Virtual MemoryContentsMemory Mapped FilesUses of Memory Mapped FilesSlide 5POSIX <sys/mman.h>Protection AttributesMap first 4kb of file and read intSlide 9munmap, msyncSharing PagesShared PagesProtectionPage Protection via PTEVirtual Memory Under MultiprogrammingTerminologyIssue: EvictionPage Replacement StrategiesPrincipal of OptimalityFrame Allocation for Multiple ProcessesMulti-Programming Frame AllocationSlide 22ThrashingPage Fault Rate vs. Size CurveWhy Thrashing?Results of ThrashingWhy?Solution: Working SetWorking set (1968, Denning)Working SetCalculating Working SetWorking Set in Action to Prevent ThrashingWorking sets of real programsWorking Set Implementation IssuesPage Fault Frequency Working SetSlide 36SummaryCS 241 Fall 2007System Programming 1Virtual MemoryLawrence Angrave and Vikram Adve2ContentsMemory mapped filesPage sharingPage protectionVirtual Memory and MultiprogrammingPage eviction policiesPage frame allocation and thrashingWorking set model and implementation3Memory Mapped FilesMemory Mapped FileIn BlocksVM of UserMmap requestsDiskFileBlocks of dataFrom file mappedTo VM4Uses of Memory Mapped FilesDynamic loading. By mapping executable files and shared libraries into its address space, a program can load and unload executable code sections dynamically. Fast File I/O. When you call file I/O functions, such as read() and write(), the data is copied to a kernel's intermediary buffer before it is transferred to the physical file or the process. This intermediary buffering is slow and expensive. Memory mapping eliminates this intermediary buffering, thereby improving performance significantly.5Uses of Memory Mapped FilesStreamlining file access. Once you map a file to a memory region, you access it via pointers, just as you would access ordinary variables and objects. Memory sharing. Memory mapping enables different processes to share physical memory pagesMemory persistence. Memory mapping enables processes to share memory sections that persist independently of the lifetime of a certain process.6POSIX <sys/mman.h> caddr_t mmap( caddress_t map_addr, /* VM address hint;0 for no preference */ size_t length, /* Length of file map*/int protection, /* types of access*/int flags, /*attributes*/int fd, /*file descriptor*/off_t offset); /*Offset file map start*/7Protection AttributesPROT_READ /* the mapped region may be read */PROT_WRITE /* the mapped region may be written */PROT_EXEC /* the mapped region may be executed */SIGSEGV signal if you reference memory with wrong protection mode.8Map first 4kb of file and read int#include <errno.h> #include <fcntl.h> #include <sys/mman.h> #include <sys/types.h> int main(int argc, char *argv[]) { int fd; void * pregion; if (fd= open(argv[1], O_RDONLY) <0) { perror("failed on open"); return –1; }9Map first 4kb of file and read int/*map first 4 kilobytes of fd*/ pregion =mmap(NULL, 4096, PROT_READ, MAP_SHARED, fd, 0); if (pregion==(caddr_t)-1) { perror("mmap failed") return –1; } close(fd); /*close physical file: we don't need it *//* access mapped memory; read the first int in the mapped file */int val= *((int*) pregion); }10munmap, msyncint munmap(caddr_t addr, int length);int msync (caddr_t addr, size_t length, int flags);addr must be multiple of page size:size_t page_size = (size_t) sysconf (_SC_PAGESIZE);11Sharing PagesCode and data can be sharedMap common page frame in two processesCode, data must be position-independent VM mappings for same code, data in different processes are different12Shared Pages13ProtectionWhy Page Protection?Implementing Page ProtectionRead, Write, eXecute bits in page table entryCheck is done by hardware during accessIllegal access generates SIGSEGVEach process can have different protection bits14Page Protection via PTELegend: Reference - page has been accessed Valid - page exists Resident - page is cached in primary memory Dirty - page has been changed since page in15Virtual Memory Under MultiprogrammingEviction of Virtual PagesOn page fault: Choose VM page to page outHow to choose which data to page out?Allocation of Physical Page FramesHow to assign frames to processes?16TerminologyReference string: memory reference sequence generated by a program:Reference = (R/W, address)Paging – moving pages to or from diskDemand Paging – moving pages only when neededOptimal – the best (theoretical) strategyEviction – throwing something outPollution – bringing in useless pages/lines17Issue: EvictionHopefully, kick out a less-useful pageDirty pages require writing, clean pages don’tGoal: kick out the page that’s least usefulProblem: how do you determine utility?Heuristic: temporal locality existsKick out pages that aren’t likely to be used again18Page Replacement StrategiesThe Principle of Optimality Replace page that will be used the farthest in the future. Random page replacement Choose a page randomly FIFO - First in First Out Replace the page that has been in primary memory the longest LRU - Least Recently Used Replace the page that has not been used for the longest time LFU - Least Frequently Used Replace the page that has been used least often NRU - Not Recently Used An approximation to LRU. Working Set Keep in memory those pages that the process is actively using.19Principal of Optimality Description: Assume each page can be labeled with number of references that will be executed before that page is first referenced.Then the optimal page algorithm would choose the page with the highest label to be removed from the memory. Impractical! Why? Provides a basis for comparison with other schemes.If future references are knownshould not use demand paging should use “pre-paging” to overlap paging with computation.20Frame Allocation for Multiple ProcessesHow are the page frames allocated to individual virtual memories of the various jobs running in a multi-programmed environment?Simple solution Allocate a minimum number (??) of frames per process. One page from the current executed instructionMost instructions require two operandsinclude an extra page for paging out and one for paging in21Multi-Programming Frame AllocationSolution 2allocate an equal number of frames per job but jobs use memory unequallyhigh priority jobs have same number of page frames as low priority jobsdegree of
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