linux异步IO--aio

简述

linux下异步方式有两种：异步通知和异步IO（AIO），异步通知请参考：linux异步通知

Linux的I/O机制经历了一下几个阶段的演进：

1. 同步阻塞I/O: 用户进程进行I/O操作，一直阻塞到I/O操作完成为止。

2. 同步非阻塞I/O: 用户程序可以通过设置文件描述符的属性O_NONBLOCK，I/O操作可以立即返回，但是并不保证I/O操作成功。

3. 异步事件阻塞I/O: 用户进程可以对I/O事件进行阻塞，但是I/O操作并不阻塞。通过select/poll/epoll等函数调用来达到此目的。

4. 异步时间非阻塞I/O: 也叫做异步I/O(AIO)，用户程序可以通过向内核发出I/O请求命令，不用等待I/O事件真正发生，可以继续做另外的事情，等I/O操作完成，内核会通过函数回调或者信号机制通知用户进程。这样很大程度提高了系统吞吐量。

The POSIX asynchronous I/O (AIO) interface allows applications to initiate one or more I/O operations that are performed asynchronously (i.e., in the background).

The application can elect to be notified of completion of the I/O operation in a variety of ways: by delivery of a signal, by instantiation of a thread, or no notification at all.

函数

The POSIX AIO interface consists of the following functions:

aio_read(3) Enqueue a read request. This is the asynchronous analog of read(2).

aio_write(3) Enqueue a write request. This is the asynchronous analog of write(2).

aio_fsync(3) Enqueue a sync request for the I/O operations on a file descriptor. This is the asynchronous analog of fsync(2) and fdatasync(2).

aio_error(3) Obtain the error status of an enqueued I/O request.

aio_return(3) Obtain the return status of a completed I/O request.

aio_suspend(3) Suspend the caller until one or more of a specified set of I/O requests completes.

aio_cancel(3) Attempt to cancel outstanding I/O requests on a specified file descriptor.

lio_listio(3) Enqueue multiple I/O requests using a single function call.

在异步非阻塞IO中，可以同时发起多个传输操作。这需要每个传输操作有唯一的上下文，这样才能在它们完成时区分到底是哪个传输操作完成了。在AIO中通过aiocb（AIO I/O control Block）结构体区分。

aiocb包含了有关传输的所有信息，以及为数据准备的用户缓冲区。在产生I/O通知（完成）时，aiocb结构就被用来唯一标识所完成的I/O操作。

           #include <aiocb.h>

           struct aiocb {
               /* The order of these fields is implementation-dependent */

               int             aio_fildes;     /* File descriptor */
               off_t           aio_offset;     /* File offset */
               volatile void  *aio_buf;        /* Location of buffer */
               size_t          aio_nbytes;     /* Length of transfer */
               int             aio_reqprio;    /* Request priority */
               struct sigevent aio_sigevent;   /* Notification method */
               int             aio_lio_opcode; /* Operation to be performed;
                                                  lio_listio() only */

               /* Various implementation-internal fields not shown */
           };

           /* Operation codes for 'aio_lio_opcode': */

           enum { LIO_READ, LIO_WRITE, LIO_NOP };

The fields of this structure are as follows:

aio_filedes The file descriptor on which the I/O operation is to be performed.

aio_offset This is the file offset at which the I/O operation is to be performed.

aio_buf This is the buffer used to transfer data for a read or write operation.

aio_nbytes This is the size of the buffer pointed to by aio_buf.

aio_reqprio This field specifies a value that is subtracted from the calling thread's real-time priority in order to determine the priority for execution of this I/O request(see pthread_setschedparam(3)). The specified value must be between 0 and the value returned by sysconf(_SC_AIO_PRIO_DELTA_MAX). This field is ignored for file synchronization operations.

aio_sigevent This field is a structure that specifies how the caller is to be notified when the asynchronous I/O operation completes. Possible values for aio_sigevent.sigev_notify are SIGEV_NONE, SIGEV_SIGNAL, and SIGEV_THREAD. See sigevent(7) for further details.

aio_lio_opcode The type of operation to be performed; used only for lio_listio(3).

应用

应用主要有两种方式：信号机制和回调函数。

参考：

1. Linux 异步IO机制

2. 使用异步 I/O 大大提高应用程序的性能