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Demand Paging CSE 120: Principles of Opera�Ng Systems Lecture 9: Demand Paging CSE 120: Principles of Operang Systems UC San Diego: Summer Session I, 2009 Frank Uyeda Announcements • Project 2 is due on Tonight • Homework 3 is due Monday. • Final Exam: 3p‐6p on Saturday, August 1 2 PeerWise • Processes are loaded and then removed from memory, creang free memory space broken into lile pieces. What type of fragmentaon is this? – Internal – External 3 PeerWise • What is the difference between "Segmenon" and "Paging"? (Choose the one with the highest truth value) – A. Le has the answer. – B. Segmentaon is non‐connous allocaon of pages of memory, where paging is connous allocaon of pages of memory. – C. Segmentaon is the use of variable page sizes, and paging is the use of discrete page sizes. – D. C – E. D & A • Really?! – None of the answers are correct… except, maybe A. 4 V Page Frame Recap: Paging 1 0x04 Process 1’s VAS 1 0x01 Physical Memory 1 0x05 Page 1 0x00000000 1 0x07 Page 1 Page 2 0x00000400 Process 2’s VAS Page 2 Page 3 MMU ….. Page 1 TLB Page 3 Page ... Page M Page 2 Table Page P Page 3 V Page Frame .. ….. 1 0x04 1 0x01 Page N Page Q 0x00040000 5 1 0x05 1 0x07 Recap: Virtual Memory • Memory Management Unit (MMU) – Hardware unit that translates a virtual address to a physical address • Translaon Table – Stored in main memory – Each Page Table Entry stores permission and bookkeeping bits. • Translaon Lookaside Buffer (TLB) – Hardware cache for the MMU’s virtual‐to‐physical translaons table Virtual Address M R V Prot Page Frame Number Translaon Physical CPU MMU Table Address Memory TLB 6 Recap: Page Tables Physical Memory 0x00000000 Virtual Address Page number Offset Page 1 Physical Address Page 2 Page frame Offset Page Table Page 3 ….. Page table entry Page N 0xFFFFFFFF Problem: Page Tables can be large! 7 Recap: 2 Level Page Table Physical Memory 0x00000000 Virtual Address Master Secondary Offset Page 1 Physical Address Page 2 Page frame Offset Page 3 ….. Page frame (PTE) Page frame (PTE) Page table entry Page N Master Secondary 0xFFFFFFFF Page Table Page Table(s) 8 Inverted Page Table Physical Memory 0x00000000 Virtual Address PID VPN Offset Page 1 Physical Address Page 2 index Offset Page 3 index ….. <PID, VPN> Page N Inverted Page Table 0xFFFFFFFF 9 Inverted Page Table • One global page table – One page table entry per physical page. – Entries keyed by PID and virtual page number. – Physical frame number = index in page table. • Advantages – Bounded amount of memory for page table(s). • 32‐bit address space, 4K pages, 4GB RAM, 4B per PTE • 1‐level page table: _____ x # processes. • Inverted Table: ______ MB • Disadvantages – Costly translaon • Using hashing can help 10 Recap: Segmentaon Heap Stack Text Data 11 Recap: Segmentaon Physical Memory Segment Table Virtual Address limit base Segment # Offset Yes? < + No? Protecon Fault 12 Goals for Today • Paging Tricks – Demand Paging – Copy on Write • Page Replacement Policies – Which pages should we keep in memory? – How many pages should each process get? 13 Paged Virtual Memory • What if there isn’t enough physical memory available to load a program? – A large poron of program’s code may be unused – A large poron of program’s code may be used infrequently • Idea: load a page only if you need it Process’s Virtual Address Space Machine’s Physical Address Space (available RAM) 14 Demand Paging • We’ve menoned before that pages can be moved between memory and disk – This process is called demand paging • OS uses main memory as a page cache of all the data allocated by processes in the system – Inially, pages are allocated from memory – When memory fills up, allocang a page in memory requires some other page to evicted from memory • Why physical memory pages are called “frames” – Evicted pages go to disk (where? The swap file paron) – The movement of pages between memory and disk is done by the OS, and is transparent to the applicaon 15 Paged Virtual Memory • A program’s pages loaded on demand Machine’s Physical Address Space • Where are pages otherwise? (available RAM) – On disk (executable) – In OS swap file (an actual file on disk) – Shared (more later) Process’s Virtual Address Space Disk ~/program.coff /swap 16 Demand Paging Example Process’s Virtual Address Space Physical Memory PC PC Page Table VPN V PPN 0x00 1 0x00 0x01 0 1 0x01 ‐‐‐ 0x02 0 ‐‐‐ 0x03 0 ‐‐‐ … Disk 0x10 0 ‐‐‐ 0x11 0 ‐‐‐ ~/program.coff /swap … … 17 Demand Paging Example Process’s Virtual Address Space Physical Memory 0x00 PC Page Table VPN V PPN 0x00 1 0x00 0x01 0 1 0x01 ‐‐‐ 0x09 0x02 1 0x02 0x0A 0x03 1 0x03 0x04 1 … 0x09 Disk 0x10 1 0 0x00 ?? 0x09 0x11 0 ‐‐‐ ~/program.coff /swap … … 18 Page Faults • What happens when a process accesses a page that has been evicted (or is otherwise not in memory)? – 1. When it evicts a page, the OS sets the PTE as invalid and stores the locaon of the page in the swap file in the PTE – 2. When a process accesses the page, the invalid PTE will cause a trap (page fault) – 3. The trap will run the OS page fault handler – 4. Handler uses the invalid PTE to locate page in swap file – 5. Reads page into a physical frame, updates PTE to point to it – 6. Restarts process • But where does it put it? Has to evict something else – OS usually keeps a pool of free pages around so that allocaons do not always cause evicons – Replace another page • Evicted page may go in swap file. Which pages need to be wrien out to swap ? 19 Recap: Page Lookups Virtual Address CPU TLB Page Table Disk Physical Memory Address 20 Recap: Page Lookups Virtual Address CPU TLB ..... Secondary Page Table Page Table Disk Physical Memory Address 21 Recap: Page Lookups Virtual Address Case 1: Virtual Address –to‐ Physical Address Mapping is in TLB CPU VPN V R M RO PFN 0x000 1 1 0 1 0x100 TLB 0x121 1 0 0 0 0x111 Secondary Page Table 0x071 Page Table 1 1 0 0 0x212 (1) How many memory references? Disk TLB_hit: 1 memory reference Physical Memory Address 22 Recap: Page Lookups Virtual Address Case 2: Virtual Address –to‐ Physical Address Mapping is not in TLB but is in Memory CPU VPN V R M RO PFN 0x000 1 1 0 1 0x100 TLB 0x121 1 0 0 0 0x111 Secondary (1) Page Table Page Table 0x071 1 1 0 0 0x212 (2) Disk How many memory references? Physical Mem_hit: 1 page table lookup Memory Address + 1 memory reference Two‐level Page table? 23 Recap: Page Lookups Virtual Address Case 3: Virtual Address –to‐ Physical Address Mapping is not in TLB and not in Memory CPU VPN V R M RO PFN 0x000 1 1 0 1 0x100 TLB 0x121 0 0 0 0 0x111 Secondary (1) Page Table (3?) Page Table 0x071 1 1 0 0 0x212 (2) (4?) How many memory references? Mem_miss: 1 page table lookup + 1 page table update(s)* Disk Physical + 1 memory reference Memory Address How much me? disk retrieval involved too 24 Memory Accesses • TLB Hit: fast – 1 TLB access + 1 memory access • Memory Hit: medium – 1 TLB access + 2+ memory access • Memory Miss: slow – 1 TLB access + 3+ memory access + 1 disk access • Want things fast as possible; minimize me spent resolving physical address. How do you do this? – Minimize number of memory references (TLB policy) – Minimize number of page faults (Page replacement policy) 25 Observaon • OSes spend a lot of me copying data – System call arguments between user/kernel space – Enre address spaces to implement fork() • How can we reduce this cost? 26 Page Sharing Emacs buffer #1 Emacs buffer #2 VPN V PPN VPN V PPN 0x00 1 0x03 0x00 1 0x03 0x04 0x010x03 0 1 0x11 ‐‐‐ 0x010x03 0 1 0x13 ‐‐‐ 0x10 1 0x12 0x10 1 0x14 0x11 1 0x15 0x11 1 0x16 Physical Memory .. What if shared page is modified? 27 Copy‐On‐Write • OSes spend a lot of me copying data – System call arguments between user/kernel space – Enre address spaces to implement fork() • Use Copy‐on‐Write (CoW) to defer large copies as long as possible, hoping to avoid them altogether – Instead of copying pages, create shared mappings of parent pages in child virtual address space – Shared pages are protected as read‐only in child • Reads happen as usual • Writes generate a protecon fault, trap to OS, copy page, change page mapping in client page table, restart write instrucon • How does this help fork()? 28 Page Sharing: CoW Emacs buffer #1 VPN V M PPN Emacs buffer #2 0x00 1 0 0x03 0x010x03 0 1 0 ‐‐‐ 0x11 0x10 1 0 1 0x12 0x11 1 0 0x15 VPN V M PPN 0x00 1 0 0x03 0x010x03 0 1 0 ‐‐‐ 0x13 0x10 1 0 0x12 0x11 1 0 0x16 Physical Memory Buffers share VPN 0x12 .. Process writes to page 29 Page Sharing: CoW Emacs buffer #1 VPN V M PPN Emacs buffer #2 0x00 1 0 0x03 0x010x03 0 1 0 ‐‐‐ 0x11 0x10 1 0 0x12 0x11 1 0 0x15 VPN V M PPN 0x00 1 0 0x03 0x010x03 0 1 0 ‐‐‐ 0x13 0x10 1 0 0x14 0x11 1 0 0x16 Physical Memory Buffers share VPN 0x12 .. Process writes to page New page is allocated 30 Paging Tricks Summary • Demand Paging – Observaon: Limited available RAM, not all pages used – Idea: Load pages only when needed • Page Sharing – Observaon: Oen mes, processes have similar virtual pages – Idea: Copy‐on‐write for efficient page sharing 31 Actual Memory Stascs 3.7 GB used, 4.1 GB available => 90+% memory used 2.1 GB swap allocated 32 Actual Memory Stascs ~10% ‐ 50% of memory is shared per process => ~370 MB – 1.85 GB mem saved 33 Actual Memory Stascs Which pages to evict? How much physical memory should each process get? 34 Page Replacement • Problem: Memory is scarce – Idea: let’s use only pages that we need • Page replacement policy: – How do we decide which pages to kick out when memory is full or near full? – How much memory do we allocate to each process? 35 Page Replacement Policy • Want to keep most acve pages in memory – Conversely, want to kick out inacve pages • What can we do? – Swapping:
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