The solution is use ondemand mode in each pool configuration.
pm = dynamic (or pm = static)
pm = ondemand
and restart fpm.
ondemand mode needs (PHP Version >= 5.3.9)
Also see the pm.max_children
directives if you choose ondemand mode
; ondemand - no children are created at startup. Children will be forked when
; new requests will connect. The following parameter are used:
; pm.max_children - the maximum number of children that
; can be alive at the same time.
; pm.process_idle_timeout - The number of seconds after which
; an idle process will be killed.