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| <h4 class="subsection">22.4.2 How to get information about the memory subsystem?</h4> |
| |
| <p>The page size of the virtual memory the process sees is essential to |
| know in several situations. Some programming interface (e.g., |
| <code>mmap</code>, see <a href="Memory_002dmapped-I_002fO.html#Memory_002dmapped-I_002fO">Memory-mapped I/O</a>) require the user to provide |
| information adjusted to the page size. In the case of <code>mmap</code> is it |
| necessary to provide a length argument which is a multiple of the page |
| size. Another place where the knowledge about the page size is useful |
| is in memory allocation. If one allocates pieces of memory in larger |
| chunks which are then subdivided by the application code it is useful to |
| adjust the size of the larger blocks to the page size. If the total |
| memory requirement for the block is close (but not larger) to a multiple |
| of the page size the kernel's memory handling can work more effectively |
| since it only has to allocate memory pages which are fully used. (To do |
| this optimization it is necessary to know a bit about the memory |
| allocator which will require a bit of memory itself for each block and |
| this overhead must not push the total size over the page size multiple. |
| |
| <p>The page size traditionally was a compile time constant. But recent |
| development of processors changed this. Processors now support |
| different page sizes and they can possibly even vary among different |
| processes on the same system. Therefore the system should be queried at |
| runtime about the current page size and no assumptions (except about it |
| being a power of two) should be made. |
| |
| <p><a name="index-g_t_005fSC_005fPAGESIZE-2779"></a>The correct interface to query about the page size is <code>sysconf</code> |
| (see <a href="Sysconf-Definition.html#Sysconf-Definition">Sysconf Definition</a>) with the parameter <code>_SC_PAGESIZE</code>. |
| There is a much older interface available, too. |
| |
| <!-- unistd.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>getpagesize</b> (<var>void</var>)<var><a name="index-getpagesize-2780"></a></var><br> |
| <blockquote><p>The <code>getpagesize</code> function returns the page size of the process. |
| This value is fixed for the runtime of the process but can vary in |
| different runs of the application. |
| |
| <p>The function is declared in <samp><span class="file">unistd.h</span></samp>. |
| </p></blockquote></div> |
| |
| <p>Widely available on System V<!-- /@w --> derived systems is a method to get |
| information about the physical memory the system has. The call |
| |
| <p><a name="index-g_t_005fSC_005fPHYS_005fPAGES-2781"></a><a name="index-sysconf-2782"></a> |
| <pre class="smallexample"> sysconf (_SC_PHYS_PAGES) |
| </pre> |
| <p class="noindent">returns the total number of pages of physical the system has. |
| This does not mean all this memory is available. This information can |
| be found using |
| |
| <p><a name="index-g_t_005fSC_005fAVPHYS_005fPAGES-2783"></a><a name="index-sysconf-2784"></a> |
| <pre class="smallexample"> sysconf (_SC_AVPHYS_PAGES) |
| </pre> |
| <p>These two values help to optimize applications. The value returned for |
| <code>_SC_AVPHYS_PAGES</code> is the amount of memory the application can use |
| without hindering any other process (given that no other process |
| increases its memory usage). The value returned for |
| <code>_SC_PHYS_PAGES</code> is more or less a hard limit for the working set. |
| If all applications together constantly use more than that amount of |
| memory the system is in trouble. |
| |
| <p>The GNU C library provides in addition to these already described way to |
| get this information two functions. They are declared in the file |
| <samp><span class="file">sys/sysinfo.h</span></samp>. Programmers should prefer to use the |
| <code>sysconf</code> method described above. |
| |
| <!-- sys/sysinfo.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: long int <b>get_phys_pages</b> (<var>void</var>)<var><a name="index-get_005fphys_005fpages-2785"></a></var><br> |
| <blockquote><p>The <code>get_phys_pages</code> function returns the total number of pages of |
| physical the system has. To get the amount of memory this number has to |
| be multiplied by the page size. |
| |
| <p>This function is a GNU extension. |
| </p></blockquote></div> |
| |
| <!-- sys/sysinfo.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: long int <b>get_avphys_pages</b> (<var>void</var>)<var><a name="index-get_005favphys_005fpages-2786"></a></var><br> |
| <blockquote><p>The <code>get_phys_pages</code> function returns the number of available pages of |
| physical the system has. To get the amount of memory this number has to |
| be multiplied by the page size. |
| |
| <p>This function is a GNU extension. |
| </p></blockquote></div> |
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