Multi-level page tables are tree-like structures to hold page tables. As an example, consider a two- level page table, again on a 32-bit architecture with 212 = 4 kbyte pages. Now, we can divide the virtual address into three parts: say 10 bits for the level-0 index, 10 bits for the level-1 index, and again 12 bits for the offset within a page.
The entries of the level-0 page table are pointers to a level-1 page table, and the entries of the level-1 page table are PTEs as described above in the single-level page table section. Note that on a 32-bit architecture, pointers are 4 bytes (32 bits), and PTEs are typically 4 bytes.
So, if we have one valid page in our process, now our two-level page table only consumes (210
level-0 entries)(22
bytes/entry)+1(210
level-1 entries)(22
bytes/entry) = 2212
bytes = 8 kbytes:
For processes with sparse virtual memory maps, this is clearly a huge savings, made possible by the additional layer of indirection.
Note that for a process which uses its full memory map, that this two-level page table would use slightly more memory than the single-level page table (4k+4M versus 4M). The worst-case memory usage, in terms of ef?ciency, is when all 210 level-1 page tables are required, but each one only has a single valid entry.
In practice, most page tables are 3-level or 4-level tables. The size of the indices for the different levels are optimized empirically by the hardware designers, then these sizes are permanently set in hardware for a given architecture.