Beyond Porting How Modern OpenGL can Radically Reduce Driver Overhead

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 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); This is so slow. offset dataSize; } }; // Now render with everything.

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)

Healthy Driver Interaction Visualized Application Driver (Client) Driver (Server) GPU Thread separator Component separator State Change Action Method (draw, clear, etc) Present

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) 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; offset dataSize; UnmapBuffer(ARRAY BUFFER); } }; // Now render with everything.

Update Loop (new hotness) void UpdateParticleData() { for particle in allParticles { dataSize GetParticleSize(particle); mParticleDst[mOffset] *particle; mOffset dataSize; // Wrapping not shown } }; // Now render with everything.

Test App

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 Freed layer uncommitted 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 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 Render Target Texture Uniform Updates Program UBO Binding Draw ROP Vertex Format

Boom! Increasing Frequency of Change Render Target (scene) Per Scene Uniform Buffer IB / VB and Input Layout 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 for obj in visibleObjectSet { update(bufferFragment, obj); } for obj in visibleObjectSet { 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 Increasing Frequency of Change Render Target (scene) Per Scene Uniform Buffer IB / VB and Input Layout 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! Increasing Frequency of Change Render Target (scene) IB / VB and Input Layout 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