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Chained command lists in DirectX 12 enable more efficient GPU utilization by allowing multiple command lists to be executed sequentially without CPU intervention between each list. This can improve performance by reducing overhead and keeping the GPU busy.

Key Concepts

• Command Lists: Represent a sequence of commands for the GPU.
• Command Queues: Where command lists are submitted for execution.
• ExecuteIndirect: Mechanism to launch GPU workloads based on data computed on the GPU.

Steps to Chain Command Lists

1. Create Multiple Command Lists: Create several command lists, each containing a portion of the rendering workload.
2. Populate Command Lists: Record commands into each command list. Ensure resource barriers are correctly set up for transitions between lists.
3. Execute Command Lists: Submit the command lists to the command queue in the desired order.

Example Scenario: Render Passes

Consider a scenario with multiple render passes:

• Pass 1: Depth pre-pass.
• Pass 2: Main color pass.
• Pass 3: Post-processing pass.

Each pass can be recorded into a separate command list, and these lists can be chained together for execution.

Code Snippet (Conceptual)

This is a simplified, conceptual snippet to illustrate the idea:

// Assume commandQueue, commandList1, commandList2, commandList3 are created and populated
ID3D12CommandList lists[] = { commandList1, commandList2, commandList3 };
commandQueue->ExecuteCommandLists(3, lists);

// Signal and wait for completion
commandQueue->Signal(fence, fenceValue);
fence->SetEventOnCompletion(fenceValue, event);
WaitForSingleObject(event, INFINITE);

Benefits

• Reduced CPU Overhead: Less CPU intervention between draw calls.
• Improved GPU Utilization: Keeps the GPU busy, maximizing throughput.
• Simplified Multi-Threading: Command list recording can be parallelized across multiple threads.

Considerations

• Resource Barriers: Ensure proper resource barriers are in place to manage resource state transitions between command lists.
• Synchronization: Manage synchronization between command lists if they access shared resources.
• Debugging: Debugging chained command lists requires careful attention to resource states and synchronization.

ExecuteIndirect and Chained Command Lists

ExecuteIndirect can be used to create command lists on the GPU, which can then be executed. This is useful for scenarios where the workload is data-dependent and determined on the GPU.

Conclusion

Chaining command lists in DirectX 12 provides a powerful mechanism to optimize rendering pipelines. Understanding the key concepts and considerations is crucial for effectively leveraging this feature to improve performance.