I have the following code that creates a file using CreateFile with the FILE_FLAG_OVERLAPPED flag, and then calls WriteFile 100 times in a loop, passing in an OVERLAPPED structure
uint64_t GetPreciseTickCount()
{
FILETIME fileTime;
GetSystemTimePreciseAsFileTime(&fileTime);
ULARGE_INTEGER large;
large.LowPart = fileTime.dwLowDateTime;
large.HighPart = fileTime.dwHighDateTime;
return large.QuadPart;
}
uint64_t g_blockedTime = 0, g_waitTime = 0;
int main()
{
auto hFile = CreateFile(
L"test.dat",
GENERIC_WRITE,
0,
NULL,
CREATE_ALWAYS,
FILE_FLAG_OVERLAPPED | FILE_FLAG_NO_BUFFERING | FILE_FLAG_WRITE_THROUGH,
NULL);
if (hFile == INVALID_HANDLE_VALUE)
{
std::cout << "CreateFile failed with err " << GetLastError() << std::endl;
return 1;
}
uint32_t bufferSize = 4*1024*1024;
char* buffer = (char*)_aligned_malloc(bufferSize, 4096);
const int loop = 100;
LARGE_INTEGER endPosition;
endPosition.QuadPart = bufferSize * loop;
auto sfpRet = SetFilePointerEx(hFile, endPosition, nullptr, FILE_BEGIN);
if (sfpRet == INVALID_SET_FILE_POINTER)
{
std::cout << "SetFilePointer failed with err " << GetLastError() << std::endl;
return 1;
}
if (0 == SetEndOfFile(hFile))
{
std::cout << "SetEndOfFile failed with err " << GetLastError() << std::endl;
return 1;
}
auto start = GetPreciseTickCount();
OVERLAPPED overlapped;
auto completionEvent = CreateEvent(nullptr, TRUE, FALSE, nullptr);
for (int i = 0; i < loop; ++i)
{
overlapped.hEvent = completionEvent;
overlapped.Offset = i * bufferSize;
overlapped.OffsetHigh = 0;
overlapped.Internal = 0;
overlapped.InternalHigh = 0;
auto writeFileStart = GetPreciseTickCount();
auto err = WriteFile(
hFile,
buffer,
bufferSize,
nullptr,
&overlapped);
auto writeFileEnd = GetPreciseTickCount();
g_blockedTime += (writeFileEnd - writeFileStart) / 10;
if (err == FALSE)
{
auto lastErr = GetLastError();
if (lastErr != ERROR_IO_PENDING)
{
std::cout << "WriteFile failed with err " << lastErr << std::endl;
return 1;
}
auto waitErr = WaitForSingleObject(overlapped.hEvent, INFINITE);
g_waitTime += (GetPreciseTickCount() - writeFileEnd) / 10;
if (waitErr != 0)
{
std::cout << "WaitForSingleObject failed with err " << waitErr << std::endl;
return 1;
}
}
}
auto end = GetPreciseTickCount();
CloseHandle(hFile);
std::cout << "Took " << (end - start) / 10 << " micros" << std::endl;
std::cout << "Blocked time " << g_blockedTime << " micros" << std::endl;
std::cout << "Wait time " << g_waitTime << " micros" << std::endl;
}
The prints the following output
Took 1749086 micros
Blocked time 1700085 micros
Wait time 48896 micros
Why does WriteFile block? (as is evidenced by g_blockedTime being significantly higher than g_waitTime). Is there any way I can force it to be non-blocking?
Update: I updated the code to use SetFilePointerEx and SetEndOfFile before the loop. Still seeing the same blocking problem.
The solution is to call SetFilePointerEx, SetEndOfFile, and SetFileValidData before the loop. Then subsequent calls to WriteFile within the loop become non-blocking.
uint64_t GetPreciseTickCount()
{
FILETIME fileTime;
GetSystemTimePreciseAsFileTime(&fileTime);
ULARGE_INTEGER large;
large.LowPart = fileTime.dwLowDateTime;
large.HighPart = fileTime.dwHighDateTime;
return large.QuadPart;
}
uint64_t g_blockedTime = 0, g_waitTime = 0;
int main()
{
HANDLE hToken;
auto openResult = OpenProcessToken(GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, &hToken);
if (!openResult)
{
std::cout << "OpenProcessToken failed with err " << GetLastError() << std::endl;
return 1;
}
TOKEN_PRIVILEGES tp;
tp.PrivilegeCount = 1;
tp.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;
auto lookupResult = LookupPrivilegeValue(NULL, SE_MANAGE_VOLUME_NAME, &tp.Privileges[0].Luid);
if (!lookupResult)
{
std::cout << "LookupPrivilegeValue failed with err " << GetLastError() << std::endl;
return 1;
}
auto adjustResult = AdjustTokenPrivileges(hToken, FALSE, &tp, 0, NULL, NULL);
if (!adjustResult || GetLastError() != ERROR_SUCCESS)
{
std::cout << "AdjustTokenPrivileges failed with err " << GetLastError() << std::endl;
return 1;
}
auto hFile = CreateFile(
L"test.dat",
GENERIC_WRITE,
0,
NULL,
CREATE_ALWAYS,
FILE_FLAG_OVERLAPPED | FILE_FLAG_NO_BUFFERING | FILE_FLAG_WRITE_THROUGH,
NULL);
if (hFile == INVALID_HANDLE_VALUE)
{
std::cout << "CreateFile failed with err " << GetLastError() << std::endl;
return 1;
}
uint32_t bufferSize = 4*1024*1024;
char* buffer = (char*)_aligned_malloc(bufferSize, 4096);
const int loop = 100;
auto start = GetPreciseTickCount();
LARGE_INTEGER endPosition;
endPosition.QuadPart = bufferSize * loop;
auto setFileErr = SetFilePointerEx(hFile, endPosition, nullptr, FILE_BEGIN);
if (setFileErr == INVALID_SET_FILE_POINTER)
{
std::cout << "SetFilePointer failed with err " << GetLastError() << std::endl;
return 1;
}
if (!SetEndOfFile(hFile))
{
std::cout << "SetEndOfFile failed with err " << GetLastError() << std::endl;
return 1;
}
if (!SetFileValidData(hFile, bufferSize * loop))
{
std::cout << "SetFileValidData failed with err " << GetLastError() << std::endl;
return 1;
}
OVERLAPPED overlapped;
auto completionEvent = CreateEvent(nullptr, TRUE, FALSE, nullptr);
for (int i = 0; i < loop; ++i)
{
overlapped.hEvent = completionEvent;
overlapped.Offset = i * bufferSize;
overlapped.OffsetHigh = 0;
overlapped.Internal = 0;
overlapped.InternalHigh = 0;
auto writeFileStart = GetPreciseTickCount();
auto err = WriteFile(
hFile,
buffer,
bufferSize,
nullptr,
&overlapped);
auto writeFileEnd = GetPreciseTickCount();
g_blockedTime += (writeFileEnd - writeFileStart) / 10;
if (err == FALSE)
{
auto lastErr = GetLastError();
if (lastErr != ERROR_IO_PENDING)
{
std::cout << "WriteFile failed with err " << lastErr << std::endl;
return 1;
}
auto waitErr = WaitForSingleObject(overlapped.hEvent, INFINITE);
g_waitTime += (GetPreciseTickCount() - writeFileEnd) / 10;
if (waitErr != 0)
{
std::cout << "WaitForSingleObject failed with err " << waitErr << std::endl;
return 1;
}
}
}
auto end = GetPreciseTickCount();
CloseHandle(hFile);
std::cout << "Took " << (end - start) / 10 << " micros" << std::endl;
std::cout << "Blocked time " << g_blockedTime << " micros" << std::endl;
std::cout << "Wait time " << g_waitTime << " micros" << std::endl;
}
This produces the following output
Took 1508131 micros
Blocked time 19719 micros
Wait time 1481362 micros
Also, check out this article.