Virtual Memory (Ch.10)

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1 Virtual Memory (Ch.10)! Background! Demand Paging! Page Faults! Page Replacement! Page Replacement Algorithms! Thrashing! Strategies for Thrashing Prevention Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 1

2 Background! Virtual memory Memory management technique virtualizes memory (RAM and disk) l Only part of the program needs to be in memory for execution (error routines, portions of arrays, lists, etc.) l Logical address space can therefore be much larger than physical address space. l Allows address spaces to be shared by several processes.! Virtual memory can be implemented via: l Demand paging l Demand segmentation Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 2

3 Virtual Memory Larger Than Physical Memory Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 3

4 Demand Paging! Bring a page into memory from secondary storage only when it is needed l Less I/O needed l Less memory needed l Faster response l More users! Page is needed reference to it l invalid reference abort l not-in-memory bring to memory Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 4

5 Valid-Invalid Bit! With each page table entry a valid invalid bit is associated (1 in-memory, 0 not-in-memory)! Initially valid invalid bit is set to 0 on all entries.! Example of a page table snapshot. 1! 1! 1! 1! 0 Frame #! valid-invalid bit!! During address translation, if valid invalid bit in page table entry is 0 page fault.! page table! 0! 0! Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 5

6 Page Table When Some Pages Are Not in Main Memory Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 6

7 Page Fault! If there is ever a reference to a page, first reference will trap to OS page fault! OS looks at another table (in PCB) to decide: l Invalid reference abort. l Just not in memory.! Get empty frame.! Copy page into frame.! Reset tables, validation bit = 1.! Restart instruction that was interrupted Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 7

8 Steps in Handling a Page Fault Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 8

9 ! Pure demand paging Paging Issues l Never bring a page into memory until it is required l Can lead to unacceptable system performance particularly when a page fault results in the access of several pages (one for instructions and several for data) l Analysis has shown that programs tend to have locality of reference which results in reasonable performance Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 9

10 What happens if there is no free frame?! Page replacement find some page in memory, but not really in use, swap it out. l algorithm l performance want an algorithm which will result in minimum number of page faults.! Same page may be brought into memory several times. Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 10

11 Demand Paging Example! Memory access time = 1 µs (microsecond)! Swap Page Time = 10 ms = µs! Page-fault rate = p! 50% of the time the page that is being replaced has been modified and therefore needs to be swapped out. EAT = (1 p) 1 + p p (in µs) Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 11

12 Need For Page Replacement Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 12

13 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. 3. Read the desired page into the (newly) free frame. Update the page and frame tables. 4. Restart the process.! 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. Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 13

14 Page Replacement Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 14

15 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 the next examples, the reference string is 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5. Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 15

16 Graph of Page Faults Versus The Number of Frames Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 16

17 First-In-First-Out (FIFO) Page replacement! What happens when we have 4 frames? Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 17

18 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) 1! 1! 4! 5! 2! 2! 1! 3! 9 page faults! 3! 3! 2! 4! 1! 1! 5! 4!! 4 frames 2! 3! 2! 3! 1! 2! 5! 10 page faults! 4! 4! 3!! This is an example of Belady s anomaly more faults with more frames, opposite is what would think is true Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 18

19 FIFO Illustrating Belady s Anamoly Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 19

20 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 1! 4! 2! 6 page faults!! How do you know this? Requires future knowledge!! Used as yardstick to evaluate how well other algorithms perform. 3! 4! 5! Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 20

21 Optimal Page Replacement Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 21

22 Least Recently Used (LRU) Algorithm! The optimal algorithm uses the time when a page is to be used next! FIFO uses the time when a page was brought into memory! LRU use the recent past as an approximation of the near future. Replace the page that has not been used for the longest period of time Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 22

23 Least Recently Used (LRU) Algorithm! Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 1! 5! 2! 3! 5! 4! 4! 3!! Counter implementation l Every page entry has a counter; every time page is referenced through this entry, copy the clock into the counter. l When a page needs to be changed, look at the counters to determine which one is the smallest value and use that frame. Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 23

24 LRU Page Replacement Note: Optimal and LRU replacement do not suffer from Belady s anomaly Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 24

25 Counting-Based Page Replacement! Keep a counter of the number of references that have been made to each page.! Least Frequently Used (LFU) Algorithm: replaces page with smallest count. l Problem: A page used heavily in the initial phase of a process will have a large count and remain in memory even though it may not be used again l Solution: Age pages as they remain in memory by shifting the counts 1 bit to the right! Most Frequently Used (MFU) Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used. Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 25

26 Global vs. Local Allocation! Global replacement process selects a replacement frame from the set of all frames; one process can take a frame from another. l A process is no longer in control of its page-fault rate l Usually results in greater system throughput and its therefore more commonly used! Local replacement each process selects from only its own set of allocated frames. l Does not take advantage of less used pages in other processes which could improve overall system throughput Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 26

27 Thrashing! If a process does not have enough pages, the pagefault rate is very high. This leads to: l low CPU utilization. l operating system thinks that it needs to increase the degree of multiprogramming. l another process added to the system to increase multiprogramming! Thrashing a process is busy swapping pages in and out rather than executing Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 27

28 Locality Model! Locality model l A locality is a set of pages that are actively used together. A program is generally composed of several different localities which may overlap l Locality is defined by the program structure and its data structures l As a process executes it migrates from one locality to another (e.g. subroutine)! Why does thrashing occur? size of locality > number of frames allocated Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 28

29 Strategies for Thrashing Prevention! Working-set strategy! Page-fault frequency scheme Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 29

30 Working-Set Model Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 30

31 Working-Set Model! Δ working-set window a fixed number of page references Example: instructions! WSS i (working set of Process P i ) = approximation of the program s locality - total number of pages referenced in the most recent Δ (varies in time) l if Δ too small will not encompass entire locality. l if Δ too large will encompass several localities. l if Δ = will encompass entire program lifetime.! D = Σ WSS i total demand for frames, m number of frames! if D > m Thrashing! Policy: if D > m, then suspend one of the processes, writing its frames to disk, and reallocate the frames to another process! Working set strategy prevents thrashing while keeping the degree of multiprogramming as high as possible Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 31

32 Other Considerations! Prepaging - prevent the high level of initial paging by bringing into memory at one time all the pages that will be needed l Use the working set whenever swap a process resumes! Page size selection l Fragmentation bigger page more wasted space l table size 4,096 pages of 1KB or 512 pages of 8KB l Locality smaller page size allows each page to match program locality more accurately, better resolution, but larger page generates less page faults Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 32

33 Other Considerations (Cont.)! Program structure l int A[][] = new int[1024][1024]; l Each row is stored in one page l Program 1 for (j = 0; j < A.length; j++) for (i = 0; i < A.length; i++) A[i,j] = 0; 1024 x 1024 page faults l Program 2 for (i = 0; i < A.length; i++) for (j = 0; j < A.length; j++) A[i,j] = 0; 1024 page faults Silberschatz / OS Concepts / 6e - Chapter 10 Virtual Memory Slide 33

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