Overview

ukoOS has multiple strategies for memory management, that manage memory at different levels.

• The kernel has a standard memory allocator, accessed with the functions alloc and free. These functions act similarly to malloc and free in ordinary C.

  This is based on the design from Mimalloc: Free List Sharding in Action; read that if you want to understand the design.

  The allocator that handles these requests is called the heap memory allocator.

• The kernel keeps track of all of RAM, and hands out pages to be mapped into userspace processes and to be used by the heap memory allocator.

  This allocator is a simple free list.

  This allocator is currently not capable of allocating more than a single contiguous page, but could be extended to support this in the future. This allocator is called the physical memory allocator.

• The kernel manages its own virtual memory, in the RAM region of the memory map.

  This allocator is a pair of treaps, one for all VMAs sorted by address, and another for only free VMAs sorted by size. If you're not already familiar with treaps, there's a Julia Evans piece about them: Data structure: the treap!

  The allocator that handles these requests is called the virtual memory allocator.

Each hart has its own root page table, since it can be running a different userspace process. However, the kernel's memory map is kept in sync between all harts.

Higher-half memory is only rarely mapped and unmapped, so relatively inefficient mechanisms (a full TLB shootdown) can be used to ensure all harts have the same view of it.