IOCP when read is busy? - c++

Question

while I am writing some IOCP server-client codes, I saw misc behavior in IOCP.
The scenario goes here,
Register Socket to IOCP
Recv event catched by GetQueuedCompletionStatus
while (!mExitFlag)
{
bool bSuccess = ::GetQueuedCompletionStatus(mIocpHandle, &dwIoSize, (PULONG_PTR)&client, (LPOVERLAPPED*)&ioData, INFINITE);
logger->st = std::chrono::steady_clock::now();
// ... queue to recv worker
}
then read buffer (char[]) related to iocp registered buffer (WSABUF)
int dataLength = recvBytes; // when iocp completed
int pktLength = Serializer::toInt32(mBuffer + mDataPos);
if (dataLength > 0 && pktLength == 0)
{
using namespace std::chrono;
char buffer[512];
if (mBuffer[mDataPos] == 0)
{
// take snapshot
memcpy(buffer, &mBuffer[mDataPos], dataLength);
}
while (mBuffer[mDataPos] == 0) { }
// elapsed < 1ms
auto elapsed_in_microseconds = CTimer::count<microseconds>(mLogger->st);
printf("elapsed %llu us", elapsed_in_microseconds);
int val = mBuffer[mDataPos]; // this gives positive value
throw std::runtime_error("serializer failed to read packet length");
}
Snapshot in buffer[512] gives always 0 padded with dataLength.
After some microseconds elapsed, the mBuffer (WSABUF registered) is retrieved with data.
I checked the recv pending and handling in the single thread with log.
I observed this only happens when client sends huge data in shortly.
When the client sends data with term (10ms?), it was fine.
Does anyoneknows this IOCP issue?
Perhaps the solution can be waiting the buffer when client recv buffer is busy.