10: Virtual Memory Management

Transcription

1 CSC400 - Operating Systems 10: Virtual Memory Management J. Sumey Introduction virtual memory management: concerned with the actual management operations of a virtual memory system fetch strategies: when to move a page from secondary to main memory demand fetch: when a process makes a reference anticipatory fetch: loads additional pages the system "thinks" a process will need ahead of time replacement strategies: which page to "kick out" when memory is full CSC400 - Virtual Memory Management 2 CSC400 Operating Systems 1

2 Locality locality of reference: execution behavior of a process wherein it only references a small subset of all its pages during various stages of its execution; thus, not all pages of a process need to be loaded into physical memory at all times of its execution i.e. processes tend to have a "favorite" subset of its pages what causes this? temporal locality: loops, function calls, variables spatial locality: sequential execution, array traversals CSC400 - Virtual Memory Management 3 Fetch Strategy: Demand Paging operation: when process starts, OS loads only the page containing the 1 st instruction OS loads additional pages only after process references them features: simple policy, easy to implement only loads pages actually needed "one-at-a-time" page accumulation causes many waits during page loads CSC400 - Virtual Memory Management 4 CSC400 Operating Systems 2

3 Fetch Strategy: Anticipatory Paging operation: OS attempts to predict which pages a process will need and preload them when memory is available thus minimizing future page faults features: difficult to predict future process behavior with additional disk I/O overhead, overall performance could actually degrade sometimes used as an enhancement to demand paging (i.e. like cache on hard drives) CSC400 - Virtual Memory Management 5 Page Replacement Algorithms deals with selection of a page (or segment) to replace when a page fault occurs and all current system memory is occupied algorithms: First-In, First-Out (FIFO) Least Recently Used (LRU) Not Recently Used (NRU) Second-Chance / Clock Replacement considerations: page usage history; referenced / modified CSC400 - Virtual Memory Management 6 CSC400 Operating Systems 3

4 but first - PTE Enhancements in order to support VM management, page table entries will need additional info: page frame address present/absent bit referenced bit used to indicate activity within its page frame, helps OS in managing memory modified (dirty) bit was the page modified since being loaded? cache disable / swap disable what about physical addresses that lead to I/O registers or PTEs? protection how is page frame used: read, write, execute? CSC400 - Virtual Memory Management 7 Optimal Page Replacement Algorithm defined as the algorithm that would yield lowest number of future page faults process would always have needed pages loaded into main memory not actually possible because it would require knowledge of future page usage (clairvoyance) CSC400 - Virtual Memory Management 8 CSC400 Operating Systems 4

5 Optimal Policy Example given 3 page frames and a process that accesses 5 page frames as follows: optimal policy produces 3 page faults after frame allocation has been filled CSC400 - Virtual Memory Management 9 FIFO Page Replacement chooses oldest page as the one to replace page usage is maintained by age; choose oldest page (Head) to replace & add new page at Tail pro: fairly low overhead con: can easily pick important (heavily used) pages; thus is not commonly used however, is the basis of other, popular algorithms CSC400 - Virtual Memory Management 10 CSC400 Operating Systems 5

6 FIFO Policy Example FIFO does not consider page use frequency results in 6 page faults CSC400 - Virtual Memory Management 11 LRU Page Replacement chooses page that has been unused for longest period of time heuristic: a process's past behavior is a good indication of its future behavior pro: performs better than FIFO con: increased overhead hardware timer & additional page table field to store reference "timestamp"; or linked-list maintained in reference time-stamp order (oldest at end) CSC400 - Virtual Memory Management 12 CSC400 Operating Systems 6

7 LRU Policy Example LRU performs nearly as well as optimal results in 4 page faults here CSC400 - Virtual Memory Management 13 NRU Page Replacement chooses page that has not been used recently small overhead, popular based on referenced (R) & modified (M) bits of PTE and "classes" 0: not ref'd, not modified 1: not ref'd, modified 2: referenced, not modified 3: referenced & modified NRU chooses a random page from the lowest nonempty class to be replaced class 1 occurs because OS uses a clock interrupt to periodically clear the R bits CSC400 - Virtual Memory Management 14 CSC400 Operating Systems 7

8 Second Chance / Clock Replacement second chance variation of FIFO where R bit of oldest page is also considered; if 0 choose it - if 1 clear it, move to end of list then continue search thus, will tend to keep active pages in memory clock replacement improvement on second chance by implementing a circular list & rotating around it with 2nd chance algorithm CSC400 - Virtual Memory Management 15 Comparison of Page Replacement Policies results are based on the execution of 0.25 x 10 6 references in a FORTRAN program, using a page size of 256 words goal: to be to the right of the knee of the curve (with a small page fault rate) while at the same time keeping a small frame allocation (to the left of the knee of the curve) CSC400 - Virtual Memory Management 16 CSC400 Operating Systems 8

9 Denning's Working Set Model working set model a paging system strategy of tracking the working set of each process and prepaging the set before allowing the process to run working set the set of favored pages currently in use by a process pages not referenced for n system clock ticks dropped from the WS consider if the entire working set fits into physical memory vs. if it needs to be paged thrashing result of page faults occurring well over 50% because a process's working set exceeds its available memory wsclock an improvement on the clock algorithm; besides finding the next page whose R bit is 0, the page is also checked for membership in the WS and spared if so CSC400 - Virtual Memory Management 17 Unix VM Management CSC400 - Virtual Memory Management 18 CSC400 Operating Systems 9

10 Windows VM Management 32-bit page sizes from 4KB to 64KB 4 GB space divided into user (2GB) and system (2GB) halves [previously shown] 64-bit increases user space from 2GB to 8TB (2 43 ) CSC400 - Virtual Memory Management 19 CSC400 Operating Systems 10