Constant HASH_THREADS 

const HASH_THREADS: u32 = 256;

The threads of a band_hash block, as the kernel sizes its reduction, and the blocks a band is split into across its columns, so a frame’s few bands fill the GPU: a P100 hashed 1080p in 0.08 ms this way and in 0.12 with a block a band, its 34 bands leaving 22 of the 56 multiprocessors idle.