Chapter 20Smaller Tables

Chien-Chung ShenCIS, UD

cshen@cis.udel.edu

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

Hybrid Approach

• Paging + segmentationPFN valid prot present dirty 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

• Any thing bad?– too many invalid PTEs

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 Seg to decide which

base/boundsSN = (VirtualAddress & SEG_MASK) >> SN_SHIFT // SegVPN = (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

Multi-level Page Tables

• How to get rid of those invalid regions in page table (instead of keeping them in memory) ?– turns linear page 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 valid prot present dirty 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

Multi-level Page Tables

Page directory entry (PDE)

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

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