void im_concurrency_set( int concurrency )
int im_concurrency_get( void )
A value of zero (the default) means to get the number of threads from the environment variable IM_CONCURRENCY. If that is not set, the number of threads defaults to one.
Setting the number of threads only affects image evaluations which start after that point, it will not change the behaviour of existing evaluations.
Most command-line vips programs support the --vips-concurrency flag, which can also be used to set the concurrency.