I/O Multiplexing

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

Non-forking concurrent serverConcurrent Server

select()

C1 C2 C3 C4 C5

listenfd

fd1 fd2 fd3 fd4 fd5Sockets

Clients

I/O Models

1. Blocking I/O

2. Non-blocking I/O

3. I/O multiplexing – select()

4. Signal driven I/O

5. Asynchronous I/O

Blocking I/O

recvfrom() No datagram ready

datagram ready

Copy datagram

Copy completeProcess datagram

System call

Application Operating system

Wait for data

Copy data to user

Return ok

Process blocks

Non-Blocking I/O

recvfrom() No datagram ready

datagram ready

Copy datagram

Copy completeProcess datagram

System call

Application Operating system

Copy data to user

Return ok

Process blocks

EWOULDBLOCK

recvfrom() No datagram readySystem call

EWOULDBLOCK

recvfrom() System call

I/O Multiplexing

select() No datagram ready

datagram ready

Copy datagram

Copy completeProcess datagram

System call

Application Operating system

Wait for data

Copy data to user

Return ok

Process blocks

Return readable

Process blocks

recvfrom() System call

Signal driven I/O

Establish SIGIOSignal handler No datagram ready

datagram ready

Copy datagram

Copy completeProcess datagram

System call

Application Operating system

Wait for data

Copy data to user

Return ok

Process blocks

Deliver SIGIO

Process continues

recvfrom() System callSignal Handler

Asynchronous I/O

aio_read() No datagram ready

datagram ready

Copy datagram

Copy completeSignal handlerProcess datagram

System call

Application Operating system

Wait for data

Copy data to user

Return ok

Process continues

return

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

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 */

}

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)

Non-forking Concurrent Server

fdset rdset, wrset;

int listenfd, connfd1, connfd2;

int maxfdp1;

………

Connection establishment etc.

………

/* initialize */

FD_ZERO(&rdset);

FD_ZERO(&wrset);

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…

}}

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.

Socket Options

Managing socket options – 3 ways

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

getsockopt() and setsockopt()

int getsockopt(int sockfd, int level, /* SOL_SOCKET, IPPROTO_IP, IPPROTO_TCP etc */

int optname, /* option name */void *opt val, /* option value */socklen_t *optlen); /* data length of option*/

int setsockopt(int sockfd, int level, int optname, const void *optval, socklen_t optlen);

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.

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)