内存映射文件在大量顺序读取时速度更快?为什么?

问题描述

我使用下面的代码来衡量读取内存映射文件的大的、顺序的读取与仅调用ReadFile相比的性能差异:

I used the code below to measure the performance difference between reading large, sequential reads of a memory-mapped file, as compared to just calling ReadFile:

HANDLE hFile = CreateFile(_T("D:\\LARGE_ENOUGH_FILE"),
    FILE_READ_DATA, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING,
    FILE_FLAG_NO_BUFFERING, NULL);
__try
{
    const size_t TO_READ = 32 * 1024 * 1024;
    char sum = 0;
#if TEST_READ_FILE
    DWORD start = GetTickCount();
    char* p = (char*)malloc(TO_READ);
    DWORD nw;
    ReadFile(hFile, p, TO_READ, &nw, NULL);
#else
    HANDLE hMapping = CreateFileMapping(hFile, NULL, PAGE_READONLY,
        0, 0, NULL);
    const char* const p = (const char*)MapViewOfFile(hMapping,
        FILE_MAP_READ, 0, 0, 0);
    DWORD start = GetTickCount();
#endif
    for (size_t i = 0; i < TO_READ; i++)
    {
        sum += p[i]; // Do something kind of trivial...
    }
    DWORD end = GetTickCount();
    _tprintf(_T("Elapsed: %u"), end - start);
}
__finally { CloseHandle(hFile); }

(我只是改变了TEST_READ_FILE的值来改变测试.)

(I just changed the value of TEST_READ_FILE to change the test.)

令我惊讶的是，ReadFile慢约 20%！为什么?

To my surprise, ReadFile was slower by ~20%! Why?

推荐答案

FILE_FLAG_NO_BUFFERING 削弱了 ReadFile.内存映射文件可以随意使用它想要的任何预读算法，并且您已经禁止 ReadFile 这样做.您仅在 ReadFile 版本中关闭了缓存.没有文件缓存，内存映射文件就无法工作.

FILE_FLAG_NO_BUFFERING cripples ReadFile. The memory-mapped file is free to use whatever read-ahead algorithm it wants, and you've forbidden ReadFile to do the same. You've turned off caching only in the ReadFile version. Memory-mapped files can't work without file cache.

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