I/O Multiplexing Chap 6

I/O Models  Blocking I/O Model  Nonblocking I/O Model  I/O Multiplexing Model  Signal Driven I/O Model  Asynchronous I/O Model

Blocking I/O Model

Nonblocking I/O Model

I/O Multiplexing Model

Signal Driven I/O Model

Asynchronous I/O Model  Synchronous vs. Asynchronous I/O  A synchronous I/O operation causes the requesting process to be blocked until that I/O operation completes.  An asynchronous I/O operation does not cause the requesting process to be blocked.

Comparison of I/O Model

Usage of I/O Multiplexing  Client  handles an interactive input and a socket  handles multiple sockets at the same time  Server  handles both a listening socket and its connected socket  handles both TCP and UDP  handles multiple services and perhaps multiple protocols (e.g., inetd daemon)

Select Functions  Wait for any one of multiple events to occur and wake up the process only when  one or more of these events occurs  or, a specified amount of time has passed  Wait possibilities  wait forever: timeout = NULL  wait up to a fixed amount of time  polling: do not wait at all: timer value = 0

Select Function(Cont’d)  Exception conditions  arrival of out-of-band data for a socket  presence of control status information to be read from the master side of pseudo terminal in packet mode  Descriptor sets(fd_set): array of integers(FD_SETSIZE)  if fdset == NULL, no interest on the condition  caution: value result arguments  macros

Conditions for Descriptor Ready (1)  A socket is ready for reading (readable)  Connected socket: read() returns  > 0 : data in socket receive buffer >= low-water mark( SO_RCVLOWAT )  by default, SO_RCVLOWAT == 1 (for TCP, UDP)  0 (end-of-file) : read-half of connection is closed(TCP has received a FIN)  -1 : socket error is pending  listening socket: # of completed connections > 0  A socket is ready for writing (writable): write returns  >= 0 : available space in socket send buffer >= low-water mark( SO_SNDLOWAT ) only if connected TCP socket or UDP socket  by default, SO_SNDLOWAT == 2048 (TCP, UDP)  -1  socket error is pending  write-half connection is closed: write() will generate SIGPIPE and return error EPIPE

Conditions for Descriptor Ready (2)  socket has an exception condition pending if  there exists out-of-band data  still at out-of-band mark

Filling the Pipe: Echo C/S  In stop-and-wait mode  response time = RTT(round-trip time) + server’s processing time(=0)  batch mode 로 (file 을 stdin 으로 redirection 해서 ) 1000 line 을 보내면 1000 x response time  Continuous Tx  Fill the pipe

str_cli Function - using I/O Multiplexing Eof on input  close socket  남은 reply 를 socket 에서 read 불가능

close versus shutdown  Close  decrease reference count  close the socket only if count == 0  terminates both directions of data transfer: reading and writing  Cannot read after close  Shutdown  initiate TCP’s normal termination regardless of reference count  terminates one direction (half of connection)  SHUT_RD: read-half of the connection is closed  Any data currently in socket receive buffer is discarded  Any data received after this call is acknowledged and then silently discarded  SHUT_WR: write-half of the connection is closed(half-close)  Any data currently in socket send buffer will be sent  SHUT_RDWR: read-half and write-half of connection are both closed  SHUT_RD + SHUT_WR

str_cli Function (Revisited)

TCP Echo Iterative Server

TCP Echo Iterative Server Program

Yet Another Problems  한 client 가 1byte data 보내고 계속 sleep 한다면 Readline() 에서 newline 도착할 때까지 blocked 됨  다른 client 도 서비스 불가  Denial of Service Attack  Possible Solutions  Use nonblocking I/O  Have each client serviced by a separate thread (or a process)  Place a timeout on the I/O operation