Rust tracing with JSON subscriber in a container without TTY uses
fully-buffered stdout — logs only flush on buffer full or process exit.
This made H100 training appear stuck for 28+ minutes with no output.
stdbuf -oL forces line buffering so epoch logs appear immediately.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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