Iced: Crash With Antialiasing Off On Dual-GPU Systems

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Iced: Crash with Antialiasing Off on Dual-GPU Systems

A bug report indicates that setting antialiasing to false in Iced applications can cause crashes on systems with dual GPUs. This article dives into the details of the issue, its potential causes, and how to reproduce it.

The Problem: Incompatible GPU Selection

The core issue revolves around the selection of the wrong GPU when antialiasing is disabled. This problem manifests specifically on machines equipped with both an integrated Intel GPU (iGPU) and a dedicated NVIDIA GPU (dGPU). The system in question runs Ubuntu 25.04, and the expectation is that the NVIDIA dGPU handles the primary graphical workload. However, when antialiasing is turned off, the application incorrectly selects the Intel iGPU, leading to a crash.

Initial Report

The bug was initially reported in the Halloy project. The application would crash upon startup due to the incorrect GPU selection. Disabling the Intel GPU in the BIOS resolved the issue, suggesting a conflict when both GPUs are active. The application functions correctly when only either the Intel or NVIDIA GPU is present.

Debugging Insights

When running Halloy with both GPUs enabled, the following sequence of events occurs:

  1. The application starts and loads configurations.
  2. It identifies available adapters, including the NVIDIA dGPU, Intel iGPU, and a software-based llvmpipe adapter.
  3. Critically, it selects the Intel iGPU instead of the NVIDIA dGPU.
  4. The application proceeds to initialize the selected GPU and attempts to set the window theme.
  5. A panic occurs during the window theme change, triggered by an XError with the description "BadMatch (invalid parameter attributes)."

Reproducing the Crash

The issue can be consistently reproduced by disabling antialiasing in an Iced application. A simple modification to the Iced todos example demonstrates this:

diff --git a/examples/todos/src/main.rs b/examples/todos/src/main.rs
index 6dee5b66..d7ef5b1f 100644
--- a/examples/todos/src/main.rs
+++ b/examples/todos/src/main.rs
@@ -26,6 +26,7 @@ fn application() -> Application<impl Program<Message = Message>> {
         .title(Todos::title)
         .font(Todos::ICON_FONT)
         .window_size((500.0, 800.0))
+        .antialiasing(false)
         .presets(presets())
 }

Running the modified todos example triggers the same crash, confirming that disabling antialiasing causes the application to select the incorrect GPU and subsequently panic.

Technical Analysis: Why Does This Happen?

The underlying cause appears to be related to how Iced and its dependencies (specifically winit and wgpu) handle GPU selection when antialiasing is disabled. Here's a breakdown of the potential factors:

1. GPU Prioritization Logic

When antialiasing is enabled, the system likely defaults to the more powerful discrete GPU (NVIDIA). However, when antialiasing is disabled, the GPU selection logic might prioritize the integrated GPU (Intel) due to power-saving considerations or other internal heuristics.

2. Driver and Backend Interactions

The interaction between the graphics drivers (NVIDIA and Intel), the graphics backend (Vulkan or OpenGL), and the windowing system (X11) could also contribute to the issue. The BadMatch error suggests an incompatibility between the selected GPU and the expected window attributes.

3. Winit's Window Theme Setting

The crash occurs during the set_theme function in winit. This function attempts to apply the system's theme to the application window. The BadMatch error indicates that the selected GPU cannot handle the theme attributes, possibly due to driver or backend limitations on the Intel iGPU.

4. wgpu-hal and Adapter Selection

The wgpu-hal crate is responsible for abstracting over different graphics APIs. It enumerates available adapters (GPUs) and allows the application to select one. The selection process might be flawed when antialiasing is disabled, leading to the iGPU being chosen incorrectly.

Potential Solutions and Workarounds

Several potential solutions and workarounds exist to address this issue:

1. GPU Selection Configuration

Expose a configuration option that allows users to explicitly specify which GPU to use. This would override the automatic selection logic and ensure the application always uses the desired GPU.

2. Conditional GPU Selection Logic

Modify the GPU selection logic in Iced to prioritize the discrete GPU, regardless of the antialiasing setting. This could involve checking for the presence of a discrete GPU and explicitly selecting it if available.

3. Driver and Backend Updates

Ensure that the latest graphics drivers are installed for both the Intel and NVIDIA GPUs. Additionally, experiment with different graphics backends (e.g., Vulkan, OpenGL) to see if one backend resolves the issue.

4. winit Patch

Investigate the winit crate's window theme setting logic and identify potential issues that cause the BadMatch error. A patch to winit might be necessary to address the underlying problem.

5. Iced Patch

Apply a patch to Iced that bypasses the set_theme function when antialiasing is disabled. This would prevent the crash but might result in the application not respecting the system's theme.

Detailed Analysis of the Code and Backtrace

To fully understand the issue, let's examine the relevant code snippets and backtrace:

Code Snippets from the Report:

The provided code snippets show the following:

  • Settings: The application settings indicate that antialiasing is explicitly set to None.
  • Available Adapters: The list of available adapters shows both the NVIDIA and Intel GPUs, as well as the llvmpipe software renderer.
  • Selected Adapter: The Intel iGPU is incorrectly selected.
  • Panic Message: The panic message indicates a failure to change the window theme due to a BadMatch XError.

Backtrace Analysis:

The backtrace provides a detailed call stack leading to the panic. Key frames include:

  • winit::platform_impl::linux::x11::window::UnownedWindow::set_theme: This is the function where the panic occurs. It attempts to set the window theme using X11.
  • winit::platform_impl::linux::Window::set_theme: This function calls the UnownedWindow::set_theme function.
  • iced_winit::run_instance::{{closure}}: This is the Iced code that triggers the window theme setting.
  • winit::event_loop::EventLoop<T>::run_app: This is the main event loop of the winit library.
  • iced_winit::run: This is the main function in iced_winit that starts the application.

By examining the code around these frames, developers can pinpoint the exact location where the error occurs and understand the sequence of events leading to the crash.

Community Contributions and Further Investigation

Community contributions and further investigation are crucial to resolving this issue. Developers can:

  • Share their experiences and insights on the Iced issue tracker.
  • Contribute patches to Iced, winit, or wgpu.
  • Provide detailed bug reports with reproducible steps.
  • Test potential solutions and workarounds.

Conclusion

The crash caused by disabling antialiasing on dual-GPU systems is a significant issue that affects Iced applications. By understanding the underlying causes and potential solutions, developers can work towards resolving this problem and ensuring that Iced applications function correctly on a wide range of hardware configurations. Further investigation, community contributions, and targeted patches are essential to achieving a robust and reliable solution.

This comprehensive analysis provides a solid foundation for addressing the bug and improving the overall stability of Iced applications on dual-GPU systems. Keep an eye on future updates and developments related to this issue as the Iced community collaborates to find a definitive fix.