Some server “stress” tests Can our TCP concurrent server handle many successive and/or simultaneous connections?

‘tcpserver3.cpp’ This version of our concurrent server used a signal-handler to call ‘wait()’ whenever a child-process disconnected from a client and invoked the ‘exit()’ system-call It seemed to eliminate ‘zombie’ processes But we didn’t yet see how it performs in case multiple simultaneous connections are initiated by a client application

‘sixlinks.cpp’ A modification of our ‘tcpclient2.cpp’ that will attempt to establish six simultaneous connections with our ‘tcpserver3.cpp’ It does seem successful in responding to these client-requests very promptly, but we need to check for ‘zombie’ processes Is there a problem? If so, why? And how can we ‘fix’ that problem?

‘waitpid()’ There’s a more general form of the UNIX ‘wait()’ function which allows the caller to request ‘non-blocking’ behavior using an option-flag: $ waitpid( -1, NULL, WNOHANG ); ‘wait’ for any child-process we don’t need any ‘status’ info return immediately to the caller without blocking The function-value returned will be 0 if no more child-processes have exited without already having been waited for; or will be equal to the PID of a child that had earlier exited and was just now ‘waited for’ by this function-call

‘tcpserver4.cpp’ Modifies one line of the signal-handler in our ‘tcpserver3.cpp’ concurrent server so that all child-processes that have exited are ‘waited for’ before resuming the task Changes:wait( NULL ); To: while ( waitpid( -1, NULL, WNOHANG ) > 0 );

‘tcpstress.cpp’ This demo modifies our ‘tcpclient2.cpp’ so that it make a large number of separate connection-requests and data-exchanges in rapid succession, to test the server’s ability to respond promptly and correctly

acknowledgement The ideas used to motivate the ‘concurrent TCP server’ here are adaptations from the presentation by W. Richard Stevens in his classic text “UNIX Network Programming, 2 nd Ed”, Prentice-Hall (1998), volume 1

In-class exercise #1 Can you modify our ‘sixlinks.cpp’ program so that it will establish a much larger set of simultaneous connections? How many can you successfully establish before the system’s resources are exhausted?

In-class exercise #2 Is there any upper limit on the number of consecutive connection-requests that our ‘tcpstress.cpp’ client can establish, given the fact that each connection-request is closed (and system resources released) before the next connection is attempted?