The page size of the virtual memory the process sees is essential to
know in several situations. Some programming interface (e.g.,
mmap, see Memory-mapped I/O) require the user to provide
information adjusted to the page size. In the case of
mmap 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.
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.
The correct interface to query about the page size is
(see Sysconf Definition) with the parameter
There is a much older interface available, too.
getpagesizefunction 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.
The function is declared in unistd.h.
Widely available on System V derived systems is a method to get information about the physical memory the system has. The call
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
These two values help to optimize applications. The value returned for
_SC_AVPHYS_PAGES 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
_SC_PHYS_PAGES 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.
The GNU C library provides in addition to these already described way to
get this information two functions. They are declared in the file
sys/sysinfo.h. Programmers should prefer to use the
sysconf method described above.
get_phys_pagesfunction 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.
This function is a GNU extension.