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I/O Multiplexing

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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 socketdescriptors

When I/O on any one descriptor can resultin blocking

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Non-forking concurrent server

Concurrent Server

select()

C1 C2 C3 C4 C5

listenfd

fd1 fd2 fd3 fd4 fd5Sockets

Clients

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I/O Models

1. Blocking I/O

2. Non-blocking I/O

3. I/O multiplexingselect()

4. Signal driven I/O

5. Asynchronous I/O

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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

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Non-Blocking I/O

recvfrom() No datagram ready

datagram ready

Copy datagram


System call


Copy data to user

Return ok

Process blocks

EWOULDBLOCK

recvfrom() No datagram readySystem call

EWOULDBLOCK

recvfrom() System call

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I/O Multiplexing

select() No datagram ready

datagram ready

Copy datagram


System call


Wait for data

Copy data to user

Return ok

Process blocks

Return readable

Process blocks

recvfrom() System call

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Asynchronous I/O

aio_read() No datagram ready

datagram ready

Copy datagram

Copy completeSignal handler

Process datagram

System call


Wait for data

Copy data to user

Return ok

Process continues

return

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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

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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 */

}

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struct fd_set

Set of descriptors that we want to wait on forevents.

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)

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Non-forking Concurrent Server

fdsetrdset, wrset;int listenfd, connfd1, connfd2;

int maxfdp1;

Connection establishment etc.

/* initialize */

FD_ZERO(&rdset);

FD_ZERO(&wrset);

f ( ) {

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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

}

}

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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.

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Managing socket options3 ways

getsockopt() and setsockopt()

fcntl()

ioctl()

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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);

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Some socket options

SO_BROADCAST - enable socket for sending broadcast

SO_KEEPALIVEregularly probes on inactive TCPconnection.

SO_LINGERlinger till all data is sent before returningfrom close()on a TCP connection.

SO_RCVBUF/ SO_SNDBUFsize of receive/sendbuffers on socket.

SO_RCVTIMEO/SO_SNDTIMEOplace timeout onreceive/send operation.

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fcntl()

int fcntl(int fd, int cmd, long arg);

Miscellaneous file control operationsNon-blocking I/O (O_NONBLOCK, F_SETFL)

Signal-driven I/O (O_ASYNC, F_SETFL)

Set socket owner (F_SETOWN)

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