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.

Download the program* from its Base64 version here:

<!-- Click "Run Snippet", then Right-Click -> Save As... -->
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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. 🙂}