IO Multiplexing

1
I/O Multiplexing
2
What is I/O multiplexing?

• When an application needs to handle multiple I/O
  descriptors at the same time
• E.g. file and socket descriptors, multiple socket
  descriptors
• When I/O on any one descriptor can result in
  blocking

3
Non-forking concurrent server
Concurrent Server select()
listenfd
Sockets ?
fd1
fd2
fd3
fd4
fd5
C1
C2
C3
C4
C5
Clients ?
4
I/O Models

• Blocking I/O
• Non-blocking I/O
• I/O multiplexing select()
• Signal driven I/O
• Asynchronous I/O

5
Blocking I/O
Application
Operating system
System call
No datagram ready
recvfrom()
Wait for data
datagram ready
Process blocks
Copy datagram
Copy data to user
Return ok
Copy complete
Process datagram
6
Non-Blocking I/O
Application
Operating system
System call
No datagram ready
recvfrom()
EWOULDBLOCK
System call
No datagram ready
recvfrom()
EWOULDBLOCK
datagram ready
System call
recvfrom()
Copy datagram
Copy data to user
Process blocks
Return ok
Copy complete
Process datagram
7
I/O Multiplexing
Application
Operating system
System call
No datagram ready
select()
Wait for data
Process blocks
Return readable
datagram ready
System call
Copy datagram
recvfrom()
Copy data to user
Process blocks
Return ok
Copy complete
Process datagram
8
Signal driven I/O
Application
Operating system
Establish SIGIO Signal handler
System call
No datagram ready
Wait for data
Process continues
Deliver SIGIO
datagram ready
Signal Handler
System call
Copy datagram
recvfrom()
Copy data to user
Process blocks
Return ok
Copy complete
Process datagram
9
Asynchronous I/O
Application
Operating system
System call
No datagram ready
aio_read()
return
Wait for data
datagram ready
Process continues
Copy datagram
Copy data to user
Return ok
Copy complete
Signal handler Process datagram
10
select() call

• Allows a process to wait for an event to occur on
  any one of its descriptors.
• Types of event
• ready for read
• ready for write
• Exception condition

11
select() call

• int select(
• int maxfdp1, / max. fd 1 /
• fd_set readfds, / read ready? /
• fd_set writefds, / write ready? /
• fd_set exceptfds, / exceptions? /
• struct timeval timeout)
• struct timeval
• long tv_sec / seconds /
• long tv_usec / microseconds /

12
struct fd_set

• Set of descriptors that we want to wait on for
  events.
• Typically holds 256 descriptor states.
• Manipulation macros
• void FD_ZERO(fd_set fds)
• void FD_SET (int fd, fd_set fds)
• void FD_CLR (int fd, fd_set fds)
• void FD_ISSET(int fd, fd_set fds)

13
Non-forking Concurrent Server

• fdset rdset, wrset
• int listenfd, connfd1, connfd2
• int maxfdp1
• Connection establishment etc.
• / initialize /
• FD_ZERO(rdset)
• FD_ZERO(wrset)

14

• for( )
• FD_SET(connfd1, rdset)
• FD_SET(connfd2, wrset)
• FD_SET(listenfd, rdset)
• maxfdp1 max(connfd1, connfd2, listenfd) 1
• / wait for some event /
• Select(maxfdp1, rdset, wrset, NULL, NULL)
• if( FD_ISSET(connfd1, rdset) )
• Read data from connfd1
• if( FD_ISSET(connfd2, wrset) )
• Write data to connfd2
• if( FD_ISSET(listenfd, rdset)
• Process a new connection

15
High resolution timer!

• Select can be used as a millisecond resolution
  timer.
• select ( 0, NULL, NULL, NULL, timeval)
• Usual sleep() call has resolution of seconds.

16
Socket Options
17
Managing socket options 3 ways

• getsockopt() and setsockopt()
• fcntl()
• ioctl()

18
getsockopt() and setsockopt()

• int getsockopt(
• int sockfd,
• int level, / SOL_SOCKET, IPPROTO_IP,
  IPPROTO_TCP etc /
• int optname, / option name /
• void optval, / option value /
• socklen_t optlen) / data length of option/
• int setsockopt(
• int sockfd,
• int level,
• int optname,
• const void optval,
• socklen_t optlen)

19
Some socket options

• SO_BROADCAST - enable socket for sending
  broadcast
• SO_KEEPALIVE regularly probes on inactive TCP
  connection.
• SO_LINGER linger till all data is sent before
  returning from close() on a TCP connection.
• SO_RCVBUF/ SO_SNDBUF size of receive/send
  buffers on socket.
• SO_RCVTIMEO/SO_SNDTIMEO place timeout on
  receive/send operation.

20
fcntl()

• int fcntl(int fd, int cmd, long arg)
• Miscellaneous file control operations
• Non-blocking I/O (O_NONBLOCK, F_SETFL)
• Signal-driven I/O (O_ASYNC, F_SETFL)
• Set socket owner (F_SETOWN)