GPU Quality and Application Portability

Transcription

1 GPU Quality and Application Portability Kalle Raita Senior Software Architect, drawelements Copyright Khronos Group, Page 1

2 drawelements Ltd. drawelements Based in Helsinki, Finland Founded in 2008 People from NVIDIA / Hybrid, Symbio Licensing Performance-critical Device Software Device and HW Manufacturers Computer Graphics & HW Run-time Compilers Embedded Software Subscription Graphics Quality Evaluation Tools drawelements Quality Program Measures quality of GPUs and drivers Currently available for OpenGL ES More APIs to be added during 2011 Enables comparison across GPU platforms Simplifies HW and SW development Reduce fragmentation Copyright Khronos Group, Page 2

3 Overview Define GPU quality Practical implications Example issues Benchmarking the quality Copyright Khronos Group, Page 3

4 GPU Quality Correctness - Behaves as stated in the specification Stability - Does not crash Performance - Fast enough for the purpose Predictability - Unrelated features should not interfere Appropriate feature set - Right tool for the job Copyright Khronos Group, Page 4

5 Implications of low quality What does low quality mean for application developers? One specific device - Increased development time, frustration - Worst case: some effects not possible or difficult to implement Closed platform (e.g. iphone) - Test on all devices and adapt more - May need to patch when a new device is released Open platform (e.g. Android) - Can only support the exact devices tested on - Constant search for the reliable subset of features GPU GPU 1 GPU 1.1 GPU 1.2 GPU X GPU X.1 GPU X.2 GPU Z GPU Z.1 GPU Z.2 GPU Y GPU Y.1 GPU Y.2 Copyright Khronos Group, Page 5

6 Example 1 OpenGL ES 2.0 shader compiler misses programming errors Will stop compiling if GPU vendor is changed or the compiler is updated A certain compiler performs dead code elimination before some checks - Leads to uniform type mismatch issues Similar to issues with web development and HTML specification Vertex Shader: varying float foo; main() { foo = 123.0; } Fragment Shader: varying integer foo; main() { /* No reference! */ } Sometimes compiles, sometimes not Copyright Khronos Group, Page 6

7 Example 2 Errors in shader programs may have dramatic consequences #define AAA AAA == application crash - on some devices Debugging is really slow as very little information is given on what actually went wrong If the application crashes, it may just indicate a fault in the shaders of the application Copyright Khronos Group, Page 7

8 Example 3 Data types are not equal Function like sqrt() can be broken exactly for vec2 inputs, others work Using matrix attributes can cause serious trouble - But replace with vectors and everything is OK Likely to show up as corrupted or missing image - Below a sample of vec2 sqrt plot Result Reference Difference Copyright Khronos Group, Page 8

9 Coding Tips Be aware of optional features in the specification Flexible application architecture - High level thinking in terms of effects, not shaders - Engine could choose shaders based on device model - Also configurable attributes, uniforms, and varyings Add new features to shaders in small increments Try modifying the shader in small steps - For example: vec2 => vec3, lowp => mediump To detect crashes, optional logging before and after - Shader compile - Shader link - Draw calls (e.g. gldrawelements, eglswapbuffers, ) Copyright Khronos Group, Page 9

10 Examples of Optional Features Vertex shader textures - Checking support from the API claims support -, but attempts to use textures in the vertex shader cause shader compilation error. Precision qualifiers - Some GPUs do not separate mediump and lowp - For example, lighting calculations won t look good when done in real lowp Complex loops in shaders - No way to query support - When no support, results vary between compiler error and a crash Copyright Khronos Group, Page 10

11 Testing Tips Test across GPU vendors - Many different devices have the same GPUs - Desktop emulators are a good reference as well Test in stages to find issues faster For example: 1. In write code / fix cycle: one device 2. Before code submission: all GPUs 3. Nightly/Weekend test: all major devices 4. Before release: all supported devices One device All GPUs All major devices All supported devices Copyright Khronos Group, Page 11

12 Benchmarking the Quality What can be measured? - Number or percentage of correctly implemented features - Number of optional features correctly implemented - Number of accurate bits in math operations - Number of iterations survived with semi-random input - Capabilities like maximum texture dimensions and texture memory size Benchmark can go beyond the specification requirements Benefits of benchmarking - Helps device manufacturers to choose high quality GPUs - GPU vendors can improve their products - Developers gain knowledge about platform capabilities Copyright Khronos Group, Page 12