Memory Management. Virtual Memory. By : Kaushik Vaghani. Prepared By : Kaushik Vaghani

Transcription

1 Memory Management Virtual Memory By : Kaushik Vaghani

2 Virtual Memory Background Page Fault Dirty Page / Dirty Bit Demand Paging Copy-on-Write Page Replacement

3 Objectives To describe the benefits of a virtual memory system To explain the concepts of demand paging, page-replacement algorithms, and allocation of page frames To discuss the principle of the working-set model

4 Background Code needs to be in memory to execute, but entire program rarely used Error code, unusual routines, large data structures like array, list and tables. Entire program code not needed at same time Consider ability to execute partially-loaded program Program no longer constrained by limits of physical memory Each program takes less memory while running -> more programs run at the same time Increased CPU utilization and throughput with no increase in response time or turnaround time Less I/O needed to load or swap programs into memory -> each user program runs faster

5 Background Virtual memory separation of user logical memory from physical memory Only part of the program needs to be in memory for execution Logical address space can therefore be much larger than physical address space Allows address spaces to be shared by several processes Allows for more efficient process creation More programs running concurrently Less I/O needed to load or swap processes Virtual address space logical view of how process is stored in memory Typically this view is that a process begins at a certain logical address- say, address 0 and exists in contiguous memory until end of space Meanwhile, physical memory organized in page frames MMU must map logical to physical Virtual memory can be implemented via: Demand paging Demand segmentation

6 Virtual Memory That is Larger Than Physical Memory

7 Virtual-address Space we allow for the heap to grow upward in memory as it is used for dynamic memory allocation. Similarly, we allow for the stack to grow downward in memory through successive function calls. The large blank space (or hole) between the heap and the stack is part of the virtual address space but will require actual physical pages only if the heap or stack grows. Virtual address spaces that include holes are known as sparse address spaces. Enables sparse address spaces with holes left for growth, dynamically linked libraries, etc

8 Virtual-address Space In addition to separating logical memory from physical memory, virtual memory allows files and memory to be shared by two or more processes through page sharing. This leads to the following benefits: System libraries shared via mapping into virtual address space Virtual memory enables processes to share memory, that allows two or more processes can communicate through the use of shared memory. Virtual memory can allow pages to be shared during fork() system call, thus speeding up process creation.

9 Shared Library Using Virtual Memory

10 Demand Paging How an executable program might be loaded form disk to memory? One option is to load entire program into memory at execution time However, a problem with this approach is that we may not initially need the entire program in memory. Or load a page into memory only when it is needed (Called Demand Paging) With demand-paged virtual memory, pages are only loaded when they are demanded during program execution; pages that are never accessed are thus never loaded into physical memory. Less memory needed Faster response More users processes

11 Demand Paging A Demand Paging system is similar to a paging system with swapping (diagram on right) where processes resides in secondary memory (disk). When we want to execute a process, we swap it into memory. Rather than swapping the entire process into memory, however, we use a lazy swapper. Lazy swapper never swaps a page into memory unless page will be needed Swapper that deals with pages is a pager. Fig. Transfer of a paged memory to contiguous disk space

12 Demand Paging Page is needed reference to it invalid reference abort not-in-memory bring to memory

13 Demand Paging Basic Concepts When a process is to be swapped in, the pager guesses which pages will be used before process swapped out again. Instead of swapping in a whole process, the pager brings in only those pages into memory. How to determine pages that are in memory and are on disk. If pages needed are already memory resident No difference from non demand-paging If page needed and not memory resident Need to detect and load the page into memory from storage Without changing program behavior Without programmer Prepared needing By : Kaushik Vaghani to change code

14 Valid-Invalid Bit we need some form of hardware support to distinguish between the pages that are in memory and the pages that are on the disk. The valid-invalid bit scheme is used for this pupose. With each page table entry a valid invalid bit is associated v in-memory memory resident, i the page either is not valid (not in logical address space of process) or is valid but is currently on the disk (not-inmemory). Initially valid invalid bit is set to i on all entries.

15 Valid-Invalid Bit Notice that marking a page invalid will have no effect if the process never attempts to access that page. During MMU address translation, if valid invalid bit in page table entry is i page fault Example of a page table snapshot

16 Page Table When Some Pages Are Not in Main Memory

17 Page Fault Access to a page marked invalid causes a Page Fault. The paging hardware, in translating the address through the page table, will notice that the invalid bit is set, causing a trap to the operating system. This trap is the result of the operating system's failure to bring the desired page into memory. The procedure for handling this page fault is straightforward

18 Page Fault 1. Operating system looks at an internal table to decide whether the reference was a valid or an invalid in memory access. 2. If Invalid reference Terminate the process Valid reference but Just not in memory page it in. 3. Find free frame 4. Swap in page into frame via scheduled disk operation 5. Reset internal table kept with process and page table to indicate page now in memory Set validation bit = v 6. Restart the instruction that caused the page fault

19 Steps in Handling a Page Fault

20 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

21 Before Process 1 Modifies Page C

22 After Process 1 Modifies Page C

23 What Happens if There is no Free Frame? Page Replacement While a user process is executing, a page fault occurs. The operating system determines where the desired page is residing on the disk but then finds that there are no free frames on the free-frame list; all memory is in use by other process pages. Also in demand from the kernel, I/O buffers, etc Page replacement find some page in memory, but not really in use, page it out Same page may be brought into memory several times

24 Page Replacement Prevent over-allocation of memory by modifying page-fault service routine to include page replacement Use modify (dirty) bit to reduce overhead of page transfers only modified pages are written to disk Page replacement completes separation between logical memory and physical memory large virtual memory can be provided on a smaller physical memory

25 Need For Page Replacement

26 Basic Page Replacement Page replacement takes the following approach. If no frame is free, we find one that is not currently being used and free it. We can free a frame by writing its contents to swap space and changing the page table (and all other tables) to indicate that the page is no longer in memory. We can now use the freed frame to hold the page for which the process faulted. We modify the page-fault include page replacement. service routine to

27 Basic Page Replacement 1. Find the location of the desired page on disk 2. Find a free frame: a) If there is a free frame, use it b) If there is no free frame, use a page replacement algorithm to select a victim frame c) Write victim frame to disk; change the page and frame tables accordingly. 3. Bring the desired page into the (newly) free frame; update the page and frame tables 4. Continue the process by restarting the instruction that caused the trap Note now potentially 2 page transfers for page fault increasing EAT (effective access time)

28 Page Replacement

29 Page and Frame Replacement Algorithms Frame-allocation algorithm determines How many frames to give each process Which frames to replace Page-replacement algorithm Want lowest page-fault rate on both first access and re-access Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string String is just page numbers, not full addresses Repeated access to the same page does not cause a page fault Results depend on number of frames available In all our examples, the reference string of referenced page numbers is 7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1

30 Graph of Page Faults Versus The Number of Frames

31 First-In-First-Out (FIFO) Algorithm The simplest page-replacement algorithm is a first-in, first-out (FIFO) algorithm. A FIFO replacement algorithm associates with each page the time when that page was brought into memory. When a page must be replaced, the oldest page is chosen. We can create a FIFO queue to hold all pages in memory. We replace the page at the head of the queue. When a page is brought into memory, we insert it at the tail of the queue.

32 First-In-First-Out (FIFO) Algorithm Reference string: 7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1 3 frames (3 pages can be in memory at a time per process) 15 page faults

33 FIFO Illustrating Belady s Anomaly Can vary by reference string: consider 1,2,3,4,1,2,5,1,2,3,4,5 Belady s Anomaly Adding more frames can cause more page faults! Notice that the number of faults for four frames (ten) is greater than the number of faults for three frames (nine)! This most unexpected result is known as Belady s anamaly. for some page-replacement algorithms, the page-fault rate may increase as the number ofallocated frames increases.

34 Optimal Page Replacement Algorithm (OPT) Replace page that will not be used for longest period of time Lowest page fault rate of all algorithm Never suffer from Belady s Anomaly 9 is optimal for the example Reference string: 7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1 9 Page Fault

35 Optimal Page Replacement Algorithm (Cont.) With only nine page faults, optimal replacement is much better than a FIFO algorithm, which results in fifteen faults. (If we ignore the first three, which all algorithms must suffer, then optimal replacement is twice as good as FIFO replacement.) In fact, no replacement algorithm can process this reference string in three frames with fewer than nine faults. Unfortunately, the optimal page-replacement algorithm is difficult to implement, because it requires future knowledge of the reference string. As a result, the optimal algorithm is used mainly for comparison studies.

36 Least Recently Used (LRU) Algorithm Use past knowledge rather than future Replace page that has not been used in the most amount of time Associate time of last use with each page 12 faults better than FIFO but worse than OPT Generally good algorithm and frequently used But how to implement? Counters, Stack

37 LRU Algorithm (Cont.) 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 find smallest value Search through table needed Stack implementation Keep a stack of page numbers: Page referenced: move it to the top But each update more expensive LRU and OPT are cases of stack algorithms that don t have Belady s Anomaly

38 Use Of A Stack to Record Most Recent Page References

39 LRU Approximation Algorithms LRU needs special hardware and still slow Some system provide no hardware support, and FIFO algorithm must be used. Many systems provide some help, however, in the form of a reference bit. Reference bit With each page associate a bit, initially = 0 When page is referenced. Bit set to 1 After some time, we can determine which pages have been used and which have not been used by examining the reference bits, although we do not know the order of use.

40 LRU Approximation Algorithms Second-chance algorithm Generally FIFO, plus hardware-provided reference bit If page to be replaced has Reference bit = 0 -> replace it reference bit = 1 then: set reference bit 0, leave page in memory replace next page, subject to same rules

41 Second-Chance (clock) Page-Replacement Algorithm

42 Enhanced Second-Chance Algorithm Not Recently Used (NRU) Improve algorithm by using reference bit and modify bit (if available) in concert Take ordered pair (reference, modify) 1. (0, 0) neither recently used not modified best page to replace 2. (0, 1) not recently used but modified not quite as good, must write out before replacement 3. (1, 0) recently used but clean probably will be used again soon 4. (1, 1) recently used and modified probably will be used again soon and need to write out before replacement When page replacement called for, use the clock scheme but use the four classes replace page in lowest non-empty class

43 Counting Algorithms Keep a counter of the number of references that have been made to each page Least Frequently Used (LFU) Algorithm: replaces page with smallest count Most Frequently Used (MFU) Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used

44 Thrashing If a process does not have enough pages, the page-fault rate is very high Page fault to get page Replace existing frame But quickly need replaced frame back This leads to: Low CPU utilization Operating system thinking that it needs to increase the degree of multiprogramming Another process added to the system Thrashing a process is busy swapping pages in and out

45 Thrashing (Cont.)

46 Extra Studies

47 Operating System Examples Windows Solaris

48 Windows 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

49 Solaris Maintains a list of free pages to assign faulting processes Lotsfree threshold parameter (amount of free memory) to begin paging Desfree threshold parameter to increasing paging Minfree threshold parameter to being swapping Paging is performed by pageout process Pageout scans pages using modified clock algorithm Scanrate is the rate at which pages are scanned. This ranges from slowscan to fastscan Pageout is called more frequently depending upon the amount of free memory available Priority paging gives priority to process code pages

50 Solaris 2 Page Scanner

51 Thank You