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OS Virtual Memory Concepts
1.
Silberschatz, Galvin and
Gagne ©2013 Operating System Concepts – 9th Edition U3_Virtual Memory
2.
8.2 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition 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
3.
8.3 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition 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 To examine the relationship between shared memory and memory-mapped files To explore how kernel memory is managed
4.
8.4 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Background Code needs to be in memory to execute, but entire program rarely used Error code, unusual routines, large data structures Entire program code not needed at same time Consider ability to execute partially-loaded program Program no longer constrained by limits of physical memory Each program takes less memory while running -> more programs run at the same time Increased CPU utilization and throughput with no increase in response time or turnaround time Less I/O needed to load or swap programs into memory -> each user program runs faster
5.
8.5 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition
6.
8.6 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Background (Cont.) Virtual 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 More programs running concurrently Less I/O needed to load or swap processes
7.
8.7 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Background (Cont.) Virtual memory: Physical memory organized in page frames MMU must map logical to physical Virtual memory can be implemented via: Demand paging Demand segmentation
8.
8.8 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Demand Paging Bring a page into memory only when it is needed Less I/O needed, no unnecessary I/O Less memory needed Faster response More users Page is needed invalid reference abort not-in-memory bring to memory and reference it Already-in-memory reference it Lazy swapper – never swaps a page into memory unless page will be needed Swapper that deals with pages is a pager
9.
8.9 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Valid-Invalid Bit With each page table entry a valid–invalid bit is associated (v in-memory – memory resident, i not-in-memory) Initially valid–invalid bit is set to i on all entries Example of a page table snapshot: During MMU address translation, if valid–invalid bit in page table entry is i page fault
10.
8.10 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Demand paging: Page Table When Some Pages Are Not in Main Memory
11.
8.11 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition 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 page table to decide: Invalid reference abort Not in memory: means page fault 2. Find free frame 3. Swap page into frame via scheduled disk operation 4. Reset tables to indicate page now in memory Set validation bit = v 5. Restart the instruction that caused the page fault
12.
8.12 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Steps in Handling a Page Fault
13.
8.13 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Steps in Handling a Page Fault
14.
8.14 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Performance of Demand Paging Stages in Demand Paging (worse case) 1. Trap to the operating system 2. Save the user registers and process state 3. Determine that the interrupt was a page fault 4. Check that the page reference was legal and determine the location of the page on the disk 5. Issue a read from the disk to a free frame: 1. Wait in a queue for this device until the read request is serviced 2. Wait for the device seek and/or latency time 3. Begin the transfer of the page to a free frame 6. While waiting, allocate the CPU to some other user 7. Receive an interrupt from the disk I/O subsystem (I/O completed) 8. Save the registers and process state for the other user 9. Determine that the interrupt was from the disk 10. Correct the page table and other tables to show page is now in memory 11. Wait for the CPU to be allocated to this process again 12. Restore the user registers, process state, and new page table, and then resume the interrupted instruction
15.
8.15 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Performance of Demand Paging (Cont.) Three major activities Service the interrupt – careful coding means just several hundred instructions needed Read the page – lots of time Restart the process – again just a small amount of time Page Fault Rate 0 p 1 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 )
16.
8.16 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition 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 7,999,800 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 want performance degradation < 10 percent 220 > 200 + 7,999,800 x p 20 > 7,999,800 x p p < .0000025 < one page fault in every 400,000 memory accesses
17.
8.17 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition What Happens if There is no Free Frame? Page replacement – find some page in memory, but not really in use, page it out Algorithm – terminate? swap out? replace the page? Performance – want an algorithm which will result in minimum number of page faults Same page may be brought into memory several times Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk
18.
8.18 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Why page replacement?
19.
8.19 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Why page replacement?
20.
8.20 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition
21.
8.21 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition
22.
8.22 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition
23.
8.23 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition
24.
8.24 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Page Replacement Algorithms Page-replacement algorithm Want lowest number of page-faults Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string String is just page numbers, not full addresses Repeated access to the same page does not cause a page fault Results depend on number of frames available In all our examples, the reference string of referenced page numbers is 7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1
25.
8.25 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Graph of Page Faults Versus The Number of Frames
26.
8.26 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition First-In-First-Out (FIFO) Algorithm Reference string: 7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1 3 frames (3 pages can be in memory at a time per process) Number of page faults: 15 To track ages of pages, just use a FIFO queue
27.
8.27 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Consider string: 1,2,3,4,1,2,5,1,2,3,4,5 Generally, adding more frames reduces page faults If adding more frames causes more page faults, then it is known as Belady’s Anomaly
28.
8.28 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition FIFO Illustrating Belady’s Anomaly
29.
8.29 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Optimal Algorithm Replace page that will not be used for longest period of time
30.
8.30 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Optimal Algorithm Replace page that will not be used for longest period of time Optimal page replacement algorithm gives minimum page faults Limitation: Can’t read the future Used for measuring how well any algorithm performs when compared with optimal algorithm. Number of page faults for the above reference string: 9
31.
8.31 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Least Recently Used (LRU) Algorithm Use past knowledge rather than future Replace page that has not been used in the most amount of time
32.
8.32 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Least Recently Used (LRU) Algorithm Use past knowledge rather than future Replace page that has not been used in the most amount of time Associate time of last use with each page 12 faults – better than FIFO Generally good algorithm and frequently used But how to implement?
33.
8.33 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition LRU Algorithm (Cont.) Counter implementation Every page entry has a counter; every time page is referenced through this entry, copy the clock into the counter When a page needs to be changed, look at the counters to find smallest value Stack implementation Keep a stack of page numbers in a double link form: But each update more expensive No search for replacement LRU and OPT are cases of stack algorithms that don’t have Belady’s Anomaly
34.
8.34 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Use Of A Stack to Record Most Recent Page References
35.
8.35 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition LRU Approximation Algorithms LRU needs special hardware and still slow Reference bit With each page associate a bit, initially = 0 When page is referenced bit set to 1 Replace any with reference bit = 0 (if one exists) We do not know the order Second-chance algorithm Generally FIFO, plus hardware-provided reference bit Known as Clock page replacement algorithm If page to be replaced has Reference bit = 0 -> replace it reference bit = 1 then: – set reference bit 0, leave page in memory – replace next page, subject to same rules
36.
8.36 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Second-Chance (clock) Page-Replacement Algorithm
37.
8.37 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Enhanced Second-Chance Algorithm Improve algorithm by using reference bit and modify bit (if available) in concert Take ordered pair (reference, modify) 1. (0, 0) neither recently used not modified – best page to replace 2. (0, 1) not recently used but modified – not quite as good, must write out before replacement 3. (1, 0) recently used but clean – probably will be used again soon 4. (1, 1) recently used and modified – probably will be used again soon and need to write out before replacement When page replacement called for, use the clock scheme but use the four classes replace page in lowest non-empty class Might need to search circular queue several times
38.
8.38 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Counting Algorithms Keep a counter of the number of references that have been made to each page Not common Lease Frequently Used (LFU) Algorithm: replaces page with smallest count 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
39.
8.39 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Allocation of Frames Each process needs minimum number of frames Maximum is total frames in the system Two major allocation schemes fixed allocation priority allocation Many variations
40.
8.40 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Fixed Allocation Equal allocation – For example, if there are 100 frames (after allocating frames for the OS) and 5 processes, give each process 20 frames Keep some as free frame buffer pool Proportional allocation – Allocate according to the size of process m S s p a m s S p s i i i i i i for allocation frames of number total process of size m = 64 s1 =10 s2 =127 a1 = 10 137 ´ 62 » 4 a2 = 127 137 ´ 62 » 57
41.
8.41 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Priority Allocation Use a proportional allocation scheme using priorities rather than size If process Pi generates a page fault, select for replacement one of its frames select for replacement a frame from a process with lower priority number
42.
8.42 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Global vs. Local Page Replacement Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another But then process execution time can vary greatly But greater throughput so more common Local replacement – each process selects from only its own set of allocated frames More consistent per-process performance But possibly underutilized memory
43.
8.43 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Thrashing If a process does not have “enough” pages, the page-fault rate is very high Page fault to get page Replace existing frame But quickly need replaced frame back This leads to: Low CPU utilization Operating system thinking that it needs to increase the degree of multiprogramming Another process added to the system Thrashing a process is busy swapping pages in and out
44.
8.44 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition Thrashing (Cont.)
45.
8.45 Silberschatz, Galvin
and Gagne ©2013 Operating System Concepts – 9th Edition How to handle thrashing: Using Page-Fault Frequency Establish “acceptable” page-fault frequency (PFF) rate and use local replacement policy If actual rate too low, process loses frame If actual rate too high, process gains frame
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