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
sysconf (_SC_NPROCESSORS_CONF)
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
sysconf (_SC_NPROCESSORS_ONLN)
returns the number of processors which are currently inline (i.e.,
available).
For these two pieces of information the GNU C library also provides
functions to get the information directly. The functions are declared
in sys/sysinfo.h.
— Function: int get_nprocs_conf (void)
The get_nprocs_conf function returns the number of processors the
operating system configured.
This function is a GNU extension.
— Function: int get_nprocs (void)
The get_nprocs function returns the number of available processors.
This function is a GNU extension.
Before starting more threads it should be checked whether the processors
are not already overused. Unix systems calculate something called the
load average. 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).
— Function: int getloadavg (double loadavg[], int nelem)
This function gets the 1, 5 and 15 minute load averages of the
system. The values are placed in loadavg. getloadavg will
place at most nelem elements into the array but never more than
three elements. The return value is the number of elements written to
loadavg, or -1 on error.
This function is declared in stdlib.h.
Published under the terms of the GNU General Public License
