Introduction to Virtual Memory Management

Transcription

1 Introduction to Virtual Memory Management Minsoo Ryu Department of Computer Science and Engineering

2 Virtual Memory Management Page X Demand Paging Page X Q & A Page X

3 Memory Allocation Three ways of memory allocation Contiguous allocation Allocate a single contiguous area of physical memory Segmentation Divide the memory area of each application into multiple segments Allocate segments that may have different sizes and need not be adjacent to another in the physical memory Paging Divide the memory area into fixed-sized small pages Allocate pages that can be scattered in the physical memory

4 4 Contiguous Allocation proc proc proc new proc 9? proc fragmentation problem! proc 7 proc 7? proc 4 Physical Memory proc 4 Physical Memory 4

5 Segmentation proc code seg. data seg. stack seg. proc proc 7 proc proc 4 proc proc 4 proc proc proc 7 proc Physical Memory segment reduced fragmentation

6 6 Paging proc proc proc 4 proc 7 page no (external) fragmentation Physical Memory 6

7 7 Two key purposes Virtual Memory Expose a single contiguous memory space to each process Provide a larger separate memory space to every process than the physically available memory space Every process is allowed to use a 4GB memory space whereas the physical memory is GB A useful trick Differentiate addresses seen by each process from the real addresses On-demand allocate memory 7

8 8 Logical vs. Physical Address Logical (virtual) address Generated by the CPU The user program deals with logical addresses Physical address Seen by the memory unit MMU (Memory Management Unit) Hardware device that translates virtual addressed to physical addresses 8

9 9 Paging and Address Translation Divide physical memory into fixed-sized frames The size is power of, between bytes and 89 bytes Typically, it is 4KB on -bit machines 8KB on 64-bit machines Divide logical address space into pages of the same size Use a page table to store the mapping between logical addresses and physical addresses Address generated by CPU is divided into page number p and page offset d 9

10 0 Paging and Address Translation 0x CPU 0x00009 page number page offset 0x x x x x x x0000 0x x0000 0x0000 0x x x x x Page Table Physical Memory 0

11 Implementation of Page Table Page table for each process is kept in main memory Page table on -bit machines is very large One million entries (4MB is required for each page table) It is not possible to use fast registers Two registers are used Page-table base register (PTBR) points to the page table Page-table length register (PRLR) indicates size of the page table

12 Two-Level Page Table A logical address (on -bit machine with 4K page size) is divided into: a page number consisting of 0 bits a page offset consisting of bits Since the page table is paged, the page number is further divided into: a 0-bit page number a 0-bit page offset Thus, a logical address is as follows: where p i is an index into the outer page table, and p is the displacement within the page of the outer page table

13 Two-Level Page Table 0x x x CPU 0x0009 p p 0x x x x x0bc00 p 0x0a4000 0x x0bc00 0x x406 p 0x4000 0x0007 0x0000 0x000 0x00 0x x x x Outer Page Table Page Table Physical Memory

14 Demand Paging

15 Page Fault 0x x x CPU 0x00009 page number 0x?????9 0x x x ? 0x0000 0x x0000 0x0000 0x x x x x Page Table Physical Memory

16 6 Demand Paging and Page Fault OS brings only necessary pages into memory How can we distinguish between those pages in memory and those pages on the disk? The valid-invalid bit scheme can be used Valid-invalid bit With each page table entry a valid invalid bit is associated ( legal and in-memory, 0 illegal or not-in-memory) Initially valid invalid but is set to 0 on all entries 6

17 7 Page Fault Handling 7

18 8 Over-Allocation and Page Replacement Suppose that We have 40 frames Processes requires 0 pages Processes actually use pages We could run 4 processes without demand paging We could run 8 processes with demand paging Over-allocation of memory Increased level of multiprogramming It is possible that Each process may suddenly try to use all ten of its pages When a page fault occurs, the system would find that there are no frames What should we do? 8

19 9 Basic Page Replacement. Find the location of the desired page on disk. Select a victim frame - Using a page replacement algorithm. Read the desired page into the (newly) frame 4. Update the page and frame tables. Restart the process page transfers doubles EAT Swapping out the victim page Swapping in the desired page 9

20 0 Page Replacement 0

21 Page Replacement Algorithms To evaluate an algorithm Use a particular string of memory references reference string Compute the number of page faults on that string Example Memory address sequence with 00bytes per page 000, 0, 00, 04, 00, 00, 004, The reference string is,,, 4,,,,,,, 4,.

22 First-In-First-Out (FIFO) Algorithm Reference string:,,, 4,,,,,,, 4, frames ( pages can be in memory at a time) There are 9 page faults with frames

23 First-In-First-Out (FIFO) Algorithm Reference string:,,, 4,,,,,,, 4, 4 frames (4 pages can be in memory at a time) There are 0 page faults with 4 frames Belady s Anomaly

24 4 FIFO Illustrating Belady s Anomaly 4

25 Optimal Algorithm Replace page that will not be used for longest period of time 4 frames example Now there are 6 page faults But this algorithm requires future knowledge of the reference string It is used mainly for comparison studies

26 6 Least Recently Used (LRU) Algorithm FIFO uses The time when a page was brought into memory OPT uses The time when a page is to be used LRU (Least Recently Used ) uses The recent past as an approximation of the near future Chooses the page that has not been used for the longest period of time Programs tend to have locality of reference Optimal backward-looking algorithm 6

27 7 Least Recently Used (LRU) Algorithm 4 frames example Now there are 8 page faults (compare with 0 and 6) 7

28 8 8