The compute_cvar_scales() was doing GPU→CPU→sort→CPU→GPU for quantile
CVaR computation. With batch_size=307 × 32 quantiles × 15 actions,
this was ~150KB of DtoH + sort + HtoD per epoch — likely the source
of the 10s/epoch overhead vs 3s baseline.
New inline CUDA kernel (iqn_cvar_kernel):
- One thread per sample (256 threads/block)
- Insertion sort of alpha_count smallest quantiles in registers
- Fast path for alpha_count=1 (just find minimum)
- Zero CPU readback, zero CPU allocation
Compiled once via OnceLock, cached for all subsequent calls.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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