IO Models

Under Unix, there are 5 I/O models:

blocking I/O
nonblocking I/O
I/O multiplexing
select
poll
signal driven I/O
SIGIO
asynchronous I/O
POSIX aio_

Two phase for input operation:

graph LR;
    S(Data Source)
    K(Kernel Space)
    A(Application Space)

    S --1 Wait for data to be ready--> K
  K --2 Move data from kernel to application--> A

1. Blocking I/O Model

the most prevalent I/O model
By default, all sockets are blocking.
recvfrom is implemented different on different platfrom.
system call on BSD
function on System V
As to system call, there is normally a switch between userspace and kernel.
With Blocking I/O, system call return when:
success
- data copied to application space
error
- interrupted by signal (most common)
- others
Process blocked when calling these kind of system call.

2. NonBlocking I/O Model

when set a socket to be nonblocking
Instead block the process, system call would return error EWOULDBLOCK
[ ] blocking I/O is described detailed within Chapter 16
call system call on nonblocking file descriptor

3. I/O Multiplexing Model

via select or poll
block on these other system calls
instead of blocking in the actual I/O system call
Prons
can wait for more than one descripter to be ready
Cons
Using select requires two system call
Another similar solution:
multithreading with blocking I/O

4. Signal-Driven I/O Model

graph LR;
    A(Application)
    K(Kernal)

    A --1 Register signal handler--> K
    K --2. Generate SIGIO when data ready--> A
    A --3. Get data via system call--> K

Application do not block while waiting for data

5. Asynchronous I/O Model

Defined by POSIX specification
reconcile different real-time function
Similar with signal-driven I/O model, but the difference is:
With asynchronous I/O, kernel notify application when data is ready
With Signal-Driven I/O, after kernel's notification, application would have to fetch data via system call

6. Comparison

The first phase is the same for the first four I/O models.
The first four I/O models are the same at the second phase.
Asynchronous I/O models handle the first and the second phase.

Referenc <[PEARSON] UNIX Network Programming (Volume 1 Third Edition)>

« Previous Next »