c++ - 是否可以缓存从 MapViewOfFile 返回的映射区域？

Question

下午好，众所周知，在处理大文件时无法映射到 Win32 中的一个 View ，创建仔细映射的代码并根据需要取消映射文件区域。 pastebin 网址是:

我创建并测试了一个处理大文件的 cMemoryMappedFile 类不能映射到 Win32 中的一个 View 。我测试了类(class)并发现虽然它功能正常，但需要很长时间(即 3 秒)随机访问。这是因为该类必须取消映射和映射文件每个随机访问的区域。我想知道是否有可能缓存从 MapViewFile 返回的映射区域以加速随机访问。

昨天，我注意到 UnMapViewOfFile 使之前的从 MapViewOfFile 返回的映射区域。有没有人有想法如何通过缓存或其他方式加速随机访问？

当前视口(viewport)为 128KB。我相信如果我放大viewport 它将减少对 UnMapViewOfFile 的调用次数和 MapViewOfFile。但是，我想知道是否可以使用其他方法。请看方法，
char* cMemoryMappedFile::GetPointer(int , bool) 看看如何视口(viewport)随文件映射移动。谢谢。

类的 pastebin url 是
> 。我在这里添加源代码以防万一没有人可以访问该 url。

// cMemoryMappedFile.Cpp
#include "cException.h"
#include "cMemoryMappedFile.h"

#define BUFFER_SIZE 10

#define MEM_BLOCK_SIZE 65536 * 2

/**
\class cMemoryMappedFile
\brief Encapsulation of the Windows Memory Management API.

The cMemoryMapped class makes some memory mapping operations easier.
*/

/**
\brief Constructor for cMemoryMappedFile object.

\param FileSize    Size of file.
\param OpenMode    File open mode 
\param AccessModes File access mode 
\param ShareMode   File sharing mode 
\param Flags       File attributes and flags 
\param ShareMode   File sharing mode 
\param Flags       File attributes and flags
\param Security    Security Attributes 
\param Template    Extended attributes tp apply to a newly created file
*/
cMemoryMappedFile::cMemoryMappedFile(long FileSize_, OpenModes OpenMode_,AccessModes AccessMode_,
    ShareModes ShareMode_,long Flags_,void *Security_,FILEHANDLE Template_) {
    FileSize = FileSize_;
char buffer[BUFFER_SIZE]; 
DWORD dwRetVal = 0;
UINT uRetVal   = 0; 
     DWORD dwPtr    = 0;
BOOL isSetEndOfFile = FALSE;
     LARGE_INTEGER Distance_;
DWORD ErrorCode = 0;

char lpTempPathBuffer[MAX_PATH]; 

     PreviousNCopy = 0;
PreviousN     = 0;

//  Gets the temp path env string (no guarantee it's a valid path).
dwRetVal = GetTempPath(MAX_PATH,          // length of the buffer
                       lpTempPathBuffer); // buffer for path 
if (dwRetVal > MAX_PATH || (dwRetVal == 0))
{
   throw cException(ERR_MEMORYMAPPING,"");  
     }

     //  Generates a temporary file name. 
     uRetVal = GetTempFileName(lpTempPathBuffer, // directory for tmp files
                          TEXT("DEMO"),     // temp file name prefix 
                          0,                // create unique name 
                          TempFileName);  // buffer for name 
     if (uRetVal == 0)
     {

      throw cException(ERR_MEMORYMAPPING,lpTempPathBuffer);  
     }
     //  Creates the new file
     hFile = CreateFile((LPTSTR) TempFileName, // file name 
                       AccessMode_,        // open for write 
                       0,                    // do not share 
                       (SECURITY_ATTRIBUTES *) Security_,  // default security 
                       OpenMode_, // CREATE_ALWAYS,       
                       Flags_,// normal file 
                       Template_);                // no template 
     if (hFile == INVALID_HANDLE_VALUE) 
     { 
       throw cException(ERR_MEMORYMAPPING,TempFileName);   
     } 
     Distance_.LowPart = (ULONG)FileSize_;
Distance_.HighPart = 0; // (ULONG)(FileSize_ >> 32);
dwPtr = ::SetFilePointer(hFile,Distance_.LowPart,
    &(Distance_.HighPart), FileBegin);

if (dwPtr == INVALID_SET_FILE_POINTER){
   throw cException(ERR_MEMORYMAPPING,TempFileName);
}
isSetEndOfFile = SetEndOfFile(hFile);
if (!isSetEndOfFile){
   ErrorCode = GetLastError();
   throw cException(ERR_MEMORYMAPPING,TempFileName);
}
hMapping=::CreateFileMapping(hFile,(SECURITY_ATTRIBUTES *)Security_,PAGE_READWRITE,0,0,0);
if (hMapping==NULL)
    throw cException(ERR_MEMORYMAPPING,TempFileName);   

MapPtr = 0;
adjustedptr = 0;
prevadjustedptr = adjustedptr;

    FilePath=new char[strlen(TempFileName)+1];
    strcpy(FilePath,TempFileName);
}



char * cMemoryMappedFile::GetPointer(int n, bool Caching){
unsigned int baseoff; 
if( n < MEM_BLOCK_SIZE / 2)
{
  baseoff = 0;
}
else
{
  baseoff = ((n + MEM_BLOCK_SIZE / 4) & 
    (~(MEM_BLOCK_SIZE / 2 - 1))) - MEM_BLOCK_SIZE / 2;

}
// the correct memory mapped view is already mapped in
     if (adjustedptr != 0 && mappedoffset == baseoff && Caching)
    return adjustedptr;
else if (Caching)
{
  /*    
   retrieve adjustedptr from cache
      */
}
// get a new memory mapped viewport
else{
    if (MapPtr){
                 UnmapViewOfFile(MapPtr);
       PreviousNCopy = PreviousN;
       prevadjustedptr = adjustedptr;
    }
    PreviousN = n;
              mappedlength = min(FileSize - baseoff, MEM_BLOCK_SIZE); 

              // MapViewOfFile should be aligned to 64K boundary

    MapPtr = (char*)::MapViewOfFile( hMapping, 
                       FILE_MAP_WRITE | FILE_MAP_READ, 0, 
        baseoff, mappedlength);
              mappedoffset =    baseoff;
    adjustedptr = MapPtr - mappedoffset; 
    printf("Value: %u n: %u\n",adjustedptr[n],n);

 /*
    cache PreviousNCopy,PreviousN, prevadjustedptr[PreviousNCopy]
 */

}
 return adjustedptr; 
}

Answer 1

你可以有一个“自由列表”风格的缓存——当你的类(class)的用户要求取消映射一个你并不真正关心的区域时，你只需将它添加到列表中即可。当他们要求映射一个新区域时，如果可能的话，你可以重用现有的映射，否则你创建一个新的映射，如果你打开了太多的映射，或者映射的大小在哪里，就从缓存中删除最近最少使用的映射缓存的映射太大。