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Chapter 20 Smaller Tables

Chapter 20 Smaller Tables. Chien -Chung Shen CIS, UD cshen@cis.udel.edu. Problem. Big page table 32-bit address space (2 32 bytes) with 4KB (2 12 byte) pages and 4-byte page table entry 2 32 /2 12 pages and 4MB page table per process Problem: how to make page table smaller?

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Chapter 20 Smaller Tables

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  1. Chapter 20Smaller Tables Chien-Chung Shen CIS, UD cshen@cis.udel.edu

  2. Problem • Big page table • 32-bit address space (232 bytes) with 4KB (212 byte) pages and 4-byte page table entry • 232/212 pages and 4MB page table per process • Problem: how to make page table smaller? • The simplest way • bigger pages • e.g., 16KB pages -> 1MB page table • internal fragmentation

  3. Hybrid Approach • Paging + segmentation PFN validprotpresentdirty 10 1 r-x 1 0 - 0 — - - - 0 — - - - 0 — - - 23 1 rw- 1 1 - 0 — - - - 0 — - - - 0 — - - - 0 — - - - 0 — - - - 0 — - - - 0 — - - - 0 — - - - 0 — - - 28 1 rw- 1 1 4 1 rw- 1 1 • Anythingbad? • toomanyinvalidPTEs

  4. Paging + Segmentation • One page table per segment • base (physical address of page table) + bounds • 3 page tables for code, heap, and stack • on TLB miss, use Segto decide which base/bounds SN = (VirtualAddress & SEG_MASK) >> SN_SHIFT // Seg VPN = (VirtualAddress & VPN_MASK) >> VPN_SHIFT AddressOfPTE= Base[SN] + (VPN * sizeof(PTE)) • Issues: • segmentation is not quite flexible (e.g., sparsely-used heap) • external fragmentation caused by page tables

  5. Multi-level Page Tables • How to get rid of those invalid regions in page table (instead of keeping them in memory) ? • turns linearpage table into something like a tree • chop up page table into page-sized units • if an entire page of PTEs is invalid, don’t allocate that page of page table at all • page directory PFN validprotpresentdirty 10 1 r-x 1 0 - 0 — - - - 0 — - - - 0 — - - 23 1 rw- 1 1 - 0 — - - - 0 — - - - 0 — - - - 0 — - - - 0 — - - - 0 — - - - 0 — - - - 0 — - - - 0 — - - 28 1 rw- 1 1 4 1 rw- 1 1

  6. Multi-level Page Tables Page directory entry (PDE)

  7. Multi-level Page Tables • Pros • page-table space allocated is proportional to amount of address space used • growing page table by getting more page frames (which could be anywhere in physical memory • Cons • on TLB miss, need two memory accesses to get translation info (one for page directory and one for PTE itself) • time-space trade-off • complexity: perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away-- Antoine de Saint-Exupery

  8. Inverted Page Table • Instead of having many page tables (one per process), a single page table that has an entry for each physical page of the system • which process is using this page (frame) • which virtual page of that process maps to this physical page

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