Comment by speedstyle

To your virtual memory comment, the kernel can only do as it's told, but allocators can indeed tell it to shuffle pages around in virtual memory. On Linux that's `mremap` [0], and `realloc` implementations [1] use it for large enough Vecs (apparently 128kiB for glibc and musl). macOS' libmalloc will try to map new pages that extend large allocations in-place, but memcpy elsewhere if existing virtual mappings are in the way. I don't think Windows' HeapReAlloc does either, but they might reserve a larger virtual region and incrementally commit it or something to similar effect.

[0] https://man7.org/linux/man-pages/man2/mremap.2.html

[1] https://git.musl-libc.org/cgit/musl/tree/src/malloc/mallocng...