First client is laggy in multi-socket winsock server - c++

Question

I have a WinSock server setup, which is properly accepting clients and relaying the appropriate information. The server takes two clients, receives a fixed size buffer of 256 bytes, stores it, and then relays the other buffer to the client. (Ie. client1 sends its buffer, server saves it, then sends client1 the buffer for client2).
Anytime client1 changes its buffer, it takes roughly 4 seconds for client2 to receive the changes. If client2 makes a change, client1 receives the update almost instantly (less than 0.1s).
Nagle's algorithm is disabled and I've tried changing the order which the server processes the requests, but client1 always lags. The data always shows up intact, but takes too long. Below is the loop the server uses to process the data:
for(;;)
{
// check if more clients can join
if (numClients < MAX_CLIENTS)
{
theClients[numClients] = accept(listeningSocket, NULL, NULL);
if (theClients[numClients] == INVALID_SOCKET)
{
nret = WSAGetLastError();
JBS::reportSocketError(nret, "server accept()");
closesocket(listeningSocket);
WSACleanup();
exit(0);
}
// disable Nagle's algorithm
int flag = 1;
int result = setsockopt(theClients[numClients], IPPROTO_TCP, TCP_NODELAY,
(char *) &flag, sizeof(int));
if (result < 0)
{
nret = WSAGetLastError();
JBS::reportSocketError(nret, "client connect()");
closesocket(theClients[numClients]);
WSACleanup();
}
// make the socket non-blocking
u_long iMode = 1;
ioctlsocket(theClients[numClients],FIONBIO, &iMode);
cout << "Client # " << numClients << " connected." << endl;
numClients++;
started = true;
}
else
{
// we've received all the connections, so close the listening socket
closesocket(listeningSocket);
}
// process client2
if (theClients[1] != INVALID_SOCKET)
{
memset(keys2, 0, 255);
// receive the updated buffer
nBytes = recv(theClients[1], keys2, sizeof(keys2), 0);
receiveResult = WSAGetLastError();
if ((receiveResult != WSAEWOULDBLOCK) && (receiveResult != 0))
{
JBS::reportSocketError(receiveResult, "server receive keys2()");
shutdown(theClients[1],2);
closesocket(theClients[1]);
WSACleanup();
exit(0);
break;
}
// send client1's buffer to client2
send(theClients[1],keys1,sizeof(keys1),0);
sendResult = WSAGetLastError();
if((sendResult != WSAEWOULDBLOCK) && (sendResult != 0))
{
JBS::reportSocketError(sendResult, "server send keys1()");
shutdown(theClients[1],2);
closesocket(theClients[1]);
WSACleanup();
exit(0);
break;
}
}
// process client1
if (theClients[0] != INVALID_SOCKET)
{
memset(keys1, 0, 255);
// receive the updated buffer
nBytes = recv(theClients[0], keys1, sizeof(keys1), 0);
receiveResult = WSAGetLastError();
if ((receiveResult != WSAEWOULDBLOCK) && (receiveResult != 0))
{
JBS::reportSocketError(receiveResult, "server receive keys1()");
shutdown(theClients[0],2);
closesocket(theClients[0]);
WSACleanup();
exit(0);
break;
}
// send client2's buffer to client1
send(theClients[0],keys2,sizeof(keys2),0);
sendResult = WSAGetLastError();
if((sendResult != WSAEWOULDBLOCK) && (sendResult != 0))
{
JBS::reportSocketError(sendResult, "server send keys2()");
shutdown(theClients[0],2);
closesocket(theClients[0]);
WSACleanup();
exit(0);
break;
}
}
Sleep((float)(1000.0f / 30.0f));
}
Client sending code:
int nError, sendResult;
sendResult = send(theSocket, keys, sizeof(keys),0);
nError=WSAGetLastError();
if((nError != WSAEWOULDBLOCK) && (nError != 0))
{
JBS::reportSocketError(sendResult, "client send()");
shutdown(theSocket,2);
closesocket(theSocket);
WSACleanup();
}

Answer 1

I've pasted your code below, with some inline comments in it, mostly because I can't fit it all reaonsably in a comment. How are you determining that it's taking four seconds for changes to get from client1 to client2? Visual inspection? Does this mean that Client1 & Client2 are running on the same machine (no different network latency issues to worry about)?
I've highlighted some blocks that look wrong. They may not be, it may be because you've tried to simplify the code that you've posted and you've missed some bits. I've also made some suggestions for where you might want to add some logging. If the sockets are really non-blocking you should be coming back from all of the calls very quickly and failing to read data, unless the client has sent it. If you've got a 4 second delay, then the problem could be:
the client hasn't sent it... is Nagle disabled on the client? If this were the case, I'd expect successive calls to recv to happen, with no data.
The recv call is taking too long... is the socket really in non-blocking mode?
The send call is taking too long... is the socket in non-blocking mode, is it buffered, is the client trying to receive the data?
Having the times each section of code takes will help to track down where your problem is.
You can get the time, using something like this (borrowed from the web):
struct timeval tv;
struct timezone tz;
struct tm *tm;
gettimeofday(&tv, &tz);
tm=localtime(&tv.tv_sec);
printf(" %d:%02d:%02d %d \n", tm->tm_hour, tm->tm_min,
m->tm_sec, tv.tv_usec);
Your code:
for(;;)
{
/* This block of code is checking the server socket and accepting
* connections, until two? (MAX_CLIENTS isn't defined in visible code)
* connections have been made. After this, it is attempting to close
* the server socket everytime around the loop. This may have side
* effects (although probably not), so I'd clean it up, just in case
*/
/* LOG TIME 1 */
// check if more clients can join
if (numClients < MAX_CLIENTS)
{
theClients[numClients] = accept(listeningSocket, NULL, NULL);
if (theClients[numClients] == INVALID_SOCKET)
{
nret = WSAGetLastError();
JBS::reportSocketError(nret, "server accept()");
closesocket(listeningSocket);
WSACleanup();
exit(0);
}
// disable Nagle's algorithm
int flag = 1;
int result = setsockopt(theClients[numClients], IPPROTO_TCP, TCP_NODELAY,
(char *) &flag, sizeof(int));
if (result < 0)
{
nret = WSAGetLastError();
JBS::reportSocketError(nret, "client connect()");
closesocket(theClients[numClients]);
WSACleanup();
}
// make the socket non-blocking
u_long iMode = 1;
ioctlsocket(theClients[numClients],FIONBIO, &iMode);
cout << "Client # " << numClients << " connected." << endl;
numClients++;
/* This started variable isn't used, is it supposed to be wrapping
* this server code in an if statement?
*/
started = true;
}
else
{
// we've received all the connections, so close the listening socket
closesocket(listeningSocket);
}
/* LOG TIME 2 */
// process client2
if (theClients[1] != INVALID_SOCKET)
{
memset(keys2, 0, 255);
// receive the updated buffer
/* LOG TIME 3 */
nBytes = recv(theClients[1], keys2, sizeof(keys2), 0);
/* LOG TIME 4 */
receiveResult = WSAGetLastError();
if ((receiveResult != WSAEWOULDBLOCK) && (receiveResult != 0))
{
JBS::reportSocketError(receiveResult, "server receive keys2()");
shutdown(theClients[1],2);
closesocket(theClients[1]);
WSACleanup();
exit(0);
break;
}
// send client1's buffer to client2
/* LOG TIME 5 */
send(theClients[1],keys1,sizeof(keys1),0);
/* LOG TIME 6 */
sendResult = WSAGetLastError();
if((sendResult != WSAEWOULDBLOCK) && (sendResult != 0))
{
JBS::reportSocketError(sendResult, "server send keys1()");
shutdown(theClients[1],2);
closesocket(theClients[1]);
WSACleanup();
exit(0);
break;
}
}
// process client1
/* If the client has been accepted (note that because this
* is part of the same block of code, and there's no protection
* around it, the first connection will process it's first
* receive/send combination before the second socket has been accepted)
*/
if (theClients[0] != INVALID_SOCKET)
{
memset(keys1, 0, 255);
// receive the updated buffer
/* You're trying a receive against a non-blocking socket. I would expect this
* to fail with WSAEWOULDBLOCK, if nothing has been sent by the client, but
* this block of data will still be sent to the client
*/
/* LOG TIME 7 */
nBytes = recv(theClients[0], keys1, sizeof(keys1), 0);
/* LOG TIME 8 */
receiveResult = WSAGetLastError();
if ((receiveResult != WSAEWOULDBLOCK) && (receiveResult != 0))
{
JBS::reportSocketError(receiveResult, "server receive keys1()");
shutdown(theClients[0],2);
closesocket(theClients[0]);
WSACleanup();
exit(0);
break;
}
// send client2's buffer to client1
/* The first time around the loop, you're sending the buffer to the
* first connected client, even though the second client hasn't connected yet.
* This will continue 30 times a second, until the second client connects. Does
* the client handle this correctly?
*/
/* LOG TIME 9 */
send(theClients[0],keys2,sizeof(keys2),0);
/* LOG TIME 10 */
sendResult = WSAGetLastError();
if((sendResult != WSAEWOULDBLOCK) && (sendResult != 0))
{
JBS::reportSocketError(sendResult, "server send keys2()");
shutdown(theClients[0],2);
closesocket(theClients[0]);
WSACleanup();
exit(0);
break;
}
}
Sleep((float)(1000.0f / 30.0f));
}
Client sending code:
int nError, sendResult;
/* There's no recv / loop in this section
*/
sendResult = send(theSocket, keys, sizeof(keys),0);
nError=WSAGetLastError();
if((nError != WSAEWOULDBLOCK) && (nError != 0))
{
JBS::reportSocketError(sendResult, "client send()");
shutdown(theSocket,2);
closesocket(theSocket);
WSACleanup();
}