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The multilevel page table may keep a few of the smaller page tables to cover just the top and bottom parts of memory and create new ones only when strictly necessary. Now, each of these smaller page tables are linked together by a master page table, effectively creating a tree data structure. There need not be only two levels, but possibly ...
This increases the importance of caching values from intermediate levels of the host and guest page tables. It is also helpful to use large pages in the host page tables to reduce the number of levels (e.g., in x86-64, using 2 MB pages removes one level in the page table). Since memory is typically allocated to virtual machines at coarse ...
A system with a smaller page size uses more pages, requiring a page table that occupies more space. For example, if a 2 32 virtual address space is mapped to 4 KiB (2 12 bytes) pages, the number of virtual pages is 2 20 = (2 32 / 2 12). However, if the page size is increased to 32 KiB (2 15 bytes), only 2 17 pages are required. A multi-level ...
For a single-level page table, addresses are broken down into a set of bits to index the page table and remaining bits that pass through to the physical address without modification, indexing a byte within the page. For a two-level page table, addresses are borken down into a set of bits to index the root level of the tree, a set of bits to ...
The page table, generally stored in main memory, keeps track of where the virtual pages are stored in the physical memory. This method uses two memory accesses (one for the page-table entry, one for the byte) to access a byte. First, the page table is looked up for the frame number.
An access descriptor contains permission bits and a 26-bit object index; the object index is an index into a table of object descriptors, giving an object type, an object length, and a physical address for the object's data, a page table for the object, or the top-level page table for a two-level page table for the object, depending on the ...
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Satisfying page faults may require freeing pages that will soon have to be re-read from disk. "Thrashing" is also used in contexts other than virtual memory systems; for example, to describe cache issues in computing or silly window syndrome in networking. A worst case might occur on VAX processors. A single MOVL crossing a page boundary could ...