Chapter 9: Virtual-Memory Management

9.2 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Chapter 9: Virtual-Memory Management 9.1 Background 9.2 Demand Paging 9.3 Copy-on-Write 9.4 Page Replacement 9.5 Allocation of Frames 9.6 Thrashing 9.7 Memory-Mapped Files ( 跳過 ) 9.8 Allocating Kernel Memory 9.9 Other Considerations 9.10 Operating-System Examples( 跳過 )

9.3 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Objectives To describe the benefits of a virtual memory system To explain the concepts of demand paging, page- replacement algorithms, and allocation of page frames To discuss the principle of the working-set model

9.4 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 9.1 Background Virtual memory – separation of user logical memory from physical memory. Only part of the program needs to be in memory for execution Logical address space can therefore be much larger than physical address space Allows address spaces to be shared by several processes Allows for more efficient process creation Virtual memory can be implemented via: Demand paging Demand segmentation

9.5 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Virtual Memory That is Larger Than Physical Memory  (page table)

9.6 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Virtual-address Space for functional calls for new objects

9.7 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Shared Library Using Virtual Memory

9.8 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 9.2 Demand Paging Bring a page into memory only when it is needed Less I/O needed Less memory needed Faster response More users Page is needed  reference to it invalid reference  abort not-in-memory  bring to memory Lazy swapper – never swaps a page into memory unless page will be needed A swapper manipulates entire processes A pager deals with pages of a process

9.9 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Transfer of a Paged Memory to Contiguous Disk Space

9.10 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Valid-Invalid Bit With each page table entry a valid–invalid bit is associated (v  in-memory, i  not-in-memory) Initially valid–invalid bit is set to i on all entries Example of a page table snapshot: During address translation, if valid–invalid bit in page table entry is i  page fault v v v v i i i …. Frame #valid-invalid bit page table

9.11 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Page Table When Some Pages Are Not in Main Memory

9.12 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Page Fault If there is a reference to a page, first reference to that page will trap to operating system: page-fault trap Steps in Handling a Page Fault: Operating system looks at an internal table If invalid reference  abort 3. If just not in memory  get an empty frame 4. Read the desired page into frame 5. Reset the internal table (Set validation bit = v )and page table. 6. Restart the instruction that caused the page fault

9.13 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Steps in Handling a Page Fault

9.14 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Page Fault A page fault in a three-address instruction (like z=x+y) will require fetching the instruction again, decoding it again, fetching the two operands again, and then adding again. Restart instruction difficulty block move of 256 bytes (p. 310) Solutions  Attempts to access both block ends first  Uses temporary registers to hold values of overwritten locations. If page fault, then written back.

9.15 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Performance of Demand Paging Page Fault Rate: 0  p  1.0 if p = 0, no page faults if p = 1, every reference is a fault Effective Access Time (EAT) EAT = (1 – p) x memory access + p ( page fault overhead + swap page out + swap page in + restart overhead )

9.16 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Demand Paging Example Memory access time = 200 nanoseconds Average page-fault service time = 8 milliseconds EAT = (1 – p) x p (8 milliseconds) = (1 – p ) x p x 8,000,000 = p x 7,999,800 If one access out of 1,000 causes a page fault, then EAT = 8.2 microseconds. This is a slowdown by a factor of 40!! If we want performance degradation less than 10% p x 7,999,800 < 200  p < To implement demand paging Need frame-allocation and page-replacement algorithm

9.17 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 9.3 Copy-on-Write During process creation, Copy-on-Write (COW) allows both parent and child processes to initially share the same pages in memory If either process modifies a shared page, only then is the page copied COW allows more efficient process creation as only modified pages are copied Free pages are allocated from a pool of zeroed-out pages ( 全部清空為 0)

9.18 Silberschatz, Galvin and Gagne ©2005 Operating System Principles

9.19 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 9.4 What happens if there is no free frame? Page replacement – find some page in memory, but not really in use, swap it out want an page replacement algorithm which will result in minimum number of page faults Same page may be brought into memory several times

9.20 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Page Replacement Prevent over-allocation of memory by modifying page-fault service routine to include page replacement Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory

9.21 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Need For Page Replacement

9.22 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Basic Page Replacement 1. Find the location of the desired page on disk 2. Find a free frame: - If there is a free frame, use it - If there is no free frame, use a page replacement algorithm to select a victim frame - Write the victim frame to the disk; change the page and frame tables 3. Bring the desired page into the (newly) free frame; update the page and frame tables 4. Restart the process

9.23 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Page Replacement

9.24 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Page Replacement Algorithms Want lowest page-fault rate Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string In all our examples, the reference string is 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

9.25 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Graph of Page Faults Versus The Number of Frames

9.26 Silberschatz, Galvin and Gagne ©2005 Operating System Principles First-In-First-Out (FIFO) Algorithm Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 3 frames (3 pages can be in memory at a time per process) 4 frames Belady’s Anomaly: more frames  more page faults page faults page faults 4 43

9.27 Silberschatz, Galvin and Gagne ©2005 Operating System Principles FIFO Page Replacement Exercise: Try the 4 frame case

9.28 Silberschatz, Galvin and Gagne ©2005 Operating System Principles FIFO Illustrating Belady’s Anomaly

9.29 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Optimal Algorithm Replace page that will not be used for longest period of time 4 frames example 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 How do you know this? Used for measuring how well your algorithm performs page faults 4 Exercise: Try the 3 frame case

9.30 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Optimal Page Replacement

9.31 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Least Recently Used (LRU) Algorithm Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 Counter implementation Add to CPU a logical clock (counter); every page entry has a time-of-use field. every time page is referenced through this entry, copy the clock into the time-of-use field. When a page needs to be replaced, look at the time-of-use fields to determine which are to be replaced Exercise: Try the 3 frame case

9.32 Silberschatz, Galvin and Gagne ©2005 Operating System Principles LRU Page Replacement

9.33 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Stack implementation (to record the most recent page references) - keep a stack of page numbers in a double link form: Page referenced:  move it to the top  Use doubly-linked list: requires 6 pointers to be changed No search for replacement

9.34 Silberschatz, Galvin and Gagne ©2005 Operating System Principles LRU Approximation Algorithms Reference bit With each page associate a bit, initially = 0 When page is referenced bit set to 1 Replace the one which is 0 (if one exists)  We do not know the order, however Second chance Need reference bit Clock replacement If page to be replaced (in clock order) has reference bit = 1 then:  set reference bit 0  leave page in memory  replace next page (in clock order), subject to same rules

9.35 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Second-Chance (clock) Page-Replacement Algorithm

9.36 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Enhanced Second-Chance Algorithm Considering the reference bit and the modification bit, we have the following four classes (0,0): neither recently used nor modified (0,1): not recently used but modified (1,0): recently used but clean (1,1): recently used and modified

9.37 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Counting Algorithms Keep a counter of the number of references that have been made to each page LFU Algorithm: replaces page with smallest count MFU Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used Page-Buffering Algorithms Keep a pool of free frames When page faults, the desired page is read into a free frame from the pool before the victim is written to disk Allow the process to restart as soon as possible 跳過 9.4.8