The hang was caused by cudarc accumulating stale CudaEvent errors
from CUDA Graph replays. EventTrackingGuard + check_err must run
every step to drain these errors. The cuStreamSynchronize is only
needed on the very first call to clear the initial backlog from
graph capture — subsequent steps rely on stream ordering.
Guard (~100ns) + check_err (~50ns) per step is negligible.
The cuStreamSynchronize (~150µs pipeline drain) is eliminated
on all but the first step.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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