)

20.25

3,240,000

Performance results
160,000 point sprites
Specified in groups of 6 vertices (one particle at a time)
Synthetic (naturally)
Method

FPS

Particles / S

Map(UNSYNCHRONIZED)

1.369

219,040

BufferSubData

17.65

2,824,000

D3D11 Map(NO_OVERWRITE)

20.25

3,240,000

Map(COHERENT|PERSISTENT)

79.9

12,784,000

Room for improvement still, but much, much better.

The other shoe
You are responsible for not stomping on data in flight.
Why 3x?
1x: What the GPU is using right now.
2x: What the driver is holding, getting ready for the GPU to use.
3x: What you are writing to.

3x should ~ guarantee enough buffer room*…
Use fences to ensure that rendering is complete before you begin
to write new data.

Fencing
Use FenceSync to place a new fence.
When ready to scribble over that memory again, use
ClientWaitSync to ensure that memory is done.
ClientWaitSync will block the client thread until it is ready
So you should wrap this function with a performance counter
And complain to your log file (or resize the underlying buffer) if you
frequently see stalls here

For complete details on correct management of buffers with
fencing, see Efficient Buffer Management [McDonald 2012]

Efficient Texture Management
Or ―how to manage all texture memory myself‖

Problem
Changing textures breaks batches.
Not all texture data is needed all the time
Texture data is large (typically the largest memory bucket for games)

Bindless solves this, but can hurt GPU performance
Too many different textures can fall out of TexHdr$
Not a bindless problem per se

Terminology
Reserve – The act of allocating virtual memory
Commit – Tying a virtual memory allocation to a physical backing
store (Physical memory)
Texture Shape – The characteristics of a texture that affect its
memory consumption
Specifically: Height, Width, Depth, Surface Format, Mipmap Level Count

Old Solution
Texture Atlases
Problems
Can impact art pipeline
Texture wrap, border filtering
Color bleeding in mip maps

Texture Arrays
Introduced in GL 3.0, and D3D 10.
Arrays of textures that are the same shape and format
Typically can contain many ―layers‖ (2048+)
Filtering works as expected
As does mipmapping!

Sparse Bindless Texture Arrays
Organize loose textures into Texture Arrays.
Sparsely allocate Texture Arrays
Introducing ARB_sparse_texture
Consume virtual memory, but not physical memory

Use Bindless handles to deal with as many arrays as needed!
Introducing ARB_bindless_texture
uncommitted
uncommitted
uncommitted

layer
layer
layer

ARB_sparse_texture
Applications get fine-grained control of physical
memory for textures with large virtual allocations
Inspired by Mega Texture
Primary expected use cases:
Sparse texture data
Texture paging
Delayed-loading assets

http://www.opengl.org/registry/specs/ARB/sparse_texture.txt

ARB_bindless_texture
Textures specified by GPU-visible ―handle‖ (really an address)
Rather than by name and binding point

Can come from ~anywhere
Uniforms
Varying
SSBO
Other textures

Texture residency also application-controlled
Residency is ―does this live on the GPU or in sysmem?‖
https://www.opengl.org/registry/specs/ARB/bindless_texture.txt

Advantages
Artists work naturally
No preprocessing required (no bake-step required)
Although preprocessing is helpful if ARB_sparse_texture is unavailable

Reduce or eliminate TexHdr$ thrashing
Even as compared to traditional texturing

Programmers manage texture residency
Works well with arbitrary streaming
Faster on the CPU
Faster on the GPU

Disadvantages
Texture addresses are now structs (96 bits).
64 bits for bindless handle
32 bits for slice index (could reduce this to 10 bits at a perf cost)

ARB_sparse_texture implementations are a bit immature
Early adopters: please bring us your bugs.

ARB_sparse_texture requires base level be a multiple of tile size
(Smaller is okay)
Tile size is queried at runtime
Textures that are power-of-2 should almost always be safe.

Implementation Overview
When creating a new texture…
Check to see if any suitable texture array exists
Texture arrays can contain a large number of textures of the same shape
Ex. Many TEXTURE_2Ds grouped into a single TEXTURE_2D_ARRAY

If no suitable texture, create a new one.

Texture Container Creation (example)
GetIntegerv( MAX_SPARSE_ARRAY_TEXTURE_LAYERS, maxLayers );

Choose a reasonable size (e.g. array size ~100MB virtual )

If new internalFormat, choose page size
GetInternalformativ( …, internalformat, NUM_VIRTUAL_PAGE_SIZES, 1, &numIndexes);

Note: numIndexes can be 0, so have a plan
Iterate, select suitable pageSizeIndex
BindTexture( TEXTURE_2D_ARRAY, newTexArray );
TexParameteri( TEXTURE_SPARSE, TRUE );
TexParameteri( VIRTUAL_PAGE_SIZE_INDEX, pageSizeIndex );

Allocate the texture’s virtual memory using TexStorage3D

Specifying Texture Data
Using the located/created texture array from the previous step
Allocate a layer as the location of our data
For each mipmap level of the allocated layer:
Commit the entire mipmap level (using TexPageCommitment)
Specify actual texel data as usual for arrays
gl(Compressed|Copy|)TexSubImage3D
PBO updates are fine too
uncommitted

Allocated layer

free
free
layer
slice
slice

Freeing Textures
To free the texture, reverse the process:
Use TexPageCommitment to mark the entire layer (slice) as free.
Do once for each mipmap level

Add the layer to the free list for future allocation

uncommitted

free
free
layer
slice
slice

uncommitted

Freed layer

layer

Combining with Bindless to eliminate binds
At container create time:
Specify sampling parameters via SamplerParameter calls first
Call GetTextureSamplerHandleARB to return a GPU-visible pointer to the
texture+sampler container
Call MakeTextureHandleResident to ensure the resource lives on the GPU

At delete time, call MakeTextureHandleNonResident
With bindless, you explicitly manage the GPU’s working set

Using texture data in shaders
When a texture is needed with the default sampling parameters
Create a GLSL-visible TextureRef object:
struct TextureRef {
sampler2DArray container;
float slice;
};

When a texture is needed with custom sampling parameters
Create a separate sampler object for the shader with the parameters
Create a bindless handle to the pair using
GetTextureSamplerHandle, then call MakeTextureHandleResident
with the new value
And fill out a TextureRef as above for usage by GLSL

C++ Code
Basic implementation (some details missing)
BSD licensed (use as you will)
https://github.com/nvMcJohn/apitest/blob/pdoane_newtests/sparse_bindless_texarray.h
https://github.com/nvMcJohn/apitest/blob/pdoane_newtests/sparse_bindless_texarray.cpp

Increasing Draw Call Count
Let’s draw all the calls!

All the Draw Calls!
Problem
You want more draw calls of smaller objects.
D3D is slow at this.
Naïve GL is faster than D3D, but not fast enough.

XY Problem
Y: How can I have more draw calls?
X: You don’t really care if it’s more draw calls, right?
Really what you want is to be able to draw more small geometry
groupings. More objects.

Well why didn’t you just say so??
First, some background.
What makes draw calls slow?
Real world API usage
Draw Call Cost Visualization

Some background
What causes slow draw calls?
Validation is the biggest bucket (by far).
Pre-validation is ―difficult‖
―Every application does the same things.‖
Not really. Most applications are in completely disjoint states
Try this experiment: What is important to you?
Now ask your neighbor what’s important to him.

Why is prevalidation difficult?
The GPU is an exceedingly complex state machine.
(Honestly, it’s probably the most complex state machine in all of CS)

Any one of those states may have a problem that requires WAR
Usually the only problem is overall performance
But sometimes not. 

There are millions of tests covering NVIDIA GPU functionality.

FINE.
How can app devs mitigate these costs?
Minimize state changes.
All state changes are not created equal!

Cost of a draw call:
Small fixed cost + Cost of validation of changed state

Feels limiting…
Artists want lots of materials, and small amounts of geometry
Even better: What if artists just didn’t have to care about this?
Ideal Programmer->Artist Interaction
―You make pretty art. I’ll make it fit.‖

Relative costs of State Changes
In decreasing cost…
Render Target
Program
ROP
Texture Bindings
Vertex Format
UBO Bindings
Vertex Bindings
Uniform Updates

~60K / s
~300K / s

~1.5M / s

~10M / s
Note: Not to scale

Real World API frequency
API usage looks roughly like this…
Increasing Frequency of Change
Render Target (scene)
Per Scene Uniform Buffer + Textures
IB / VB and Input Layout
Shader (Material)
Per-material Uniform Buffer + Textures
Per-object Uniform Buffer + Textures
Per-piece Uniform Buffer + Textures
Draw

Draw Calls visualized

Render Target

Texture

Uniform Updates

Program

UBO Binding

Draw

ROP

Vertex Format

Draw Calls visualized (cont’d)

Read down, then right
Black—no change

Render Target

Texture

Uniform Updates

Program

UBO Binding

Draw

ROP

Vertex Format

Goals
Let’s minimize validation costs without affecting artists
Things we need to be fast (per app call frequency):
Uniform Updates and binding
Texture Updates and binding

These happen most often in app, ergo driving them to ~0 should
be a win.

Textures
Using Sparse Bindless Texture Arrays (as previously described)
solves this.
All textures are set before any drawing begins
(No need to change textures between draw calls)

Note that from the CPU’s perspective, just using bindless is
sufficient.

That was easy.

Eliminating Texture Binds -- visualized

Per Scene Uniform Buffer + Textures
Shader (Material)
Per-material Uniform Buffer + Textures
Per-object Uniform Buffer + Textures
Per-piece Uniform Buffer + Textures
Draw
Render Target

Texture

Uniform Updates

Program

UBO Binding

Draw

ROP

Vertex Format

Boom!

Per Scene Uniform Buffer
Shader (Material)
Per-material Uniform Buffer
Per-object Uniform Buffer
Per-piece Uniform Buffer
Draw
Render Target

Texture

Uniform Updates

Program

UBO Binding

Draw

ROP

Vertex Format

Buffer updates (old and busted)
Typical Scene Graph Traversal
for obj in visibleObjectSet {
update(buffer, obj);
draw(obj);
}

Buffer updates (new hotness)
Typical Scene Graph Traversal
update(bufferFragment, obj);
}
draw(obj);
}

bufferFragma-wha?
Rather than one buffer per object, we share UBOs for many
objects.
ie, given struct ObjectUniforms { /* … */ };
// Old (probably not explicitly instantiated,
// just scattered in GLSL)
ObjectUniforms uniformData;
// New
ObjectUniforms uniformData[ObjectsPerKickoff];

Use persistent mapping for even more win here!
For large amounts of data (bones) consider SSBO.
Introducing ARB_shader_storage_buffer_object

SSBO?
Like ―large‖ uniform buffer objects.
Minimum required size to claim support is 16M.

Accessed like uniforms in shader
Support for better packing (std430)
Caveat: They are typically implemented in hardware as textures
(and can introduce dependent texture reads)
Just one of a laundry list of things to consider, not to discourage use.
http://www.opengl.org/registry/specs/ARB/shader_storage_buffer_object.txt

Eliminating Buffer Update Overhead

Per Scene Uniform Buffer
Shader (Material)
Per-material Uniform Buffer
Per-object Uniform Buffer
Per-piece Uniform Buffer
Draw
Render Target

Texture

Uniform Updates

Program

UBO Binding

Draw

ROP

Vertex Format

Sweet!

Shader (Material)
Draw ( * each object )

Hrrrrmmmmmm….
Render Target

Texture

Uniform Updates

Program

UBO Binding

Draw

ROP

Vertex Format

So now…
It’d be awesome if we could do all of those kickoffs at once.
Validation is already only paid once
But we could just pay the constant startup cost once.
If only…….

So now…
It’d be awesome if we could do all of those kickoffs at once.
Validation is already only paid once
But we could just pay the constant startup cost once.
If only…….

Introducing ARB_multi_draw_indirect

ARB_multi_draw_indirect
Allows you to specify parameters to draw commands from a
buffer.
This means you can generate those parameters wide (on the CPU)
Or even on the GPU, via compute program.
http://www.opengl.org/registry/specs/ARB/multi_draw_indirect.txt

ARB_multi_draw_indirect cont’d
void MultiDrawElementsIndirect(enum mode,
enum type
const void* indirect,
sizei primcount,
sizei stride);

ARB_multi_draw_indirect cont’d
const ubyte * ptr = (const ubyte *)indirect;
for (i = 0; i < primcount; i++) {
DrawArraysIndirect(mode,
(DrawArraysIndirectCommand*)ptr);
if (stride == 0)
{
ptr += sizeof(DrawArraysIndirectCommand);
} else {
ptr += stride;
}
}

DrawArraysIndirectCommand
typedef struct {
uint count;
uint primCount;
uint first;
uint baseInstance;
} DrawArraysIndirectCommand;

Knowing which shader data is mine
Use ARB_shader_draw_parameters, a necessary companion to
ARB_multi_draw_indirect
Adds a builtin to the VS: DrawID (InstanceID already available)
This tells you which command of a MultiDraw command is being
executed.
When not using MultiDraw, the builtin is specified to be 0.

Caveat: Right now, you have to pass this down to other shader
stages as an interpolant.
Hoping to have that rectified via ARB or EXT extension ―real soon now.‖
http://www.opengl.org/registry/specs/ARB/shader_draw_parameters.txt

Applying everything
CPU Perf is massively better
5-30x increase in number of distinct objects / s

Interaction with driver is decreased ~75%
Note: GPU perf can be affected negatively (although not too
badly)

As always: Profile, profile, profile.

Previous Results

Render Target

Texture

Uniform Updates

Program

UBO Binding

Draw

ROP

Vertex Format

Visualized Results

Render Target

Texture

Uniform Updates

Program

UBO Binding

Draw

ROP

Vertex Format

MultiDraw

Where we came from

Render Target

Texture

Uniform Updates

Program

UBO Binding

Draw

ROP

Vertex Format

Conclusion
Go forth and work magnify.

Questions?
jmcdonald at nvidia dot com
cass at nvidia dot com

Beyond porting

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Editor's Notes