Memory Management - II

1. CS423UG Operating Systems Memory Management - II Indranil Gupta Lecture 14 Sep 26, 2005

2. Agenda • Storage Management • Virtual Memory and Paging CS 423UG - Operating Systems, Indranil Gupta

3. Review • Memory Manager • Monitor used and free memory • Allocate memory to processes • Reclaim (De-allocate) memory • Swapping between main memory and disk • Mono-programming memory management • Overlay • Multi-programming memory management • Fixed partition • Relocation and protection CS 423UG - Operating Systems, Indranil Gupta

4. Swapping • Move a part of or the whole process to disk • Allows several processes to share a fixed partition • Processes that grow can be swapped out and swapped back in a bigger partition CS 423UG - Operating Systems, Indranil Gupta

5. Variable Partitions and Fragmentation Free 1 Monitor Job 1 Job 2 Job 3 Job 4 2 Free Monitor Job 1 Job 3 Job 4 3 Free Monitor Job 1 Job 5 Job 3 Job 4 Free 4 Monitor Job 5 Job 3 Job 4 Job 6 Free 5 Monitor Job 7 Job 5 Job 3 Job 8 Job 6 Memory wasted by External Fragmentation CS 423UG - Operating Systems, Indranil Gupta

6. Memory Management with Bitmaps • Memory is divided into fixed-size allocation units (e.g., 512 B). • One allocation unit corresponds to 1bit in the bitmap • 0: free, 1: allocated • Use bitmaps for free lists. • Size of allocation unit • The smaller the allocation unit, the larger the bitmap. • Problem: allocation • When a new process arrives, the manager must find consecutive 0 bits in the map. • Searching a bitmap for a run of a given length is a slow operation. 1 0 1 0 1 0 0 0 1 0 1 0 0 1 0 CS 423UG - Operating Systems, Indranil Gupta

7. Memory Management with Linked Lists A,0,10 • Use a linked list of allocated and free memory segments (“holes”) • sorted by the address or by the size h,10,5 B,15,8 Four neighbor combinations for the terminating process X CS 423UG - Operating Systems, Indranil Gupta

8. Storage Placement Strategies • Analogy: • Shoe Fitting • Valet parking • First Fit • Scan; use the first available hole whose size is sufficient to meet the need. • Problem: Creates average size holes; more fragementation closer to scan start. • Next Fit • Minor variation of first fit: keep track of where last search ended, restart from there. • Problem: slightly worse performance than first fit. • Best Fit • Use the hole whose size is equal to the need, or if none is equal, the smallest hole that is large enough. • Problem: Creates small holes that can't be used. • Worst Fit • Use the largest available hole. • Problem: Gets rid of large holes, making it difficult to run large processes. • Quick Fit • maintains separate lists for some of the more common sizes requested. • When a request comes for placement, it finds the closest fit. • This is a very fast scheme, but a merge is expensive. If merge is not done, memory will quickly fragment into a large number of holes. CS 423UG - Operating Systems, Indranil Gupta

9. Compaction (Similar to Garbage Collection) • Assumes programs are all relocatable (how supported?) • Processes must be suspended during compaction • Needed only when fragmentation gets very bad Free 5 Monitor Job 7 Job 5 Job 3 Job 8 Job 6 Free 6 Monitor Job 7 Job 5 Job 3 Job 8 Job 6 Free 7 Monitor Job 7 Job 5 Job 3 Job 8 Job 6 Free 8 Monitor Job 7 Job 5 Job 3 Job 8 Job 6 Free 9 Monitor Job 7 Job 5 Job 3 Job 8 Job 6 CS 423UG - Operating Systems, Indranil Gupta

10. Multiple Base Registers • Break programs into smaller units because they will fit better • Use multiple base registers, one for each unit • Examples • Code/Data • Constants/variables CS 423UG - Operating Systems, Indranil Gupta

11. Storage Management Problems • Fixed partitions suffer from internal fragmentation • Process may require only, say, 800 B, but blocks allocated in multiples of 512 B’s. • Variable partitions suffer from external fragmentation • Compaction suffers from overhead • Partitions must be less in size than real memory • Overlays are painful to program efficiently • Swapping requires writing to disk sectors CS 423UG - Operating Systems, Indranil Gupta

12. How Bad Is Fragmentation? • Statistical arguments - Random sizes • First-fit • Given N allocated blocks • 0.5N blocks will be lost because of fragmentation • Known as 50% RULE CS 423UG - Operating Systems, Indranil Gupta

13. Alternative Approach: Virtual Memory • Provide user with virtual memory that is as big as user needs • Store virtual memory on disk • Store in real memory those parts of virtual memory currently under use • Load and store cached virtual memory without user program intervention (“transparently”) CS 423UG - Operating Systems, Indranil Gupta

14. Benefits of Virtual Memory • Use secondary storage($) • Extend DRAM($$$) with reasonable performance • Protection • Processes do not step on each other • Convenience • Flat address space • Processes have the same view of the world • Load and store cached virtual memory without user program intervention • Reduce fragmentation: • make cacheable units all the same size (page=allocation unit) • Remove memory deadlock possibilities: • permit pre-emption of real memory CS 423UG - Operating Systems, Indranil Gupta

15. Real Memory Request Page 3 Page Table 1 VM Frame Memory 3 1 2 3 4 1 2 3 4 Virtual Memory Stored on Disk Disk 1 2 3 4 5 6 7 8 Paging – What’s that? Per-process Frames Pages CS 423UG - Operating Systems, Indranil Gupta

16. Real Memory Request Page 1 Page Table 1 VM Frame Memory 3 1 2 1 2 3 4 1 2 3 4 Virtual Memory Stored on Disk Disk 1 2 3 4 5 6 7 8 Paging – it’s about Pages and Frames Per-process CS 423UG - Operating Systems, Indranil Gupta

17. Real Memory Request Page 6 Page Table 1 VM Frame Memory 3 1 2 1 2 3 6 3 4 1 2 3 4 Virtual Memory Stored on Disk Disk 1 2 3 4 5 6 7 8 Paging Per-process CS 423UG - Operating Systems, Indranil Gupta

18. Real Memory Request Page 2 Page Table 1 VM Frame Memory 3 1 2 1 2 3 6 3 2 4 4 1 2 3 4 Virtual Memory Stored on Disk Disk 1 2 3 4 5 6 7 8 Paging Per-process CS 423UG - Operating Systems, Indranil Gupta

19. Real Memory Request Page 8: Swap page 2 to disk first Page Table 1 VM Frame Memory 3 1 2 1 2 3 6 3 4 2 4 1 2 3 4 Virtual Memory Stored on Disk Disk 1 2 3 4 5 6 7 8 Paging Per-process CS 423UG - Operating Systems, Indranil Gupta

20. Real Memory Load Page 8 to Memory Page Table 1 VM Frame Memory 3 1 2 8 2 3 6 3 4 2 4 1 2 3 4 Virtual Memory Stored on Disk Disk 1 2 3 4 5 6 7 8 Paging Per-process CS 423UG - Operating Systems, Indranil Gupta

21. Virtual Memory Virtual Address (P,D) P Page Table P D 0 D P 1 Contents(P,D) 0 P, F 1 1 0 Physical Memory 1 F D F Physical Address (F,D) D Contents(F,D) Paging: Hardware Required! CPU P=page number F=frame number D=offset within page CS 423UG - Operating Systems, Indranil Gupta

22. Translating Pages into Frames CPU Virtual Memory Virtual Address (003006) 003 Page Table 003 006 0 006 4 1 Contents(3006) 0 003, 004 1 1 0 Physical Memory 1 004 006 004 Physical Address (F,D) Page size 1000 Number of Possible Virtual Pages 1000 Number of Page Frames 8 006 Contents(4006) CS 423UG - Operating Systems, Indranil Gupta

23. Page Fault • Access a virtual page that is not mapped into any physical page • A fault/TRAP is triggered by hardware • Page fault handler (by VM Software, a part of OS) • Find if there is any free physical frame available • If no, evict some resident page to disk (either permanently, or temporarily into swap space) • Allocate a free physical frame • Load the faulted virtual page to the prepared physical frame • Modify the page table CS 423UG - Operating Systems, Indranil Gupta

24. Paging Issues • Page size is 2n • usually 512, 1k, 2k, 4k, or 8k • Too small is bad. E.g. 32 bit VM address may have 220 (1MB) pages with 4k (212 ) bytes per page • 220 page entries take 222 bytes (4MB) • Too large is bad – internal fragmentation • Page table: • Page table must be stored in memory! • Page Table Base Register (points to physical address of page table of “running” process). Must be changed for context switch. • NO External fragmentation, Internal fragmentation on last page ONLY CS 423UG - Operating Systems, Indranil Gupta

25. PTBR=Page Table Base Register (in hardware) CPU Virtual Memory Virtual Address (003006) PTBR=001 003 Page Table 003 006 0 006 4 1 Contents(3006) 0 003, 004 1 1 0 Physical Memory 1 004 006 004 Physical Address (F,D) Page size 1000 Number of Possible Virtual Pages 1000 Number of Page Frames 8 006 Contents(4006) CS 423UG - Operating Systems, Indranil Gupta

26. Address Translation: Birth to Death Virtual address (P,D) 1 (PTBR,P*4)=phy. address of page table entry (4 B per entry) CPU 2 PTBR Return via address bus 3 6 Get page table entry (P,F) (F,D) 4 Access (F,D)’th byte of phy. memory Physical address 5 CS 423UG - Operating Systems, Indranil Gupta

27. Reminder • Reading for this lecture: Until 4.3.2 • Reading for next lecture: 4.0-4.3 • Midterm: October 10, Monday, 10am-10:50am, 1404 SC • HW3 due THIS WEEK Friday • MP2 out soon CS 423UG - Operating Systems, Indranil Gupta