Why is iterating though `std::vector` faster than iterating though `std::array`?

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The difference is due to memory pages of array not being resident in process address space (global scope array is stored in .bss section of the executable that hasn’t been paged in, it is zero-initialized). Whereas vector has just been allocated and zero-filled, so its memory pages are already present.

If you add

std::fill_n(v.data(), n, 1); // included in <algorithm>

as the first line of main to bring the pages in (pre-fault), that makes array time the same as that of vector.

On Linux, instead of that, you can do mlock(v.data(), v.size() * sizeof(v[0])); to bring the pages into the address space. See man mlock for full details.

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