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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); }
(I just changed the value of TEST_READ_FILE to change the test.)
To my surprise, ReadFile was slower by ~20%! Why?
536
Accessing files via memory map is faster than using I/O functions such as fread and fwrite . Data are read and written using the virtual memory capabilities that are built in to the operating system rather than having to allocate, copy into, and then deallocate data buffers owned by the process.
Performance: Memory mapped writing is often fast as no stream/file buffers are used. OS does the actual file writing, usually in blocks of several kilo bytes at once. One downside would be, unless you're writing sequentially there could be page faults slowing down your program.
What mmap helps with is that there is no extra user space buffer involved, the "read" takes place there where the OS kernel sees fit and in chunks that can be optimized. This may be an advantage in speed, but first of all this is just an interface that is easier to use.
Benefits. The benefit of memory mapping a file is increasing I/O performance, especially when used on large files. For small files, memory-mapped files can result in a waste of slack space as memory maps are always aligned to the page size, which is mostly 4 KiB.
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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