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| <h4 class="subsection">13.10.5 How to optimize the AIO implementation</h4> |
| |
| <p>The POSIX standard does not specify how the AIO functions are |
| implemented. They could be system calls, but it is also possible to |
| emulate them at userlevel. |
| |
| <p>At the point of this writing, the available implementation is a userlevel |
| implementation which uses threads for handling the enqueued requests. |
| While this implementation requires making some decisions about |
| limitations, hard limitations are something which is best avoided |
| in the GNU C library. Therefore, the GNU C library provides a means |
| for tuning the AIO implementation according to the individual use. |
| |
| <!-- aio.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Data Type: <b>struct aioinit</b><var><a name="index-struct-aioinit-1309"></a></var><br> |
| <blockquote><p>This data type is used to pass the configuration or tunable parameters |
| to the implementation. The program has to initialize the members of |
| this struct and pass it to the implementation using the <code>aio_init</code> |
| function. |
| |
| <dl> |
| <dt><code>int aio_threads</code><dd>This member specifies the maximal number of threads which may be used |
| at any one time. |
| <br><dt><code>int aio_num</code><dd>This number provides an estimate on the maximal number of simultaneously |
| enqueued requests. |
| <br><dt><code>int aio_locks</code><dd>Unused. |
| <br><dt><code>int aio_usedba</code><dd>Unused. |
| <br><dt><code>int aio_debug</code><dd>Unused. |
| <br><dt><code>int aio_numusers</code><dd>Unused. |
| <br><dt><code>int aio_reserved[2]</code><dd>Unused. |
| </dl> |
| </p></blockquote></div> |
| |
| <!-- aio.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>aio_init</b> (<var>const struct aioinit *init</var>)<var><a name="index-aio_005finit-1310"></a></var><br> |
| <blockquote><p>This function must be called before any other AIO function. Calling it |
| is completely voluntary, as it is only meant to help the AIO |
| implementation perform better. |
| |
| <p>Before calling the <code>aio_init</code>, function the members of a variable of |
| type <code>struct aioinit</code> must be initialized. Then a reference to |
| this variable is passed as the parameter to <code>aio_init</code> which itself |
| may or may not pay attention to the hints. |
| |
| <p>The function has no return value and no error cases are defined. It is |
| a extension which follows a proposal from the SGI implementation in |
| Irix 6<!-- /@w -->. It is not covered by POSIX.1b or Unix98. |
| </p></blockquote></div> |
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