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| <h3 class="section">22.5 Learn about the processors available</h3> |
| |
| <p>The use of threads or processes with shared memory allows an application |
| to take advantage of all the processing power a system can provide. If |
| the task can be parallelized the optimal way to write an application is |
| to have at any time as many processes running as there are processors. |
| To determine the number of processors available to the system one can |
| run |
| |
| <p><a name="index-g_t_005fSC_005fNPROCESSORS_005fCONF-2787"></a><a name="index-sysconf-2788"></a> |
| <pre class="smallexample"> sysconf (_SC_NPROCESSORS_CONF) |
| </pre> |
| <p class="noindent">which returns the number of processors the operating system configured. |
| But it might be possible for the operating system to disable individual |
| processors and so the call |
| |
| <p><a name="index-g_t_005fSC_005fNPROCESSORS_005fONLN-2789"></a><a name="index-sysconf-2790"></a> |
| <pre class="smallexample"> sysconf (_SC_NPROCESSORS_ONLN) |
| </pre> |
| <p class="noindent">returns the number of processors which are currently inline (i.e., |
| available). |
| |
| <p>For these two pieces of information the GNU C library also provides |
| functions to get the information directly. The functions are declared |
| in <samp><span class="file">sys/sysinfo.h</span></samp>. |
| |
| <!-- sys/sysinfo.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>get_nprocs_conf</b> (<var>void</var>)<var><a name="index-get_005fnprocs_005fconf-2791"></a></var><br> |
| <blockquote><p>The <code>get_nprocs_conf</code> function returns the number of processors the |
| operating system configured. |
| |
| <p>This function is a GNU extension. |
| </p></blockquote></div> |
| |
| <!-- sys/sysinfo.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>get_nprocs</b> (<var>void</var>)<var><a name="index-get_005fnprocs-2792"></a></var><br> |
| <blockquote><p>The <code>get_nprocs</code> function returns the number of available processors. |
| |
| <p>This function is a GNU extension. |
| </p></blockquote></div> |
| |
| <p><a name="index-load-average-2793"></a>Before starting more threads it should be checked whether the processors |
| are not already overused. Unix systems calculate something called the |
| <dfn>load average</dfn>. This is a number indicating how many processes were |
| running. This number is average over different periods of times |
| (normally 1, 5, and 15 minutes). |
| |
| <!-- stdlib.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>getloadavg</b> (<var>double loadavg</var>[]<var>, int nelem</var>)<var><a name="index-getloadavg-2794"></a></var><br> |
| <blockquote><p>This function gets the 1, 5 and 15 minute load averages of the |
| system. The values are placed in <var>loadavg</var>. <code>getloadavg</code> will |
| place at most <var>nelem</var> elements into the array but never more than |
| three elements. The return value is the number of elements written to |
| <var>loadavg</var>, or -1 on error. |
| |
| <p>This function is declared in <samp><span class="file">stdlib.h</span></samp>. |
| </p></blockquote></div> |
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