coalescing

A warp issues one load instruction and the memory system turns it into some number of 32-byte sector requests; that number is the whole story. The arithmetic for contiguous, stride-2 and stride-32 access, the row-major matrix where a loop order change gives 8x, and the two Nsight Compute counters that show the waste.

Thirty-two addresses per warp instruction, thousands of instructions: say what the pattern is and how many sectors it cost. The address-delta test for contiguous and strided, the sector count that measures the damage, the code that does both, and the edge cases (misalignment, inactive lanes, mixed widths).

A transpose moves every byte exactly once, so a device copy is the honest ceiling. Why the naive kernel pays eight times the write traffic, why staging through shared memory does nothing until you pad, and the counter that tells the two problems apart.