Meaning of child exit/start entries in PHP-FPM logs

php-fpm: hundreds of seconds in the log, Can anyone tell me, what are these seconds like after 51.320215 seconds from start and what does it mean? [14-Jun-2018 21:46:39] NOTICE: [pool mysite.com] child 4296 exited with code 0 after 51.320215 seconds from start [14-Jun-2018 21:46:39] NOTICE: [pool mysite.com] child 4316 started
My config is pm = ondemand pm.max_children = 500 pm.max_requests = 20 pm.max_spare_servers = 5 pm.min_spare_servers = 1 pm.process_idle_timeout = 10 pm.start_servers = 0