Summary: Debugging API performance issues can be challenging, especially when dealing with complex graphics and GPU-related problems. NVIDIA provides a suite of tools to help developers identify and resolve these issues. This article will delve into the main ideas presented in NVIDIA’s guide on advanced API performance debugging, focusing on practical steps and tools to improve debugging efficiency.

Advanced API Performance Debugging: A Practical Guide

Understanding the Challenges

Debugging API performance issues, particularly those related to graphics and GPU, can be daunting. The complexity of modern graphics APIs and the variety of hardware configurations make it difficult to pinpoint the source of problems. NVIDIA’s suite of debugging tools is designed to help developers overcome these challenges.

Tools for Graphics Debugging

NVIDIA offers several tools for graphics debugging, each serving a specific purpose:

  • NVIDIA Nsight System: This tool is primarily used for CPU debugging, providing detailed insights into system-level issues.
  • Nsight Graphics: Designed for GPU debugging, this tool helps developers identify and resolve GPU-related problems.
  • Nsight Aftermath: This tool is particularly useful for analyzing crash dumps, providing detailed information about the type of error that occurred during a crash.

Practical Steps for Debugging

1. Isolate Problems Using Debug Checkpoints

Debug checkpoints are a powerful feature that allows developers to insert checkpoints in the GPU command stream. This helps narrow down crashes to specific subsections of the command stream, making it easier to identify the source of the problem.

  • Use Nsight Aftermath API: This API supports inserting debug checkpoints and analyzing crash dumps. For more information and samples, refer to the NVIDIA/nsight-aftermath-samples GitHub repository.
  • DirectX 12 Cross-Vendor Solution: Use ID3D12GraphicsCommandList2::WriteBufferImmediate or DRED to insert checkpoints.

2. Build Shaders with Debug Info

Compiling shaders with debug information embedded can significantly improve debugging efficiency.

  • Compile with /Zi: This flag embeds debug information into the shader binary, which is helpful when using debugging tools like NVIDIA Nsight Graphics.
  • Nsight Aftermath: This tool can provide source-level GPU crash information using the embedded debug information.

3. Analyze Crash Dumps

Crash dumps are invaluable for identifying the type of error that occurs during a crash.

  • Device Hung: These can occur due to a single command list taking longer than a few seconds to execute, leading to a Timeout Detection and Recovery (TDR) by Microsoft Windows.
  • Page Faults: These can be identified using the DirectX 12 debug layer, but GPU-based validation must be enabled, which can significantly slow down the application.
  • Validate Input Data: Ensure that input vertex or index data are valid. Invalid indices can crash the GPU builder kernel, while invalid vertices can affect acceleration structures and degrade performance.
  • Simplify Code: Disabling textures or reducing shader permutations can help isolate issues. Having a debug view showing barycentrics only (no shader binding table requirements) can be useful.
  • Check Geometry: Visually verify output from dynamic sources, such as deformed geometry or skinned meshes in a ray tracing-only view. Being able to fully disable dynamic geometry can help isolate these kinds of issues.

5. Serialize GPU/CPU Operations

Adding flags in the application to serialize GPU/CPU operations can simplify debugging.

  • Serialize at Queue Level: Serialize GPU/CPU operations at the queue level.
  • Serialize at Command List Level: Serialize GPU/CPU operations at the command list level.
  • Disable Async Compute: Disable asynchronous compute operations.
  • Disable Async Copies: Disable asynchronous copy operations.
  • Add Full Barriers: Add full barriers between compute, dispatch, and copy calls in the command lists.

Best Practices

  • Use Debug Checkpoints Sparingly: Excessive use of debug checkpoints can have a significant CPU and GPU performance cost. Aim for approximately 100 checkpoints per frame.
  • Avoid Assuming CPU Call Stack: Crashes with a call stack pointing to the driver usually manifest as a random graphics API call failing due to an internal device lost event. Use Nsight Aftermath crash dumps or debug checkpoints to pinpoint where the fault occurs.

Conclusion

Debugging API performance issues, especially those related to graphics and GPU, requires a systematic approach and the right tools. NVIDIA’s suite of debugging tools, including Nsight System, Nsight Graphics, and Nsight Aftermath, provides developers with powerful resources to identify and resolve these issues. By following the practical steps outlined in this guide, developers can improve their debugging efficiency and deliver high-performance applications.