Chapter 9: Virtual Memory Chapter 9: Virtual Memory „ Background „ Demand Paging „ Copy-on-Write „ Page Replacement „ Allocation of Frames „ Thrashing „ Memory-Mapped Files „ Allocating Kernel Memory „ Other Considerations „ Operating-System Examples Operating System Concepts – 7th Edition, Feb 22, 2005 9.2 Silberschatz, Galvin and Gagne ©2005 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 Operating System Concepts – 7th Edition, Feb 22, 2005 9.3 Silberschatz, Galvin and Gagne ©2005 Background „ Virtual memory – separation of user logical memory from physical memory z Only part of the program needs to be in memory for execution z Logical address space can therefore be much larger than physical address space z Allows address spaces to be shared by several processes z Allows for more efficient process creation „ Virtual memory can be implemented via: z Demand paging z Demand segmentation Operating System Concepts – 7th Edition, Feb 22, 2005 9.4 Silberschatz, Galvin and Gagne ©2005 Virtual Memory That is Larger Than Physical Memory ⇒ Operating System Concepts – 7th Edition, Feb 22, 2005 9.5 Silberschatz, Galvin and Gagne ©2005 Virtual-address Space Operating System Concepts – 7th Edition, Feb 22, 2005 9.6 Silberschatz, Galvin and Gagne ©2005 Shared Library Using Virtual Memory Operating System Concepts – 7th Edition, Feb 22, 2005 9.7 Silberschatz, Galvin and Gagne ©2005 Demand Paging „ Bring a page into memory only when it is needed z Less I/O needed z Less memory needed z Faster response z More users „ Page is needed ⇒ reference to it z invalid reference ⇒ abort z not-in-memory ⇒ bring to memory „ Lazy swapper – never swaps a page into memory unless page will be needed z Swapper that deals with pages is a pager Operating System Concepts – 7th Edition, Feb 22, 2005 9.8 Silberschatz, Galvin and Gagne ©2005 Transfer of a Paged Memory to Contiguous Disk Space Operating System Concepts – 7th Edition, Feb 22, 2005 9.9 Silberschatz, Galvin and Gagne ©2005 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: Frame # valid-invalid bit v v v v i … i i page table „ During address translation, if valid–invalid bit in page table entry is I ⇒ page fault Operating System Concepts – 7th Edition, Feb 22, 2005 9.10 Silberschatz, Galvin and Gagne ©2005 Buddy System „ Allocates memory from fixed-size segment consisting of physically- contiguous pages „ Memory allocated using power-of-2 allocator z Satisfies requests in units sized as power of z Request rounded up to next highest power of z When smaller allocation needed than is available, current chunk split into two buddies of next-lower power of  Continue until appropriate sized chunk available Operating System Concepts – 7th Edition, Feb 22, 2005 9.56 Silberschatz, Galvin and Gagne ©2005 Buddy System Allocator Operating System Concepts – 7th Edition, Feb 22, 2005 9.57 Silberschatz, Galvin and Gagne ©2005 Slab Allocator „ Alternate strategy „ Slab is one or more physically contiguous pages „ Cache consists of one or more slabs „ Single cache for each unique kernel data structure z Each cache filled with objects – instantiations of the data structure „ When cache created, filled with objects marked as free „ When structures stored, objects marked as used „ If slab is full of used objects, next object allocated from empty slab z If no empty slabs, new slab allocated „ Benefits include no fragmentation, fast memory request satisfaction Operating System Concepts – 7th Edition, Feb 22, 2005 9.58 Silberschatz, Galvin and Gagne ©2005 Slab Allocation Operating System Concepts – 7th Edition, Feb 22, 2005 9.59 Silberschatz, Galvin and Gagne ©2005 Other Issues Prepaging „ Prepaging z To reduce the large number of page faults that occurs at process startup z Prepage all or some of the pages a process will need, before they are referenced z But if prepaged pages are unused, I/O and memory was wasted z Assume s pages are prepaged and α of the pages is used cost of s * α save pages faults > or < than the cost of prepaging s * (1- α) unnecessary pages?  Is α near zero ⇒ prepaging loses Operating System Concepts – 7th Edition, Feb 22, 2005 9.60 Silberschatz, Galvin and Gagne ©2005 Other Issues – Page Size „ Page size selection must take into consideration: z fragmentation z table size z I/O overhead z locality Operating System Concepts – 7th Edition, Feb 22, 2005 9.61 Silberschatz, Galvin and Gagne ©2005 Other Issues – TLB Reach „ TLB Reach - The amount of memory accessible from the TLB „ TLB Reach = (TLB Size) X (Page Size) „ Ideally, the working set of each process is stored in the TLB z Otherwise there is a high degree of page faults „ Increase the Page Size z This may lead to an increase in fragmentation as not all applications require a large page size „ Provide Multiple Page Sizes z This allows applications that require larger page sizes the opportunity to use them without an increase in fragmentation Operating System Concepts – 7th Edition, Feb 22, 2005 9.62 Silberschatz, Galvin and Gagne ©2005 Other Issues – Program Structure „ Program structure z Int[128,128] data; z Each row is stored in one page z Program for (j = 0; j [...]... 4 4 3 9 page faults 4 frames 10 page faults Belady’s Anomaly: more frames ⇒ more page faults Operating System Concepts – 7th Edition, Feb 22, 2005 9. 28 Silberschatz, Galvin and Gagne ©2005 FIFO Page Replacement Operating System Concepts – 7th Edition, Feb 22, 2005 9. 29 Silberschatz, Galvin and Gagne ©2005 FIFO Illustrating Belady’s Anomaly Operating System Concepts – 7th Edition, Feb 22, 2005 9. 30 Silberschatz,... „ Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory Operating System Concepts – 7th Edition, Feb 22, 2005 9. 22 Silberschatz, Galvin and Gagne ©2005 Need For Page Replacement Operating System Concepts – 7th Edition, Feb 22, 2005 9. 23 Silberschatz, Galvin and Gagne ©2005 Basic Page Replacement 1 Find the... caused the page fault Operating System Concepts – 7th Edition, Feb 22, 2005 9. 12 Silberschatz, Galvin and Gagne ©2005 Page Fault (Cont.) „ Restart instruction z block move z auto increment/decrement location Operating System Concepts – 7th Edition, Feb 22, 2005 9. 13 Silberschatz, Galvin and Gagne ©2005 Steps in Handling a Page Fault Operating System Concepts – 7th Edition, Feb 22, 2005 9. 14 Silberschatz,... EAT = 8.2 microseconds This is a slowdown by a factor of 40!! Operating System Concepts – 7th Edition, Feb 22, 2005 9. 16 Silberschatz, Galvin and Gagne ©2005 Process Creation „ Virtual memory allows other benefits during process creation: - Copy-on-Write - Memory- Mapped Files (later) Operating System Concepts – 7th Edition, Feb 22, 2005 9. 17 Silberschatz, Galvin and Gagne ©2005 Copy-on-Write „ Copy-on-Write...Page Table When Some Pages Are Not in Main Memory Operating System Concepts – 7th Edition, Feb 22, 2005 9. 11 Silberschatz, Galvin and Gagne ©2005 Page Fault „ If there is a reference to a page, first reference to that page will trap to operating system: page fault 1 Operating system looks at another table to decide: z z Invalid reference ⇒ abort Just not in memory 2 Get empty frame 3 Swap page into... – p) x memory access + p (page fault overhead + swap page out + swap page in + restart overhead ) Operating System Concepts – 7th Edition, Feb 22, 2005 9. 15 Silberschatz, Galvin and Gagne ©2005 Demand Paging Example „ Memory access time = 200 nanoseconds „ Average page-fault service time = 8 milliseconds „ EAT = (1 – p) x 200 + p (8 milliseconds) = (1 – p x 200 + p x 8,000,000 = 200 + p x 7 ,99 9,800... and frame tables 4 Restart the process Operating System Concepts – 7th Edition, Feb 22, 2005 9. 24 Silberschatz, Galvin and Gagne ©2005 Page Replacement Operating System Concepts – 7th Edition, Feb 22, 2005 9. 25 Silberschatz, Galvin and Gagne ©2005 Page Replacement Algorithms „ Want lowest page-fault rate „ Evaluate algorithm by running it on a particular string of memory references (reference string)... 2, 5, 1, 2, 3, 4, 5 Operating System Concepts – 7th Edition, Feb 22, 2005 9. 26 Silberschatz, Galvin and Gagne ©2005 Graph of Page Faults Versus The Number of Frames Operating System Concepts – 7th Edition, Feb 22, 2005 9. 27 Silberschatz, Galvin and Gagne ©2005 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... 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 Operating System Concepts – 7th Edition, Feb 22, 2005 9. 18 Silberschatz, Galvin and Gagne ©2005 Before Process 1 Modifies Page C Operating System Concepts – 7th Edition, Feb 22, 2005 9. 19. .. 2, 3, 4, 5 1 4 2 6 page faults 3 4 5 „ How do you know this? „ Used for measuring how well your algorithm performs Operating System Concepts – 7th Edition, Feb 22, 2005 9. 31 Silberschatz, Galvin and Gagne ©2005 Optimal Page Replacement Operating System Concepts – 7th Edition, Feb 22, 2005 9. 32 Silberschatz, Galvin and Gagne ©2005 Least Recently Used (LRU) Algorithm „ Reference string: 1, 2, 3, 4, 1, ... Physical Memory ⇒ Operating System Concepts – 7th Edition, Feb 22, 2005 9. 5 Silberschatz, Galvin and Gagne ©2005 Virtual- address Space Operating System Concepts – 7th Edition, Feb 22, 2005 9. 6 Silberschatz,... Considerations „ Operating- System Examples Operating System Concepts – 7th Edition, Feb 22, 2005 9. 2 Silberschatz, Galvin and Gagne ©2005 Objectives „ To describe the benefits of a virtual memory system. .. pages Operating System Concepts – 7th Edition, Feb 22, 2005 9. 18 Silberschatz, Galvin and Gagne ©2005 Before Process Modifies Page C Operating System Concepts – 7th Edition, Feb 22, 2005 9. 19 Silberschatz,