@zappazack found a real regression: pin_memory page-locks RAM that
NVIDIA's Windows driver wants for shared-memory VRAM-overflow fallback
when VRAM saturates. Page-locked pages can't be relocated, so the driver
falls back to PCIe thrashing - step times went from 1.5s to 20+s when
VRAM saturated on RTX 5090 + Qwen LoRA + 1024x1024.
The original "strict-improvement" claim was wrong. Convert to opt-in:
- Add `pin_memory: bool` field on DatasetConfig (default False)
- Only pin when both CUDA is available AND user opts in via config
Users with stable VRAM headroom can opt in for the transfer speedup.
Users at the VRAM ceiling stay safe with the default.
Refs #758.
Booyaka101
0c8793ff47