Operating Systems CSE 410, Spring Virtual Memory. Stephen Wagner Michigan State University

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1 Operating Systems CSE 410, Spring 2004 Virtual Memory Stephen Wagner Michigan State University

2 Virtual Memory Provide User an address space that is larger than main memory Secondary storage is used to augment main memory Transparent to the user A single process may be larger than main memory Allows the OS to support more processes

3 Hardware and Control Structures Memory references are dynamically translated into physical addresses at run time. A process may be swapping in and out of memory and does not need to occupy the same region of memory. A process can be broken up into pieces that do not need to be located contiguously in memory. All pieces of a program do not need to be loaded into main memory during execution.

4 Virtual Memory and Paging Each process has its own virtual address space A virtual address consists of a virtual page # and an offset A virtual page can be stored in physical memory or on disk Page table is used to indicate where a virtual page is stored

5 Execution of a Program Operating system brings into memory a few pieces of the process Resident Set: memory portion of the process that is currently in main An interrupt is generated when an address is needed that is not in main memory Operating system places the process in a blocked state

6 Page Faults If a page is not in memory when accessed, it must be read from disk to memory This is a page fault Page faults are very expensive Cache memory access : 1 cycle Main memory access : 10 cycles Disk access : 1,000,000 cycles Usually some other page will need to be evicted from main memory.

7 Page Faults Process is put in a blocked state Operating system issues a disk I/O read request Another process is dispatched to run while the I/O takes place When I/O completes, an interrupt is generated and the process is put back in the ready queue The instruction that cause the page fault needs to be executed again, or completed

8 Advantages of Virtual Memory More processes may be supported Each process only needs some of its pages in memory With more processes available, it is more likely some process will be in the Ready state A process can have an address space larger than main memory A program is not restricted by a particular architecture

9 Types of Memory Real Memory: Physical main memory Virtual Memory: Physical memory plus memory on disk Allows for effective multiprogramming and relieves the user of tight memory constraints

10 Thrashing What happens if there are too many processes in the system? Working Set: the set of pages a process needs to perform some useful work The working set is usually much smaller than the set of all pages in the process The working set can change throughout a process s lifetime If the working sets of all processes do not fit into memory, thrashing will occur The OS will spend most of its time swapping pages in and out

11 Thrashing Thrashing will severely impair system performance Working Set Model: keep track of each process s working set and only let the process run when its working set is in memory Prepaging: load the pages of a process before letting the process run normally you wait until a request is made to the page before loading it in prepaging you load the page before any request is made

12 Principle of Locality Program and data references with a process tend to cluster Only a few pieces of a process will be needed over a relatively long period of time Possible to make intelligent guesses about which pieces will be needed in the future Principle of Locality is what makes virtual memory work efficiently

13 Support Needed for Virtual Memory Hardware must support paging or segmentation Operating system must be able to manage the movement of pages/segments between secondary memory and main memory.

14 Page Tables Each process has its own page table Each page table entry contains the frame number of the corresponding page in main memory A bit is needed to indicate whether the page is in main memory or not If a page is not in main memory, we need to know where on disk it is stored

15 Page Tables and the Modify Bit When a page is swapped out of main memory, we need to save any changes made to the page we have to write the evicted page to disk we have to read in the new page To minimize writes, we only write a page if it has been modified since it was last loaded into main memory Keep track of this information with a Modify Bit If the page has not been changed, we do not have to write it to disk

16 Page Table Entry Virtual Address Page Number Offset Page Table Entry P MOther Control Bits Frame Number (a) Paging only Virtual Address Segment Number P: present bit Segment Table Entry P MOther Control Bits M: modify bit Length Offset (b) Segmentation only Segment Base Virtual Address Segment Number Page Number Offset

17 Virtual Address Page # Offset Frame # Offset Register Page Table Ptr Page Table + Page# Offset Page Frame Frame # Program Paging Mechanism Main Memory Figure 8.3 Address Translation in a Paging System

18 Size of Page Table Assume a 32 bit address and 4K pages 2 20 virtual pages, Page table requires 2 20 entries If each page table entry (PTE) requires 4 bytes, this is 4MB. Each process requires its own page table Where do we store the page table?

19 Virtual Page Tables Store the page table in virtual memory Each page access has potential for two page faults Base address of page table is a virtual address in system space Virtual address of PTE = Base address of page table + Logical Page # * size of PTE Virtual address of PTE needs to be converted into physical address

20 Two Level Paging A method to support storing pages in virtual memory A root page table maps an address to a page containing part of the page table The virtual address can be divided into: index into root page table : index into page of PTEs : offset in page

21 Two Level Paging 4-kbyte root page table 4-Mbyte user page table 4-Gbyte user address space Figure 8.4 A Two-Level Hierarchical Page Table [JACO98a]

22 Translation Look Aside Buffer Each memory access results in two accesses of physical memory One access to read the page table One access to read the requested data To speed up process a Translation Look Aside Buffer (TLB) is used The TLB is implemented in hardware and contains page number to frame number mappings The TLB is fast, but small, and only contains a small subset of the page table

23 Translation Look Aside Buffer Is a cache of recently used virtual addresses Given a virtual address, processor examines the TLB If page table entry (PTE) is present, the frame is retrieved and the real address is formed If the PTE is not present, the page number is used to access the process s page table The TLB is updated to include the new PTE

24 Return to Faulted Instruction Start CPU checks the TLB Page Table Entry in TLB? Yes No Page Fault Handling Routine OS Instructs CPU to Read the Page from Disk CPU Activates I/O Hardware No Access Page Table Page in Main Memory? Yes Update TLB Page Transferred from Disk to Main Memory CPU Generates Physical Address Memory Full? Yes No Page Tables Updated Page Tables Updated Figure 8.8 Operation of Paging and Translation Lookaside Buffer (TLB) [FURH87]

25 Virtual Address Main Memory Secondary Memory Page # Offset Translation Lookaside Buffer TLB hit Offset Page Table Load page TLB miss Frame # Offset Real Address Page fault Figure 8.7 Use of a Translation Lookaside Buffer

26 Virtual Address Page # Offset Virtual Address Page # Offset Page # PT Entries Frame # Offset Real Address Translation Lookaside Buffer Frame # Offset Real Address Page Table (a) Direct mapping (b) Associative mapping Figure 8.9 Direct Versus Associative Lookup for Page Table Entries

27 Inverted Page Table The number of physical pages (frames) is much smaller than the number of virtual pages The memory requirements are much smaller if we store one entry per frame, instead of per virtual page The TLB and/or a hash table is used to map virtual page # to an entry in the inverted page table Because multiple virtual pages may map to same location, the entries are chained in the page table

28 Inverted Page Table Example: 64-bit virtual address, 4K pages and 32MB of RAM 2 52 (45 quadrillion) possible pages 8192 frames Inverted Page Table is much smaller Used on some IBM and Hewlett Packard work stations

29 Virtual Address Inverted Page Table Page # Offset Page # Inverted Page Table entry Chain Frame # Hash Table Frame # Offset Physical Address

30 Page Size Smaller page size, less internal fragmentation Smaller page size, more pages required per process More pages per process means larger page tables Larger page tables means large portion of page tables in virtual memory Secondary memory is designed to efficiently transfer large blocks of data so large page size is better

31 Page Size Small page size means a large number of pages will be found in main memory As time progresses, each page in memory will contain portions of the process near recent references With larger pages, each page will contain portions of the process farther from recent references Number of page faults is related to the percent of the process in memory that is near recent references

32 Page Fault Rate Page Fault Rate (a) Page Size P W (b) Number of Page Frames Allocated N P = size of entire process W = working set size N = total number of pages in process The page fault rate drops as the number of pages in memory increases Figure 8.11 Typical Paging Behavior of a Program

33 Page Size Multiple page sizes would allow greater flexibility Large page sizes can be used for program instructions or data with high spatial locality Smaller page sizes can be used for threads Variable page sizes not supported by mainstream OSes

34 Computer Atlas Honeywell-Multics IBM 370/XA and 370/ESA VAX family IBM AS/400 DEC Alpha MIPS UltraSPARC Pentium PowerPC Page Sizes Page Size bit words bit words 4 Kbytes 512 bytes 512 bytes 8 Kbytes 4 Kbytes to 16 Mbytes 8 Kbytes to 4 Mbytes 4 Kbytes to 4 Mbytes 4 Kbytes

35 Segmentation Processes are divided into different segments Segments can be different sizes Segments are visible to the programmer Allows parts of a program to be altered and compiled independently (minimal linking) Lends itself to sharing and protection

36 Combined Paging and Segmentation Paging is transparent to the programmer Paging eliminates external fragmentation Segmentation is visible to the programmer Segmentation allows for growing data structures, modularity, and support for sharing and protection Each segment is broken into fixed-size pages

37 P MOther Control Bits Length Segment Base (b) Segmentation only Paging and Segmentation Virtual Address Segment Number Page Number Offset Segment Table Entry Other Control Bits Length Segment Base Page Table Entry P MOther Control Bits Frame Number (c) Combined segmentation and paging P= present bit M = Modified bit Figure 8.2 Typical Memory Management Formats

38 Locality Only a small number of pages will be in memory If pages were accessed uniformly, most page accesses would be page faults and performance will be poor Real programs tend to reused a small set of data Temporal locality: if a process accesses a data item, it is likely to access the same data again Spatial locality: if a process accesses a data item, it is likely to access a nearby data item

39 Locality The use of pages takes advantage of spatial locality When a data item is accessed, the entire page containing the data is read into memory Other items in the page will likely be accessed Too large of a page size will not take advantage of spatial locality Pages already in memory will tend to be accessed again due to temporal locality The replacement policy can try to take further advantage of temporal locality

40 Locality The fact that real programs exhibit spatial and temporal locality makes the memory hierarchy practical Consider a 32 bit virtual address and 512 MB of real memory Virtual memory is 8 times as big real memory What percentage of memory references will cause page faults? If memory references were random, 87.5% of memory references would cause page faults In practice, less than 1% of memory references cause page faults

41 Fetch Policy When should pages be brought into memory? Demand Paging: requested. only bring a page into memory when it is Only brings in pages that are needed Lots of page faults when a process first starts Prepaging: bring in pages before they are requested Requires some guess work Bringing in pages that are physically contiguous on disk is very efficient

42 Replacement Policies When a new page is brought in, we usually have to evict a page so that there is a free frame Which place should be replaced? We want to evict something that will not be used soon

43 Replacement Policies Optimal Replacement policy Replace page that will not be used for the longest time. Requires knowledge of the future. Not possible to implement. Least Recently Used (LRU) First In First Out (FIFO) Clock

44 LRU When temporal locality is in affect, the near future looks like the recent past LRU replaces the page that was used least recently Requires keeping track of when each page was last accessed Tends to outperform other replacement policies, but it is expensive to implement

45 FIFO Replace the page that has been in memory the longest Very trivial to implement fifo_replacement() { frame_number = next_frame next_frame=(next_frame+1)%number_of_frames return frame_number } Does not take advantage of temporal locality

46 Page address stream OPT F F F LRU F F F F FIFO F F F F F F CLOCK 2* 2* 3* 2* 3* 2* 3* 1* 5* 3 1 F 5* 2* 1 F 5* 2* 4* F 5* 2* 4* 3* 2 4 F 3* 2* 4 3* 2 5* F 3* 2* 5* Figure 8.15 Behavior of Four Page-Replacement Algorithms

47 Clock LRU works well but is expensive to implement FIFO is easy to implement, but does not take advantage of temporal locality Clock is a policy that attempts to cheaply simulate LRU Keep track of an additional use bit that is set to 1 whenever the page is referenced Scan the frames looking for a frame with a use bit set to 0 Set use bit to 0 as frames are scanned Many similar replacement policies

48 Clock clock_replacement() { while (set[next_frame] == 1) { set[next_frame]=0 frame_number = next_frame next_frame=(next_frame+1)%number_of_frames } frame_number = next_frame next_frame=(next_frame+1)%number_of_frames return frame_number }

49 n 1 page 9 use = 1 0 page 19 use = 1 next frame pointer page 1 use = 1 1 First frame in circular buffer of frames that are candidates for replacement page 45 use = page 222 use = 0 page 191 use = page 33 use = 1 page 67 use = 1 page 556 use = 0 page 13 use = (a) State of buffer just prior to a page replacement 5 n 1 0 page 9 use = 1 page 19 use = 1 page 1 use = 0 1

50 (a) State of buffer just prior to a page replacement n 1 0 page 9 use = 1 page 19 use = 1 page 1 use = 0 1 page 45 use = page 222 use = 0 page 191 use = page 33 use = 1 page 67 use = 1 page 13 use = 0 page 727 use = (b) State of buffer just after the next page replacement 5 Figure 8.16 Example of Clock Policy Operation

51 Improved Clock Algorithm Evicting a modified (dirty) page is more expensive than evicting a clean page Have to write out the modified page then read in the new page We can modify clock to favor dirty pages

52 Improved Clock Algorithm On first scan look for u bit = 0 and m bit = 0. Do not change u bit. If first scan does not find a frame, on second scan look for u bit = 0 and m bit = 1. Change u bit to 0. Repeat above two steps until a free frame is found. Other modifications including additional use bits to better approximate LRU.

53 n 1 First frame in circular buffer for this process 0 Page 7 not accessed recently; modified Page 9 not accessed recently; modified Page 94 not accessed recently; not modified 1 9 Page 13 not accessed recently; not modified Page 95 accessed recently; not modified 2 8 Next replaced Page 47 not accessed recently; not modified 7 Page 46 not accessed recently; modified Page 121 accessed recently; not modified Page 45 accessed recently; not modified Page 97 not accessed recently; modified Page 96 accessed recently; not modified 4 3 Last replaced 6 5 Figure 8.18 The Clock Page-Replacement Algorithm [GOLD89]

54 Aging Clock algorithms do not distinguish between old references and recent ones. Approximate LRU by aging Each PTE has a counter and a use bit (like in clock) Every k ticks, shift each counter to the right, and insert the use bit as the new high order bet Reset the use bit Choose as victim the PTE with lowest count.

55 Ref. 1 Frames Use Counter Page Ref. 1 Frames Use Counter Page Use Counter Page Use Counter Page Use Counter Page Use Counter Page

56 Ref 5 Frames Use Counter Page Ref 4 Frames Use Counter Page Use Counter Page Use Counter Page Use Counter Page Use Counter Page

57 TLB and Replacement Policies The TLB only contains a subset of the page table entries To take advantage of temporal locality, the TLB should be updated to keep track of recently used pages The TLB is implemented in hardware, so the replacement policy needs to be simple

58 Frame Locking In some cases we want to lock a frame in memory to guarantee it is not swapped out Kernel pages I/O buffers Control Structures Associate a lock bit with each frame

59 Page Buffering Writing single pages out to disk is inefficient A replaced page is added to one of two lists a free page list if the page has not been modified a modified page list if the page has been modified Evicted pages actually remain in memory. Similar to a victim cache System tries to keep some free pages. Modified pages can be written out in clusters

60 Fixed-set Allocation Resident Set Management gives a process a fixed number of frames within which to execute when a page fault occurs, one of the pages of that process must be replaced Variable-allocation number of frames allocated to process varies over the lifetime of the process

61 Fixed Allocation Variable Allocation Local Replacement Number of frames allocated to a process is fixed. Page to be replaced is chosen among the frames allocated to the process. The number of frames allocated to a process may be changed from time to time. Page to be replaced is chosen among the frames allocated to the process. Global Replacement Not possible Page to be replaced is chosen among all available frames. Number of frames used by a process varies.

62 Fixed Allocation, Local Scope A page in the same process will be replaced Replacement algorithms are applied The amount of allocation needs to be decided Too small an allocation means a high number of page faults Too large an allocation limits the number of resident processes

63 Variable Allocation, Global Scope Easiest to implement Adopted by many operating systems Operating system keeps list of free frames Free frame is added to resident set of process when a page fault occurs If no free frame, replace one from another process

64 Variable Allocation, Global Scope When a new process is created, allocate number of page frames based on application type, program request, or other criteria When page fault occurs, select a page among the resident set of the process that suffered the fault Reevaluate allocation from time to time

65 Working Set Strategy Working Set, W (t, ): the set of pages of that process that have been referenced in the last virtual time units. is a window of time over which the process is observed. Working set size will be a nondecreasing function of the window size. The large the window size the larger the working set. The working set size can vary over time for a fixed.

66 - tsequence of Page References Window Size, Figure 8.19 Working Set of Process as Defined by Window Size

67 Working Set Strategy Monitor the working set of each process Periodically remove from the resident set of a process those pages that are not in its working set. A process may only execute if its working set is in main memory.

68 Working Set Problems The past does not always accurately predict the future A true measurement of the working set for each process is not practical The optimal value of is not known and will vary

69 Cleaning Policy Demand cleaning a page is written to disk only when it is selected for replacement Precleaning pages are written out in batches can take advantage of I/O parallelism can write out pages in a more efficient manner

70 Cleaning Policy Best approach used page buffering Replaced pages are put into two lists: modified and unmodified Pages in the modified list are periodically written out in batches Pages in the unmodified list are either reclaimed if referenced again or the frame is assigned to another page

71 Virtual Memory and Scheduling Page faults are expensive. A process that generates a page fault should be blocked until the page fault is resolved With virtual memory, more processes can be supported If too many processes are in memory, thrashing may result Each process needs a subset of its pages (the working set) to work efficiently If the working sets of the current processes do not all fit in memory, excessive page faults will occur If there are too many processes, some may be suspended

72 Process Suspension Lowest priority process Faulting process this process does not have its working set in memory so it will be blocked anyways Last process activated this process is least likely to have its working set resident

73 Process Suspension Process with smallest resident set this process will be the easiest to reload Process with largest resident set obtains the most free frames

74 UNIX and Solaris Memory Management Paging System Kernel Memory Management

75 UNIX and Solaris Paging System Page Table : one per each process Disk block descriptor : associated with each page of a process is an entry that describes the disk copy of the virtual page Page frame data table : describes each frame of real memory and is indexed by frame number Swap-use table: one for each swap device, with one entry for each page on the device

76 Reference count Page/storage unit number Page frame number Age Reference Valid Modify Copy on write Protect (a) Page table entry Swap device number Device block number Type of storage (b) Disk block descriptor Page state Reference count Logical device Block number Pfdata pointer (c) Page frame data table entry

77 Page state Reference count Logical device Block number Pfdata pointer (c) Page frame data table entry Reference count Page/storage unit number (d) Swap-use table entry Figure 8.22 UNIX SVR4 Memory Management Formats

78 UNIX Paging Page Replacement refinement of the clock policy known as the two-handed clock algorithm. The algorithm Set to 0 when the page is first brought in Set to 1 when the page is reference for a read or write

79 fronthand handspread backhand Fronthand: sweeps through pages and sets reference bits to 0. Figure 8.23 Two-Handed Clock Page-Replacement Algorithm Backhand: sweeps and checks references bit. Pages with 0 are placed on a list to be paged out. Scanrate: the rate at which the hands move in pages per second Handspread: the gap between the hands

80 UNIX Kernel Memory Allocation Most allocations are smaller than a page size Using pages would waste memory Lazy Buddy System (SRV4) Defers coalescing until it is needed, and then combine as many blocks as possible This avoids unnecessary work In the steady state the kernel uses many small blocks

81 Linux Memory Managment Three level page table for Virtual Memory Addressing Page directory: a process has a single page directory in memory. Each entry points to a page in the page middle directory. Page middle directory: spans multiple pages. Each entry points to one page in the page table. Page table: each entry refers to one virtual page

82 Linux Virtual Memory Managment Page allocation uses the buddy system Kernel maintains a list of contiguous page frame groups of fixed size, eg. 1, 2, 4, 8, 16, 32 As pages are allocated and deallocated in main memory, the available groups are split and merged as needed

83 Linux Page Replacement Variant of clock and aging algorithm Use bit is replaced by an 8 bit age variable Age variable is incremented each time page is accessed Periodically sweep through global page pool and decrements age Lowest age page is best candidate for replacement

84 Linux Kernel Memory Managment Uses paging A variation of the buddy algorithm is used to allocate smaller chunks within the page

85 W2K Virtual Memory Each user process sees a 32-bit (4 GB) address space A portion of memory is reserved for OS Each user has 2GB of memory Can be increased to 3GB, leaving 1GB for the OS

86 64-Kbyte region for NULL-pointer assignments (inaccessible) 0 2-Gbyte user address space (unreserved, usable) 64-Kbyte region for bad pointer assignments (inaccessible) 2-Gbyte region for the operating system (inacessible) 0xFFFFFFFF Figure 8.25 Windows 2000 Default Virtual Address Space

87 W2K Paging A page can be in one of three states Available : not currently used by this process Reserved: set aside for a process, but does not count against the processes s memory quota until used Committed : pages for which the system has set aside space in its paging file

88 W2K Paging Variable allocation, local scope Assign a certain number of pages to a process as its working set when it is first activated Working sets are adjusted if memory is available, processes are allowed to grow otherwise LRU pages are removed from the resident set

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