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I/O Multiplexing

I/O multiplexing is the ability to serve multiple I/O channels (or anything that can be referenced via a file descriptor / handle) simultaneously. If a given application, such a server, has multiple sockets on which it serves connection, it may be the case that operating on one socket blocks the server. One solution is using asynchronous operations, with different backends. The other solution is using I/O multiplexing.

With I/O multiplexing, the API provides a structure / an array to list all used channels. And then it provides an API to operate on that channel. And a (blocking) function to retrieve the channel that is ready for interaction (or channels). So instead of waiting for a given channel (while others may be ready), you now simultaneously wait on all channels, and the ready channel is returned by the function.

The classical functions for I/O multiplexing are select and poll. Due to several limitations, modern operating systems provide advanced (non-portable) variants to these:

Note that I/O multiplexing is orthogonal to asynchronous operations. You could tie them together if the completion of the asynchronous operation sends a notification that can be handled via a file descriptor / handle. This is the case with Windows asynchronous I/O (called overlapped I/O).

Practice​

Enter the multiplex/ directory. See the implementation of an epoll()-based server in epoll_echo_server.c.

  1. Build the server:

    student@os:~/.../lab/support/multiplex$ make
    gcc -g -Wall -Wextra -I../../../../../common/makefile/..  -c -o epoll_echo_server.o epoll_echo_server.c
    gcc -g -Wall -Wextra -I../../../../../common/makefile/..  -c -o ../../../../../common/makefile/../utils/log/log.o ../../../../../common/makefile/../utils/log/log.c
    gcc -g -Wall -Wextra -I../../../../../common/makefile/..  -c -o ../../../../../common/utils/sock/sock_util.o ../../../../../common/utils/sock/sock_util.c
    gcc epoll_echo_server.o ../../../../../common/makefile/../utils/log/log.o ../../../../../common/utils/sock/sock_util.o  -o epoll_echo_server

    And run it:

    student@os:~/.../lab/support/multiplex$ ./epoll_echo_server
    08:41:07 INFO  epoll_echo_server.c:252: Server waiting for connections on port 42424

    It listens for connection on port 42424.

    Connect using netcat on another console and send messages:

    $ nc localhost 42424
    aaa
    aaa
    bbb
    bbb

    Keep the connection open and, on the third console, initiate another netcat connection. The server is now multiplexing both connections.

  2. Create a script and / or a program to exercise the server. Create many connections to the server and continuously send messages to the server. See it multiplex the I/O channels (one for each connection - actually two: one for receiving and one for sending).