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1.1 虚拟内存管理机制
windows操作系统使用虚拟内存技术#xff0c;将磁盘文件#xff0c;通过映射对象#xff08;存储在物理内存#xff09;关联#xff0c;映射到虚拟内存作为文件试图。即用户操作虚拟内存中File View Objec…一 windows系统的内存管理涉及哪些
1.1 虚拟内存管理机制
windows操作系统使用虚拟内存技术将磁盘文件通过映射对象存储在物理内存关联映射到虚拟内存作为文件试图。即用户操作虚拟内存中File View Object- 物理内存中File Mapping - 磁盘文件。
1.2 分页和分段机制
windows采用分页机制将内存划分为固定大写的页面通过页表来映射虚拟地址和物理地址分段机制是将内存划分为不同大小的段通过段表来管理。
1.3 内存保护
windows通过硬件机制保护系统内存资源防止程序对内存的非法访问确保系统的稳定性和安全性
1.4 高级内存管理
windows还提供内存压缩、内存回收、内存优化等功能 内存压缩可以节省生存空间内存回收能释放不再使用的内存资源而内存优化则旨在提高系统性能和响应速度。
二 虚拟内存的实现技术 - 文件映射
2.1 磁盘文件文件映射对象文件视图的关系
MSDN - 文件映射 注 文件映射 将文件内存File on Disk)于进程的一部分虚拟地址空间关联。 文件映射对象调用CreateFileMapping创建文件映射对象维护磁盘文件和虚拟地址关联。 文件视图: 进程用来访问磁盘文件的虚拟地址空间部分。如果使用指针从文件视图读取和写入文件视图的过程就像使用动态分配的内存一样。使用文件映射可提高可提高效率因为文件驻留在磁盘上但文件视图驻留在内存中。
2.2 应用场景为什么要用文件映射
1解决大文件频繁读写效率问题 直接加载文件到内存传统方式 C读取txt文件
1 逐行读取
void readTxt(string file)
{ifstream infile; infile.open(file.data()); //将文件流对象与文件连接起来 assert(infile.is_open()); //若失败,则输出错误消息,并终止程序运行 string s;while(getline(infile,s)){coutsendl;}infile.close(); //关闭文件输入流
}传统方式将磁盘文件加载到内存中频繁操作磁盘文件。 文件映射方式将磁盘文件通过关联到文件映射对象中读取IO效率更高。减少磁盘文件访问次数。 2 解决多个进程访问共享磁盘文件场景 多个进程都可以访问文件映射对象。解决IPC通信数据复制的开销。
2.3 原理
通过文件映射机制进程可以通过访问内存的方式直接读写磁盘文件修改内容后由操作系统自动同步的文件进行更新。使用FlushViewOfFile可刷新缓存区将映射在虚拟内存当中的数据立即回写到磁盘文件。 过程如下所示 1. CreateFileMapping创建文件映射对象内核对象多进程可访问关联磁盘文件文件句柄hFile 2. MapViewOfFile将文件映射对象映射到进程虚拟地址
2.4 代码
/*This program demonstrates file mapping, especially how to align aview with the system file allocation granularity.
*/#include windows.h
#include stdio.h
#include tchar.h#define BUFFSIZE 1024 // size of the memory to examine at any one time#define FILE_MAP_START 138240 // starting point within the file of// the data to examine (135K)/* The test file. The code below creates the file and populates it,so there is no need to supply it in advance. */TCHAR * lpcTheFile TEXT(fmtest.txt); // the file to be manipulatedint main(void)
{HANDLE hMapFile; // handle for the files memory-mapped regionHANDLE hFile; // the file handleBOOL bFlag; // a result holderDWORD dBytesWritten; // number of bytes writtenDWORD dwFileSize; // temporary storage for file sizesDWORD dwFileMapSize; // size of the file mappingDWORD dwMapViewSize; // the size of the viewDWORD dwFileMapStart; // where to start the file map viewDWORD dwSysGran; // system allocation granularitySYSTEM_INFO SysInfo; // system information; used to get granularityLPVOID lpMapAddress; // pointer to the base address of the// memory-mapped regionchar * pData; // pointer to the dataint i; // loop counterint iData; // on success contains the first int of dataint iViewDelta; // the offset into the view where the data//shows up// Create the test file. Open it Create Always to overwrite any// existing file. The data is re-created belowhFile CreateFile(lpcTheFile,GENERIC_READ | GENERIC_WRITE,0,NULL,CREATE_ALWAYS,FILE_ATTRIBUTE_NORMAL,NULL);if (hFile INVALID_HANDLE_VALUE){_tprintf(TEXT(hFile is NULL\n));_tprintf(TEXT(Target file is %s\n),lpcTheFile);return 4;}// Get the system allocation granularity.GetSystemInfo(SysInfo);dwSysGran SysInfo.dwAllocationGranularity;// Now calculate a few variables. Calculate the file offsets as// 64-bit values, and then get the low-order 32 bits for the// function calls.// To calculate where to start the file mapping, round down the// offset of the data into the file to the nearest multiple of the// system allocation granularity.dwFileMapStart (FILE_MAP_START / dwSysGran) * dwSysGran;_tprintf (TEXT(The file map view starts at %ld bytes into the file.\n),dwFileMapStart);// Calculate the size of the file mapping view.dwMapViewSize (FILE_MAP_START % dwSysGran) BUFFSIZE;_tprintf (TEXT(The file map view is %ld bytes large.\n),dwMapViewSize);// How large will the file mapping object be?dwFileMapSize FILE_MAP_START BUFFSIZE;_tprintf (TEXT(The file mapping object is %ld bytes large.\n),dwFileMapSize);// The data of interest isnt at the beginning of the// view, so determine how far into the view to set the pointer.iViewDelta FILE_MAP_START - dwFileMapStart;_tprintf (TEXT(The data is %d bytes into the view.\n),iViewDelta);// Now write a file with data suitable for experimentation. This// provides unique int (4-byte) offsets in the file for easy visual// inspection. Note that this code does not check for storage// medium overflow or other errors, which production code should// do. Because an int is 4 bytes, the value at the pointer to the// data should be one quarter of the desired offset into the filefor (i0; i(int)dwSysGran; i){WriteFile (hFile, i, sizeof (i), dBytesWritten, NULL);}// Verify that the correct file size was written.dwFileSize GetFileSize(hFile, NULL);_tprintf(TEXT(hFile size: %10d\n), dwFileSize);// Create a file mapping object for the file// Note that it is a good idea to ensure the file size is not zerohMapFile CreateFileMapping( hFile, // current file handleNULL, // default securityPAGE_READWRITE, // read/write permission0, // size of mapping object, highdwFileMapSize, // size of mapping object, lowNULL); // name of mapping objectif (hMapFile NULL){_tprintf(TEXT(hMapFile is NULL: last error: %d\n), GetLastError() );return (2);}// Map the view and test the results.lpMapAddress MapViewOfFile(hMapFile, // handle to// mapping objectFILE_MAP_ALL_ACCESS, // read/write0, // high-order 32// bits of file// offsetdwFileMapStart, // low-order 32// bits of file// offsetdwMapViewSize); // number of bytes// to mapif (lpMapAddress NULL){_tprintf(TEXT(lpMapAddress is NULL: last error: %d\n), GetLastError());return 3;}// Calculate the pointer to the data.pData (char *) lpMapAddress iViewDelta;// Extract the data, an int. Cast the pointer pData from a pointer// to char to a pointer to int to get the whole thingiData *(int *)pData;_tprintf (TEXT(The value at the pointer is %d,\nwhich %s one quarter of the desired file offset.\n),iData,iData*4 FILE_MAP_START ? TEXT(is) : TEXT(is not));// Close the file mapping object and the open filebFlag UnmapViewOfFile(lpMapAddress);bFlag CloseHandle(hMapFile); // close the file mapping objectif(!bFlag){_tprintf(TEXT(\nError %ld occurred closing the mapping object!),GetLastError());}bFlag CloseHandle(hFile); // close the file itselfif(!bFlag){_tprintf(TEXT(\nError %ld occurred closing the file!),GetLastError());}return 0;
}
