Chapter 13. Virtual Memory

1. Chapter 13. Virtual Memory • Introduction • Demand Paging • Hardware Requirements • 4.3 BSD Virtual Memory • 4.3 BSD Memory Management Operations

2. Introduction • Memory management unit (MMU) • Responsible for getting data to and from main memory • Virtual memory • The notion of an address space as distinct from memory locations • Translation tables • Page-based

3. Introduction (cont) • Memory management in the Stone Age • Software overlay • Swapping • Demand paging • Segmentation

4. Demand Paging • Primary goal • To allow a process to run in a virtual address space • To perform translations from virtual to physical addresses transparently to the process • Functional requirements • Address space management • fork( ), exec( ) • Address translation • Virtual address  Virtual page number + offset

5. Demand Paging (cont) • Physical memory management • Memory protection • Memory sharing • Monitoring system load • Other facilities • Memory-mapped files • Dynamically linked shared libraries • etc.

6. Demand Paging (cont) • Page swapping uninitialized data pages zero-filled on first access dirty pages saved initialized data main memory swap area on disk text and initialized data executable file subsequent faults on outswapped pages stack and heap pages allocated on first access

7. Demand Paging (cont) address space map backup store map • Translation maps page fault hardware address translations Data process + virtual page number physical page number physical memory map

8. Demand Paging (cont) • Page replacement policies • Local v.s. global replacement policy • Locality of reference • Concept of working set • Least recently used (LRU) policy

9. Hardware Requirements • MMU • Translation of virtual addresses • Using page tables or TLBs • Page table entry • Physical page frame number • Protection information • a valid bit • a modified bit • a referenced bit

10. Hardware Requirements (cont) • Page table • Hardware-prescribed • Located in main memory • MMU uses only the active tables, whose locations are loaded in h/w page table registers • Typically, on a uniprocessor, there are two active page tables - one for kernel and one for the currently running process • Paging the page table to reduce the required memory

11. Hardware Requirements (cont) • Address translation may fail for three reasons • Bounds error • Validation error • Protection error • MMU caches • A high-speed cache that is searched before each memory access • Translation lookaside buffer (TLB)

12. Hardware Requirements (cont) • Intel x86 • Unix implementations on the x86 hides the notion of segmentation from user processes, who see a flat address space • Two-level page table • 4k byte page size • Control register CR3 stores the physical page number of the current page directory • CR3 register needs to be reset on each context switch

13. Hardware Requirements (cont) • Page table entry • Physical page number, protection field • Valid, referenced, and modified bit • x86 supports 4 privilege levels of which UNIX uses only two • The kernel runs in the innermost ring, which is the most privileged • User code runs in the outermost, least privileged ring

14. Address Translation on Intel x86 page directory of current process CR3 31 11 0 one of the page tables of current process PFN 31 11 0 PFN 31 21 11 0 DIR PAGE OFFSET virtual address 31 11 0 PFN OFFSET physical address

15. Intel x86 Page Table Entry 31 12 6 5 2 1 0 PFN D A U W P PFN Page Frame Number D Dirty A Accessed (Referenced) U User (0) / Supervisor (1) W Read (0) / Write (1) P Present (valid)

16. 4.3BSD • Target platform: VAX-11 • Several BSD-based implementations emulate the VAX memory architecture in software, including its address space layout and page table entry format • Core map • Describes physical memory • Page tables • Describe virtual memory

17. 4.3BSD (cont) • Disk maps • Describe the swap areas • Resource maps • Manage allocation of resources such as page tables and swap space

18. 4.3BSD Physical Memory error buffer nonpaged pool paged pool cmap[ ] (in nonpaged pool)

19. 4.3BSD Physical Memory (cont) • Core map: struct cmap • Is a kernel data structure, allocated at boot time and resident in the nonpaged pool • One entry for each frame in the paged pool • Core map entry • Name • <type, owner, virtual page number> • Text page cache • Synchronization

20. 4.3BSD Physical to Virtual address Translation proc[ ] cmap[ ] type = data owner VPN … page table type = data owner VPN … page table text[ ]

21. 4.3BSD Address Space • Uses VAX-11 address space model • 32-bit machine with 512-byte page size • 4 Giga byte address space is divided into four regions of equal size • P0 • Program region • Text and data section of the process

22. 4.3BSD Address Space (cont) • P1 • Control region • User stack, u area, kernel stack • S0 • System region • Kernel text and data • 4th region • Reserved and not supported by current VAX h/w

23. 4.3BSD Page Tables • Single system page table • Map the kernel text and data • Each process has two page tables to map its P0 and P1 • Mapped by a set of contiguous PTEs in the Userptmap section of the system page table • State of a particular page of a process • Resident • The page is in physical memory, and the page table entry contains its physical page frame number

24. 4.3BSD Page Tables (cont) • Fill-on-demand • Fill-from-text: Text and initialized data pages are read in from the executable file upon first access • Zero-fill: Uninitialized data, heap, and stack pages are created and filled with zero when required • Outswapped • These pages may be recovered from their swap area locations

25. 4.3BSD Page Tables (cont) • Kernel maintains information about all nonresident pages • For swapped out pages • Kernel must stores their location on the swap device • For zero-fill pages • Kernel only needs to recognize them as such • For fill-from-text pages • Kernel must determine their location in the filesystem

26. 4.3BSD Page Tables (cont) • Page table entry • Since all nonresident pages have the valid bit clear, other fields can be replaced by other information that tracks these pages • Kernel maintains separate swap maps to locate those pages on the swap device

27. 4.3BSD Page Tables Entry (a) VAX-11 page table entry format 31 26 20 0 V PROT M unused Page Frame Number (PFN) valid modified (b) Ordinary page table entry 31 26 25 20 0 V PROT M 0 Page Frame Number (PFN) valid modified fill-on-demand fill-from-text (1) or zero-fill (0) V PROT 1 F File System Block Number 31 26 23 0 (c) Fill-on-demand page table entry

28. 4.3BSD Swap Space • Per region dmap structure swap space data region 0 dmap dmmin 2*dmmin 4*dmmin

29. 4.3BSD Swap Space (cont) • Text pages • Once the page is brought into memory, the fill-on-demand PTE is overwritten by the page frame number • As a result, retrieving the page from the file involving recomputing its location and perhaps, accessing one or more indirect blocks • To avoid that, such pages are saved in swap as well

30. 4.3BSD Swap Space (cont) • U area holds the maps for the data and stack region • The text region swap map is part of the text structure

31. 4.3BSD Memory Management Operations • Process creation • Swap space, u area • Page tables • Kernel must allocate contiguous PTEs in Userptmap to map page tables for their process • Text region • The child is added to the list of processes sharing the text structure used by the parent • Data and Stack • Data and stack must be copied one page at a time • Copy-on-write, vfork( )

32. 4.3BSD Memory Management Operations (cont) • Page table handling • Two types of page faults • validation, protection • Validation • No PTE for that page (bounds error) • PTE is marked invalid • Pagein( ) is called to handle the fault • The faulting virtual address  PTE

33. Pagein( ) (1/2) start No Fill on demand? Yes No PFN == 0? (next page) Yes in transit text page? Yes text page onhash queue? Yes set wanted flag No (next page) No on free list? No allocate new page sleep on text struct Yes take page off free list read page from swap start over when woken up set valid bit

34. Pagein( ) (2/2) Fill on demand? Yes (previous page) Yes zero-fill? (previous page) No allocate new page Yes text page onhash queue? fill it with zeros No take page off free list Yes page in buffer cache? flush cache copy to disk No Read page from file mark page modified

35. 4.3BSD Memory Management Operations (cont) • Free page list • Ideally, to keep all garbage pages at the head of the free list, followed by some useful pages in LRU order • 4.3BSD replaces the least recently used policy by a not recently used policy • Uses referenced bit and two passes over each page • VAX-11 does not support a referenced bit in the h/w, so BSD simulates the referenced bit in software • Pagedaemon process is responsible for page replacement

36. 4.3BSD Memory Management Operations (cont) • Swapping • Problem of thrashing • Swapper process monitors the system load, and swap processes in and out when needed • Swapper will swap out a process in the following cases • Userptmap fragmentation • Memory shortfall • Inactive processes

37. 4.3BSD Memory Management Operations (cont) • Swapper performs the following task when swapping out a process • Allocates swap space for the u area, kernel stack, and page tables • Detach the process from its text region • Save the resident data and stack on swap • Release the system PTEs in Userptmap • Record the swap location of the u area in the proc structure

38. Analysis of 4.3BSD • No support for execution of remote programs • No support for sharing of memory • vfork( ) is not a true substitute for fork( ) • The lack of copy-on-write hurts the performance of Ap that rely extensively on fork • No support for memory mapped files

39. Analysis of 4.3BSD (cont) • Reserves enough swap space in advance to page out very single page in the process address space • No support for using swap space on remote nodes