Windows WriteFile blocks even with FILE_FLAG_OVERLAPPED - c++

Question

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.

Answer 1

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.