windows内存管理

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
在这里插入图片描述
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))
    {
        cout<<s<<endl;
    }
    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 a
   view 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 manipulated

int main(void)
{
  HANDLE hMapFile;      // handle for the file's memory-mapped region
  HANDLE hFile;         // the file handle
  BOOL bFlag;           // a result holder
  DWORD dBytesWritten;  // number of bytes written
  DWORD dwFileSize;     // temporary storage for file sizes
  DWORD dwFileMapSize;  // size of the file mapping
  DWORD dwMapViewSize;  // the size of the view
  DWORD dwFileMapStart; // where to start the file map view
  DWORD dwSysGran;      // system allocation granularity
  SYSTEM_INFO SysInfo;  // system information; used to get granularity
  LPVOID lpMapAddress;  // pointer to the base address of the
                        // memory-mapped region
  char * pData;         // pointer to the data
  int i;                // loop counter
  int iData;            // on success contains the first int of data
  int 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 below
  hFile = 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 isn't 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 file

  for (i=0; 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 zero
  hMapFile = CreateFileMapping( hFile,          // current file handle
                NULL,           // default security
                PAGE_READWRITE, // read/write permission
                0,              // size of mapping object, high
                dwFileMapSize,  // size of mapping object, low
                NULL);          // name of mapping object

  if (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 object
                               FILE_MAP_ALL_ACCESS, // read/write
                               0,                   // high-order 32
                                                    // bits of file
                                                    // offset
                               dwFileMapStart,      // low-order 32
                                                    // bits of file
                                                    // offset
                               dwMapViewSize);      // number of bytes
                                                    // to map
  if (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 thing
  iData = *(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 file

  bFlag = UnmapViewOfFile(lpMapAddress);
  bFlag = CloseHandle(hMapFile); // close the file mapping object

  if(!bFlag)
  {
    _tprintf(TEXT("\nError %ld occurred closing the mapping object!"),
             GetLastError());
  }

  bFlag = CloseHandle(hFile);   // close the file itself

  if(!bFlag)
  {
    _tprintf(TEXT("\nError %ld occurred closing the file!"),
           GetLastError());
  }

  return 0;
}