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The assistant continued optimizing the DFlash training pipeline, focusing on the `pack_hidden` and CPU copy path. After diagnosing NaN losses from unsafe GPU packing on a second CUDA stream, the assistant implemented a safe async-copy path where the target thread retains GPU packing, and a background thread handles D2H completion and queue publishing. The user then provided a set of directives: keep `hs-min-ready` at 10 for sequence-length mixing, remove gradient norm W&B logging, defer drafter metrics sync, pre-allocate target buffers, enable expandable segments, and warm up FLA autotune. The assistant committed the state, implemented these changes, fixed a bug in the async metric copy, and launched a stable run. The user requested additional W&B metrics that wouldn't impact GPU performance. The assistant added low-overhead observability including profile timing snapshots, NVML GPU telemetry (utilization, memory, power, temperature), queue health ratios, per-worker counters, and CUDA allocator stats. The user then decided to change the hidden state buffer defaults to `min_ready=30` and `max_depth=90` to improve training signal smoothness, as the previous setting often resulted in pulling from only the long-sequence bucket. The assistant deployed these changes and restarted training from scratch. The user requested an evaluation of the latest checkpoint against the eval harness. The assistant ran the step-4000 checkpoint and the z-lab baseline on a 10-task coding set. Results showed the current model significantly behind z-lab (DDTree-8: 7.28 vs 11.26). Based on this, the user directed the assistant to kill the training process and deploy the z-lab DFlash model on the Pro6000 hardware with DDTree and a draft length of 16. The assistant stopped the training and began investigating the SGLang server configuration to prepare for the z-lab DFlash deployment.

The Optimization, Evaluation, and Pivot: A Complete Arc in DFlash Training 3572 words

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