Graphics performance balancing the rendering pipeline
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Graphics Performance: Balancing the Rendering Pipeline Cem Cebenoyan and Matthias Wloka Introduction At a minimum, PC is a 2 processor system CPU GPU Maximum efficiency IFF All processors are busy All the time GPU CPU AGP Bus Actually, It’s Worse GPU Vertex Processing CPU AGP Bus

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Introduction l.jpg
Introduction

  • At a minimum, PC is a 2 processor system

    • CPU

    • GPU

  • Maximum efficiency IFF

    • All processors are busy

    • All the time

GPU

CPU

AGP Bus


Actually it s worse l.jpg
Actually, It’s Worse

GPU

Vertex Processing

CPU

AGP Bus

Application

Triangle Setup

API

Large Cache

Fragment Shading

Framebuffer Access


Multi processor system l.jpg
Multi-Processor System

  • Conceptually, 5 processors

    • CPU

    • Vertex-processor(s)

    • Setup processor(s)

    • Fragment processor(s)

    • Blending processor(s)

  • All connected via some form of cache

    • To smooth data flow

    • To keep things humming


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MP Systems Become Inefficient If…

  • One or more processors sync to each other

  • For example, frame-buffer lock

    • Insures that all caches drain

    • Insures that all processors idle (CPU and GPU!)

    • Overhead in restarting the processors

  • A single processor bottlenecks all others


Overview l.jpg
Overview

  • CPU

  • AGP Bus

  • Vertex Processing

  • Triangle Setup

  • Rasterization

  • Memory bandwidth

    • Writing to and blending with video memory


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Overview: For Each Stage

  • What are its characteristics?

    • How does it behave?

  • How to measure whether it is the bottleneck

  • How to influence it


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CPU Characteristics

  • Stay within on-chip cache for maximum performance

  • Use CPU for

    • Collision detection

    • Physics

    • AI

    • Etc.


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CPU Characteristics (cont.)

  • Note that graphics is capable of

    • 20+ MTri/s (2 year old high-end)

    • 20+ MTri/s (integrated graphics)

    • 100+ MTri/s (current high-end)

  • CPU also responsible for pushing data to GPU

    • Cannot look at every triangle

    • Don’t limit graphics with CPU processing


Cpu measurement l.jpg
CPU Measurement

  • Use VTune

    • Or any other profiler

  • Most games are CPU-limited

  • Little to no time in the graphics driver:

    • CPU is the bottleneck

    • Faster GPU will NOT result in faster graphics

    • Use VTune to track where you spend your time

      • Optimize those places


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CPU Measurement (cont.)

  • But even if most time is spent in graphics driver:

    • CPU might still be the bottleneck

    • Faster GPU will NOT result in faster graphics

    • Use Nvidia Stats-driver (NVTune) to trace into the GPU

  • Timing graphics calls is pointless

    • Remember the large cache between CPU/GPU

    • Use Nvidia Stats-driver (NVTune) instead

    • NVTune available from Nvidia’s registered developer site


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CPU Common Problems

  • Small batches of geometry being sent to the GPU

  • 100 triangles per batch should be your minimum

    • Would like to see ~500 triangles/batch

    • Up to 10,000 triangles/batch

  • Combination of causes kill your performance

    • Runtime

    • Driver

    • Hardware



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CPU: Batching Solutions

  • Sort by render-state

  • Texture switches

    • Combine textures into one large (4kx4k) texture

    • Modify uv-coordinates accordingly

    • Tessellate geometry to overcome mirroring and wrapping

    • Mip-mapping works just fine

  • Transform switches

    • Pre-transform on the CPU into world-space

    • Replicate data into VBs (costs AGP memory)


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Other Common CPU Problems

  • Specify vertex buffers as WRITEONLY

  • Minimize state changes

    • consider using a PURE device, iff you are optimal

  • Do not lock and read data from GPU

    • Multi-processor sync!


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AGP Bus Characteristics

  • AGP 4x supports 20+ MTri/s

    • Even if all vertices and indices are dynamic

    • BenMark5 does just that

    • http://developer.nvidia.com/view.asp?IO=BenMark5

  • Too often AGP 4x support is busted

    • Use BenMark5 to test for AGP 4x support

  • AGP Bus through-put influenced by

    • Size of vertex format of dynamically written vertices

    • How many vertices are dynamically written


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AGP Bus Characteristics (cont.)

  • But if frame-buffer and textures exceed video-memory, AGP is also used

    • to transfer STATIC vertices to GPU every frame

    • to transfer textures to GPU every frame

  • Make sure you avoid partial writes

    • See “Fast AGP Writes for Dynamic Vertex Data” by Dean Macri for details

    • Always modify all vertex-data,

      • even if only some data changes

    • Pentium 3: write in 32 byte chunks

    • Pentium 4: write in 64 byte chunks


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AGP Bus Characteristics (cont.)

  • GPU caches vertex fetches

    • Hitting this cache causes no data to cross the bus

  • Cache has 32-byte lines

    • Vertex sizes that are multiples of 32 are beneficial

    • See also http://developer.nvidia.com/view.asp?IO=Vertex_Buffer_Statistics



Agp bus measurement l.jpg
AGP Bus Measurement

  • You can tell you’re bound by the bus if:

    • Increasing/decreasing vertex format size significantly impacts performance

    • Best to increase vertex format size using components not needed by rasterizer

      • for example, normals


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Increasing AGP Bus Performance

  • Make sure frame buffer and textures fit into video-memory

  • Decrease number of dynamic objects (vertices)

    • Use vertex-shaders to animate static VBs!

  • Decrease vertex size

    • Let vertex-shader generate vertex-components!

    • Compress components and use vertex shader to decompress

      • For example, use 16bit short normals

  • Reorder vertices in VB to be sequential in use

    • Can use NVTriStrip to do this

    • Pad to multiples of 32-bytes


Vertex processing characteristics l.jpg
Vertex Processing Characteristics

  • Each vertex is transformed and lit

  • Performance correlates directly to

    • Number of vertices processed

    • Length of vertex shader or

    • Fixed-function factors, such as

      • Number of active lights

      • Type of lights

      • Specular on/off

      • LOCALVIEWER on/off

      • Texgen on/off

    • GPU core clock frequency



Vertex processing characteristics24 l.jpg
Vertex Processing Characteristics

  • After processing, vertices land in post-TnL FIFO

    • GeForce1/2/4 MX: effectively 10 entries

    • GeForce3/4 Ti: effectively 18 entries

  • Cache-hit saves:

    • all TnL work!

    • Everything before TnL in the pipeline

  • Only works with indexed primitives


Vertex processing performance l.jpg
Vertex Processing Performance

  • Do not be afraid to use triangles

    • Rarely the bottleneck

      • Even if it is, it would make us happy

    • A lot of vertex processing power available

      • 6 * 6 pixel-quad with 2 tris is not vertex bound

    • If you can tell an object is made from triangles, you are not using enough triangles

  • ~10k triangles/frame is off by 2 (two!) orders of magnitude


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Code Creatures Demo

  • Grass scenes are NOT vertex-bound

  • In excess of 1,000,000 tris/frame for opening scene

    • ~250k tris/frame minimum

  • CodeCreatures demo available from: http://www.codecult.de/


Vertex processing measurement l.jpg
Vertex Processing Measurement

  • You are bound by vertex processing if:

    • Increasing/decreasing vertex shader length significantly influences performance

      • Adding unnecessary instructions may be optimized out by driver, though

      • Instead, use instructions that access constant memory to add zero to a result, for example

    • Fixed-function TnL performance improves when

      • Reducing number of lights

      • Turning off texgen

      • Simplifying light types


Improving vertex processing l.jpg
Improving Vertex Processing

  • Optimize for the post-TnL vertex cache

    • Use indexed primitives

    • Access vertices mostly sequentially, revisiting only recently accessed vertices

    • Let NVTriStrip or ID3DXMesh do the work

  • Turn off unnecessary calculations

    • LOCALVIEWER often unnecessary for specular

    • Prefer cheap approximations for lighting and other math when using vertex shaders


Improving vertex processing cont l.jpg
Improving Vertex Processing (cont.)

  • Optimize your vertex shaders

    • Use swizzling/masking extensively

    • Question all MOV instructions

    • Storing lookup tables in constant memory

      • for example, to compute sin/cos

  • See “Implementation of ‘Missing’ Vertex Shader Instructions” for more ideas

    • http://developer.nvidia.com/view.asp?IO=Implementation_Missing_Instructions


Improving vertex processing cont30 l.jpg
Improving Vertex Processing (cont.)

  • Consider moving per-vertex work to per-pixel

  • Consider using ‘shader-LODing’

    • Do far-away objects really need 4-bone skinning?

  • Can always increase screen-res/use AA to NOT be vertex-processing bound!


Triangle setup characteristics l.jpg
Triangle Setup Characteristics

  • Triangle setup is never the bottleneck

    • Except when rating the GPU

    • Since it is the fastest stage

  • Setup speed influenced by:

    • Number of triangles

    • Vertex attributes needed by rasterization

  • Extremely small triangles running very simple TnL

    • i.e., degenerate triangles!

      • No TnL cost, since most likely hits post-TnL cache

      • No fill-cost, since rejected in setup


Measuring improving triangle setup l.jpg
Measuring/Improving Triangle Setup

  • Has never come up

  • Reduce ratio of degenerate triangles to real triangles

  • Reduce unnecessary components written out from the vertex shader


Rasterization characteristics l.jpg
Rasterization Characteristics

  • Prefer the term “fragment” to “pixel”

    • May not correspond to any pixel in framebuffer, for example, due to z/stencil/alpha tests

    • May correspond to more than one pixel due to multisampling

  • Commonly referred to as “fill-rate”


Fill rate characteristics l.jpg
Fill-Rate Characteristics

  • Fill-rate is function of

    • number of fragments filled

    • cost of each fragment

    • GPU’s core clock

  • Parallel SIMD operation, processes

    • Up to 4 pixels per clock on GeForce1/2/3/4 Ti

    • Up to 2 pixels per clock on GeForce2 MX / 4 MX

  • Broken into a number of parts:

    • Texture fetching

    • Texture addressing operations

    • Color blending operations


Texture fetching characteristics l.jpg
Texture Fetching Characteristics

  • Texture fetches are

    • From AGP to local video-memory, only if frame-buffer and textures exceed video-memory (to be avoided), then

    • From local video-memory to on-chip cache


Texture fetching characteristics cont l.jpg
Texture Fetching Characteristics (cont.)

  • Minimize cache-misses:

    • Use mip-mapping!

      • Avoid LOD bias to sharpen: it hurts caching and adds aliasing

        • Prefer anisotropic filtering for sharpening

    • Use DXT everywhere you can

    • Texture size as big as needed and no bigger

    • Texture format as small as possible

      • 16 vs. 32 bit

    • Localize texture access

      • E.g., normal texture reads

      • Dependent texture reads are less local

        • Per-pixel reflection potentially really bad


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Texture Fetching Characteristics (cont.)

  • Number of samples taken also affects performance:

    • Trilinear filtering cuts fillrate in half

    • Anisotropic even worse

      • Depending on level of anisotropy

      • The hardware is intelligent in this regard, you only pay for the anisotropy you use


Texture addressing characteristics l.jpg
Texture Addressing Characteristics

  • Different texture addressing operations have wildly different performance characteristics

    • But texture cache hits/misses more significant


Texture addressing characteristics39 l.jpg
Texture Addressing Characteristics

  • Also, every two textures cuts fill-rate in half:

    • 1 or 2 textures runs at full speed

    • 3 or 4 textures runs at half speed (two clocks)


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Color Blending Characteristics

  • Color blending operations also called ‘Register Combiners’

    • 1 or 2 instructions (combiners) – full speed

    • 3 or 4 instructions (combiners) – half speed

    • 5 or 6 instructions (combiners) – one third speed

    • 7 or 8 instructions (combiners) – one quarter speed

      • These numbers are for GF3 / 4 Ti

  • But if using 4 textures

    • Already at half-speed or less

    • Using up to 4 combiners is free


Fill rate measurement l.jpg
Fill-Rate Measurement

  • You are bound by fill-rate, if

    • Reducing texture sizes

      • Or better turning off texturing

    • Increases performance significantly

    • Turning on / off trilinear affects performance

    • Increasing texture units used to 4, but not actually fetching from any textures (using pixel shader instructions like texcoord), causes you to slow down


Improving fill rate l.jpg
Improving Fill-Rate

  • Render z-only pass first

    • Because z-optimizations happen before rasterization

    • Helps with memory bandwidth as well

      • Even for older chips without z-optimizations

  • Do everything to reduce texture cache misses

  • Turn on anisotropic, but turn off trilinear filtering

    • Mip-map transitions are less visible with anisotropic filtering on


Improving fill rate cont l.jpg
Improving Fill-Rate (cont.)

  • Consider palletized normal maps for compression

  • Consider moving per-pixel work to per-vertex

  • Consider ‘shader LODing’

    • Turn off detail map computations in the distance


Memory bandwidth characteristics l.jpg
Memory Bandwidth Characteristics

  • Memory bandwidth is often the bottleneck

    • especially at high resolutions

  • Memory bandwidth influenced by:

    • Screen and render-target resolutions

    • Render-target color / z bit depth

    • FSAA

    • Texture sizes and formats (texture fetching)

    • Overdraw complexity

    • Alpha blending

    • GPU’s memory-interface width

    • Memory clock


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Memory Bandwidth Characteristics

  • FSAA hits memory bandwidth exclusively

    • no fill-rate hit with multi-sample

  • Failing the z/stencil/alpha test means

    • Pixel color is not written

    • Z is not written


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Measuring Memory Bandwidth

  • Switch frame-buffer format to 16bit

  • Switch all render-targets to 16bit

  • If performance doubles

    • App was 100% memory-bandwidth bound

  • If performance unchanged

    • App is not memory-bandwidth bound


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Improving Memory Bandwidth

  • Overdraw

    • Reduce as much as possible

    • Lightly sort objects front to back

      • All architectures benefit, since z-test fails

    • Reduce blending as much as possible

      • Always enable alpha-test when blending

        • Tweak test-value as much as possible

      • Consider using 2-pass alpha-test/-blend technique

  • Always clear z/stencil (using clear())

    • Do not clear color if not necessary

    • Writing z from shader destroys early z


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Improving Memory Bandwidth (cont.)

  • Prefer FSAA over high resolution

  • Consider using z-only pass

    • Turn off z-writing for all subsequent passes


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Conclusion

  • A lot of different performance bottle-necks

    • Know which one to tweak

    • Use suggestions here to

      • make things faster w/o making it visibly worse

      • Make things prettier for free!


Questions l.jpg
Questions…

?

cem@nvidia.com

mwloka@nvidia.com

http://developer.nvidia.com


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