Memory Management: Review of Paging and Virtual Memory

1. Paging and Virtual Memory

2. Memory management: Review • Fixed partitioning, dynamic partitioning • Problems • Internal/external fragmentation • A process can be loaded only if a contiguous memory chunk is available to accommodate the process • Process size is limited by the main memory size • Advantage: simplicity

3. Paging • Process memory is divided into fixed size chunks of the same size, called pages • Pages are mapped onto frames in the main memory • Process pages can be scattered all over the main memory

4. 0 A.0 1 A.1 2 0 A.2 3 1 A.3 4 2 D.0 5 3 D.1 6 4 D.2 7 C.0 8 C.1 9 C.2 10 C.3 11 D.3 12 D.4 13 14 Paging example 0 0 0 --- 1 1 1 --- 2 2 2 --- 3 3 Process B Process A 0 7 4 13 1 8 5 14 2 9 6 Free Frame List 3 10 11 12 Process C Process D

5. Paging support • Page table maintains mapping of process pages onto frames • Hardware support is needed to support translation of relative addresses within a program (logical addresses) into the memory addresses

6. Address translation • Page (frame) size is a power of 2 • with page size = 2r, a logical address of l+r bits is interpreted as a tuple (l,r) • l = page number, r = offset within the page • Page number is used as an index into the page table

7. Hardware support Logical address Page # Offset Frame # Offset Register Page Table Ptr Page Table Offset Page Frame P# + Frame # Program Paging Main Memory

8. Virtual Memory • Paging makes virtual memory possible • Logical to physical address mapping is dynamic => It is not necessary that all of the process pages be in main memory during execution

9. Benefits • More processes may be maintained in the main memory • Better system utilization and throughput • The process size is not restricted by the physical memory size: the process memory is virtual • But what is the limit anyway? • Less disk I/O to swap/load programs

10. How does this work? • CPU can execute a process as long as some portion of its address space is mapped onto the physical memory • E.g., next instruction and data addresses are mapped • Once a reference to an unmapped page is generated (page fault): • Page fault interrupt transfers control to the OS handler

11. Page Fault Handler • Put the process into blocking state • Program disk controller to read the page from disk into the memory • Later on: I/O interrupt signals completion • Resume the process

12. Why is this practical? • Observation: Program branching and data access patterns are not random • Principle of locality: program and data references tend to cluster => Only a fraction of the process virtual address space need to be resident to allow the process to execute for sufficiently long

13. Virtual memory implementation • Efficient run-time address translation • Hardware support, control data structures • Fetch policy • Demand paging: page is brought into the memory only when page-fault occurs • Pre-paging: pages are brought in advance • Page replacement policy • Which page to evict when a page fault occurs?

14. Thrashing • A condition when the system is engaged in moving pages back and forth between memory and disk most of the time • Bad page replacement policy may result in thrashing • Programs with non-local behavior

15. Address translation • Virtual address is divided into page number and offset • Mapping of virtual pages onto physical frames are facilitated by page table(s) • Forward-mapped page tables (FMPT) • Inverted page tables (IPT) Virtual Address Page Number Offset

16. Forward-mapped page tables (FMPT) • Page table entry (PTE) structure • Page table is an array of the above • Index is the virtual page number Frame Number P M Other Control Bits P: present (valid) bit M: modified bit Page Table Page # Frame #

17. Address Translation using FMPT Virtual address Page # Offset Frame # Offset Register Page Table Ptr Page Table Offset Page Frame P# + Frame # Program Paging Main Memory

18. Handling large address spaces • One level FMPT is not suitable for large virtual address spaces • 32 bit addresses, 4K (212) page size, 232 / 212 = 220 entries ~4 bytes each => 4Mbytes resident page table per process! • What about 64 bit architectures?? • Solutions: • multi-level FMPT • Inverted page tables (IPT)

19. Multilevel FMPT • Use bits of the virtual address to index a hierarchy of page tables • The leaf is a regular PTE • Only the root is required to stay resident in main memory • Other portions of the hierarchy are subject to paging as regular process pages

20. Two-level FMPT page number page offset pi p2 d 10 10 12

21. Two-level FMPT

22. Inverted page table (IPT) • A single table with one entry per physical page • Each entry contains the virtual address currently mapped to a physical page (plus control bits) • Different processes may reference the same virtual address values • Address space identifier (ASID) uniquely identifies the process address space

23. Address translation with IPT • Virtual address is first indexed into the hash anchor table (HAT) • The HAT provides a pointer to a linked list of potential page table entries • The list is searched sequentially for the virtual address (and ASID) match • If no match is found -> page fault

24. Address translation with IPT Virtual address page number offset frame number register HAT ASID page number ASID + hash Frame# + IPT base HAT base register register IPT

25. Translation Lookaside Buffer (TLB) • With VM accessing a memory location involves at least two intermediate memory accesses • Page table access + memory access • TLB caches recent virtual to physical address mappings • ASID or TLB flash is used to enforce protection

26. TLB internals • TLB is associative, high speed memory • Each entry is a pair (tag,value) • When presented with an item it is compared to all keys simultaneously • If found, the value is returned; otherwise, it is a TLB miss • Expensive: number of typical TLB entries: 64-1024 • Do not confuse with memory cache!

27. Address translation with TLB

28. Bits in the PTE: Present (valid) • Present (valid) bit • Indicates whether the page is assigned to frame or not • A reference to an invalid page generates page fault which is handled by the operating system

29. Bits in PTE: modified, used • Modified (dirty) bit • Indicates whether the page has been modified • Unmodified pages need not be written back to the disk when evicted • Used bit • Indicates whether the page has been accessed recently • Used by the page replacement algorithm

30. Bits in PTE • Access permissions bit • indicates whether the page is read-only or read-write • UNIX copy-on-write bit • Set whether more than one process shares a page • If one of the processes writes into the page, a separate copy must first be made for all other processes sharing the page • Useful for optimizing fork()

31. Protection with VM • Preventing processes from accessing other process pages • Simple with FMPT • Load the process page table base address into a register upon context switch • ASID with IPT

32. Page size considerations • Small page size • better approximates locality • large page tables • inefficient disk transfer • Large page size • internal fragmentation • Most modern architectures support a number of different page sizes • a configurable system parameter

33. Next: Page replacement