29x Kernel Speedup Yields Only 10% End-to-End Gain Due to Memory Wall
shifu_legend · reddit · 2026-08-31
An author optimized a matmul kernel for a C99 BitNet inference engine using AVX-512BW, achieving a 29x speedup in isolation. However, profiling revealed that the workload is memory-bound, hitting 95% of DRAM bandwidth ceiling, meaning the optimized kernel only contributes a 6-10% end-to-end performance improvement. The author shares this deflating realization and asks if others have encountered similar DRAM ceilings on other hardware.
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