How can I invalidate the file system cache?

Question

I want to measure/optimize the "cold boot" startup performance of an application, and it's difficult to do this without an actual reboot, which is obviously not an ideal solution.

Is there a way I could invalidate entire system's file cache, so that mapped page accesses actually cause a disk access, so that I can measure the time my program takes to start up?

Information:

I pretty much need FSCTL_DISMOUNT_VOLUME's functionality, but for the system volume.

Answer 1

At least on Windows 7, it seems that attempting to open a volume handle without FILE_SHARE_WRITE sharing permissions causes the file system cache to be invalidated, even if the creation fails.

Thus I made a program that simply calls CreateFile to this end.

Base64 encoding of the program*:

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Source:

// Usage: ClearCache C: D: #include <tchar.h> #include <stdio.h> #include <windows.h>  int _tmain(int argc, LPTSTR argv[]) {     LPCTSTR DOS_PREFIX = _T("\\\\.\\");     for (int i = 1; i < argc; i++) {         LPTSTR arg = argv[i];         LPTSTR path = (LPTSTR)calloc(             _tcslen(arg) + _tcslen(DOS_PREFIX) + 1, sizeof(*arg));         __try {             if (_istalpha(arg[0]) && arg[1] == _T(':') &&                (arg[2] == _T('\0') ||                 arg[2] == _T('\\') && arg[3] == _T('\0')))             { _tcscat(path, DOS_PREFIX); }             _tcscat(path, arg);             HANDLE hFile = CreateFile(path,                 FILE_READ_DATA, FILE_SHARE_READ, NULL, OPEN_EXISTING, 0, NULL);             if (hFile != INVALID_HANDLE_VALUE) { CloseHandle(hFile); }             else {                 DWORD le = GetLastError();                 if (le != ERROR_SHARING_VIOLATION && le != ERROR_ACCESS_DENIED)                 {                     _ftprintf(stderr, _T("Error %d clearing %s\n"), le, argv[i]);                     return le;                 }             }         } __finally { free(path); }     }     return 0; } 

_{*Just for fun, see if you can figure out what the executable does by disassembling it. It's not your typical executable. :)}

Answer 2

I've written a simple command-line utility to do that: FlushFileCache

It relies on the undocumented NtSetSystemInformation functions, and can flush the various other memory pools as well.