Type alignment (understanding memory layout)

[luca3s]

Program to continuously allocate memory for testing purposes

simple code that constructs a doubly-linked list of pages, ensuring they're allocated and not duplicates, so linux has to actually use that many physical pages: use std::cell::Cell; #[derive(Default)] #[repr(align(4096))] struct Page { next: Cell<Option<&'static Page>>, prev: Cell<Option<&'static Page>>, } fn main() { let head = &*Box::leak(Box::new(Page::default())); let mut tail = head; for memory_usage in (2u64 * 4096..).step_by(4096) { let next = &*Box::leak(B…

This should be the relevant part. This forces the OS to allocate one page per object, without having to write a lot to the page (so it stays fast).
Alingment here is equal to the page size, so that the object can only be placed at the beginning of a page.

similarly I have seen some concurrent data structures which put an alignment equal to the size of a cache line on their locks, so that there is no "false sharing" and each lock gets its own cache line.