Multi-level page tables
To reduce the memory overhead of a single large page table, modern systems use a hierarchical paging structure called a multi-level page table. Instead of a single, flat table mapping all virtual pages to physical frames, the multi-level approach breaks this into a series of smaller nested page tables.
At the top level, an outer page table contains pointers to lower-level page tables, continuing down the hierarchy until reaching the final level (the “inner page table”), which directly maps virtual pages to physical frames.
This approach optimizes memory usage by allocating only the page tables that are needed—a technique known as “on-demand allocation” or “sparse paging”. If a virtual memory region is never accessed, its corresponding page tables are never created, saving memory.
Adding more levels increases granularity, reducing wasted space, but comes at the cost of more memory lookups per access, potentially increasing TLB misses and reducing performance.
