Transcript Some server “stress” tests Can our TCP concurrent server simultaneous connections?

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,
2nd 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?