Presentation is loading. Please wait.

Presentation is loading. Please wait.

Silberschatz, Galvin and Gagne 2002 10.1 Operating System Concepts Virtual Memory Virtual memory – separation of user logical memory from physical memory.

Similar presentations


Presentation on theme: "Silberschatz, Galvin and Gagne 2002 10.1 Operating System Concepts Virtual Memory Virtual memory – separation of user logical memory from physical memory."— Presentation transcript:

1 Silberschatz, Galvin and Gagne Operating System Concepts Virtual Memory Virtual memory – separation of user logical memory from physical memory. Only part of the program needs to be in memory for execution - some can be on disk Disk address can be stored in place of frame number A valid–invalid bit is associated with each page table entry (1 in-memory, 0 not-in-memory) During address translation, if valid–invalid bit in page table entry is 0 page fault bring in to memory

2 Silberschatz, Galvin and Gagne Operating System Concepts Page Table When Some Pages Are Not in Main Memory Sum logical address space can therefore be much larger than physical address space. More processes

3 Silberschatz, Galvin and Gagne Operating System Concepts Page Faults Bringing a Page into Memory Get empty frame. Swap page into frame. Reset tables, validation bit = 1. Restart instruction

4 Silberschatz, Galvin and Gagne Operating System Concepts Need For Page Replacement

5 Silberschatz, Galvin and Gagne Operating System Concepts What happens if there is no free frame? Page replacement – find some page in memory, but not really in use, swap it out. Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk.

6 Silberschatz, Galvin and Gagne Operating System Concepts Basic Page Replacement 1. Find the location of the desired page on disk. 2. Find a free frame: 1. If there is a free frame, use it. 2. If there is no free frame, use a page replacement algorithm to select a victim frame. 3. If victim is dirty, write it out to disk 3. Read the desired page into the (newly) free frame. Update the page and frame tables. 4. Restart the process

7 Silberschatz, Galvin and Gagne Operating System Concepts Page Replacement Policies 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 - only for crucial OS processes Local replacement – each process selects from only its own set of allocated frames - the normal case I/O interlock Pages that are used for copying a file from a device must be locked

8 Silberschatz, Galvin and Gagne Operating System Concepts Thrashing VM works because of locality Process migrates from one locality to another. Localities may overlap What happens if size of localities > total memory size

9 Silberschatz, Galvin and Gagne Operating System Concepts Thrashing If a process does not have enough pages for its locality, the page-fault rate is very high. Thrashing a process is busy swapping pages in and out. Thrashing may lead to: low CPU utilization. operating system thinks that it needs to increase the degree of multiprogramming. another process added to the system It is important to allocate enough frames to each process If thats not possible, the process must be swapped out

10 Silberschatz, Galvin and Gagne Operating System Concepts Allocation of Frames Each process needs minimum number of frames Example: IBM 370 – 6 pages to handle SS MOVE instruction: instruction is 6 bytes, might span 2 pages. 2 pages to handle from. 2 pages to handle to. Two major allocation schemes. fixed allocation priority allocation

11 Silberschatz, Galvin and Gagne Operating System Concepts Fixed Allocation Equal allocation – e.g., if 100 frames and 5 processes, give each 20 frames. Proportional allocation – Allocate according to the size of process.

12 Silberschatz, Galvin and Gagne Operating System Concepts Priority Allocation Use a proportional allocation scheme using priorities rather than size. If process P i generates a page fault, select for replacement one of its frames. select for replacement a frame from a process with lower priority number.

13 Silberschatz, Galvin and Gagne Operating System Concepts Page-Fault Frequency Scheme Establish acceptable page-fault rate. If actual rate too low, process loses frame. If actual rate too high, process gains frame.

14 Silberschatz, Galvin and Gagne Operating System Concepts Initial Load Load pages predicted to be needed (compiler flags) or Load no pages Allows for more efficient process creation

15 Silberschatz, Galvin and Gagne Operating System Concepts Performance of Demand Paging Page Fault Rate 0 p 1.0 (hopefully low, e.g. 5%) 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) Values Assume memory access roughly 1 (with TLB hits) Page fault overhead is small Swap is about Restart is small

16 Silberschatz, Galvin and Gagne Operating System Concepts Page Replacement Algorithms Want lowest page-fault rate (disk is slow) Evaluate algorithm by running it on a particular string of page references (reference string) and computing the number of page faults on that string. A page fault means a disk access (or two), and thats where the time goes. Assume no initial load, local replacement, no I/O (no interlock issues)

17 Silberschatz, Galvin and Gagne Operating System Concepts Optimal Algorithm Replace page that will not be used for longest period of time in future. 3 frames example: (9) 4 frames example: (6) But the future is unknown (like SJF scheduling)

18 Silberschatz, Galvin and Gagne Operating System Concepts First-In-First-Out (FIFO) Algorithm 3 frames example: (15)

19 Silberschatz, Galvin and Gagne Operating System Concepts FIFO Illustrating Beladys Anamoly 4 frames example: (10) 3 frames example: (9) Beladys Anomaly - more frames does not imply less page faults

20 Silberschatz, Galvin and Gagne Operating System Concepts Counting Algorithms Keep a counter of the number of references that have been made to each page. Count is reset each time a page is brought in LFU Algorithm: replaces page with smallest count. May delay start of counting to avoid bias from an initial rush MFU Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used. See example in Geoffs notes

21 Silberschatz, Galvin and Gagne Operating System Concepts LRU Page Replacement 3 frames example: (11) 4 frames example: (?)

22 Silberschatz, Galvin and Gagne Operating System Concepts Least Recently Used (LRU) Algorithm 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 determine which are to change. Stack implementation – keep a stack of page numbers in a double link form: Page referenced: move it to the top requires 6 pointers to be changed No search for replacement Both need HW support, as they are done very very often

23 Silberschatz, Galvin and Gagne Operating System Concepts Use Of A Stack to Record The Most Recent Page References

24 Silberschatz, Galvin and Gagne Operating System Concepts LRU Approximation Algorithms Usually not enough HW support for full LRU, so … Reference bit in the page table With each page associate a bit, in HW 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. Clear when all 1 Reference bytes in the page table Make top bit the reference bit Shift right reference byte at regular intervals Victim is page with smallest value byte Second chance bit associated with each frame Looks through frames in order, with wrap-around, starting at frame after last loaded If frame has bit = 1 then: set bit 0. leave page in memory. replace next page, subject to same rules.

25 Silberschatz, Galvin and Gagne Operating System Concepts Second-Chance (clock) Page-Replacement Algorithm

26 Silberschatz, Galvin and Gagne Operating System Concepts Enhanced Second Chance Keep a reference bit and a modified bit with each frame 0,0 => not recently used or modified => best to replace 0,1 => not recently used but modified => must be written out 1,0 => recently used but not modified => may be used again soon 1,1 => Recently used and modified => best keep this one in RAM Victim selection After each load, the current position is set to the frame after the loaded one. Scan from current position round for a 0,0 frame If not found, scan round for a 0,1 frame, reseting the use bit (ala 2nd chance) If not found, start again (a victim must be found) Used by Mac OS

27 Silberschatz, Galvin and Gagne Operating System Concepts Page Buffering Keep some frames free -a frame pool Immediately swap in on page fault Write out while first process uses CPU Remember what pages are in the frame pool, in case they are requested again - used by VMS with FIFO to recover from preemptive removal Write out modified pages whenever the swap disk is idle

28 Silberschatz, Galvin and Gagne Operating System Concepts Other Techniques Pre-paging - predicting page needs Windows NT clustering Copy-on-Write 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.

29 Silberschatz, Galvin and Gagne Operating System Concepts Other Considerations Program structure int A[ ][ ] = new int[1024][1024]; Each row is stored in one page 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 Program 2 for (i = 0; i < A.length; i++) for (j = 0; j < A.length; j++) A[i,j] = 0; 1024 page faults

30 Silberschatz, Galvin and Gagne Operating System Concepts Windows NT Uses demand paging with clustering. Clustering brings in pages surrounding the faulting page. Processes are assigned working set minimum and working set maximum. Working set minimum is the minimum number of pages the process is guaranteed to have in memory. A process may be assigned as many pages up to its working set maximum. When the amount of free memory in the system falls below a threshold, automatic working set trimming is performed to restore the amount of free memory. Working set trimming removes pages from processes that have pages in excess of their working set minimum. On single x86 CPUS, uses a second chance style algorithm to select victims On Alpha and SMP, uses a modified FIFO

31 Silberschatz, Galvin and Gagne Operating System Concepts Solaris 2 Maintains a list of free pages to assign faulting processes. Lotsfree – threshold parameter to begin pageout. Pageout is called more frequently depending upon the amount of free memory available. Pageout scans pages using variation on the second chance algorithm (2 handed clock algorithm) Scanrate is the rate at which pages are scanned. This ranged from slowscan to fastscan. This increases as free memory decreases Handspread affects time process has to reuse a page before the big-hand send the page out Pages from shared libraries are kept in memory (recent variation)


Download ppt "Silberschatz, Galvin and Gagne 2002 10.1 Operating System Concepts Virtual Memory Virtual memory – separation of user logical memory from physical memory."

Similar presentations


Ads by Google