I Built a C++ Backend So My GPU Would Stop Eating Air

WarpGroup-Backend is a C++ engine that eliminates padding waste in LLM inference by implementing First-Fit Decreasing (FFD) bin packing for variable-length sequences. Instead of padding all sequences to the longest one (wasting up to 48% of GPU compute on zeros), it concatenates sequences into dense 1D ribbons and feeds them to FlashAttention-2's varlen kernel. Key engineering components include: empirical VRAM autotuning (Phase 0 probes until OOM then backs off 10%), 16-token Tensor Core alignment, GIL-free async C++ dispatch via PyBind11, and pinned-memory async DMA transfers. Benchmarks show 2.08× throughput on H100 with Qwen2.5-7B and 5.89× on GTX 1080 with SmolLM2-360M. The author draws a compelling parallel between this GPU bin-packing approach and 5G MAC scheduling, noting both solve the same problem of packing variable-size payloads into fixed hardware budgets under alignment constraints.