Operating System Concepts and Techniques Lecture 10

1. Operating System Concepts and Techniques Lecture 10 Memory Management-3 M. Naghibzadeh Reference M. Naghibzadeh, Operating System Concepts and Techniques, First ed., iUniverse Inc., 2011. To order: www.iUniverse.com, www.barnesandnoble.com, or www.amazon.com

2. Virtual Memory • Goals • Reduce memory waste • Eliminate program size restriction • Increase number of live programs • Move inactive parts of data, program, and results out of main memory

3. Page-Based Virtual MM • Extended Page memory management to support virtual memory • With virtual memory, disk becomes a virtual main memory • During execution of a program, disk virtual area is the permanent place for the program • Main memory is a temporary place for the program • To load a program, any page can take any page frame • Executable files from slow devices such as CD has to be moved to virtual area to improve performance

4. Page-Based Virtual MM... • Page table knows what page is where • No need to bring all pages to start running it • Can start running even if no page is brought to main memory • Prefetch is possible, if you like and there is space • Page table is extended to show which page is in main memory and which page is not • This is done by adding a column called present/absent • In theory, a very small main memory is adequate to run a very large program • When the system wants to execute an instruction the instruction and its operands have to be in main memory

5. Page-Based Virtual MM... • Otherwise, a page fault occurs • the process is suspended until the page is brought to main memory by memory manager • If the processor spends most of its time suspending and resuming processes due to frequent page faults, then a thrashing condition occurs • A reference to a logical address which has already been loaded into main memory is called page success • In short time intervals, the program usually refers to a small number of pages; this phenomenon is called locality of reference • Page Frame Table (PFT) knows what page frames are free

6. Ready to go • Put the first executable address of the program in PC and we are ready to go, even if no page is brought to main memory • Every logical address has to be translated to a physical address • Next we talk about actual address translation

7. Page number Offset Logical address Page map table of the running process with present/absent field in each entry Offset is directly copied from logical address to physical address No Present? Yes Interrupt Page frame number Offset Physical address aaddress Address translation Using PT • Address translation using page table within MMU • Page table is too big to fit in MMU; use cache memory

8. Address translation Using PFT • Have to use a content addressable memory called Translation Lookaside Buffer (TLB) • The content addressable memory has to house page frame table • We cannot have such a huge associative memory • Will use Inverted Page Table (IPT)

9. Virtual page# Offset Logical address TLB Offset is directly copied from logical address to physical address No Present? Yes Virtual page# Page frame# Protection Free/Allocated Other fields Interrupt Page frame number Offset Physical address aaddress Address translation Using IPT • Rows of IPT are created as needed

10. Concepts before page removal • If a running program need a page, OS must bring it to main memory • If there is no free page frame, a page must be removed • We have to respect: • Working set; every process is given a credit of MM • Per process or overall; do we remove pages from the same process or from any process • Page locking; some pages are locked which means they cannot be removed • What to do with removed pages? If necessary copy them back to disk

11. First-In-First-Out Page removal FIFO behavior with three page frames, H=3, M=9, h=3/12, and m=9/12. • Simple to implement: use a single linked list with two pointers

12. FIFO (Belady’s) Anomaly • Sometimes by increasing memory frames page faults increase FIFO behaviour with four page frames, H=2, M=10, h=2/12, and m=10/12 FIFO does not have stack property

13. Clock policy/Second chance policy • Use reference bit Second chance behavior with three page frame, H=3, M=6, h=3/9, and m=6/9

14. Least Recently Used LRU with a main memory of three frames, H=2, M=10, h=2/12, and m=10/12 • Stack algorithm, but not practical

15. Some other page removals • Not Recently Used, uses modified bit and reference bit • Least Frequently Used, uses a reference counter • Most Frequently Used, Uses a reference counter • Not Frequently used, uses a reference bit and a reference bit update interval

16. Aging • Aging shifts every R bit to its corresponding shift counter at the end of interval • It forgets far history Aging with a main memory of three frames, H=4, M=6, h=4/10, and m=6/10

17. Summary • Virtual memory management is the de facto MM policy of contemporary computers • It has many good properties such as being able to run extremely large programs and running numerous simultaneous programs • With virtual memory management, comes a variety of page removal methods with different complexities and efficiencies • It has disadvantages too; increased execution time and the need for special hardware module called MMU are two main ones

18. Find out • The smallest size main memory which can run any program though inefficiently • Optimal page size • The main property which makes aging to become a good page replacement algorithm • The total space needed for all page tables of say 1000 programs of size 4giga bytes each • The format of a logical address in a four level page table environment • The effective main memory access time with TLB and a non-virtual page table

19. Any questions?