Here is the output from /var/log/php-fpm/error.log when log_level string is set to debug.
When php-fpm is stopped all is normal:
[14-Nov-2014 09:06:02] NOTICE: Terminating ...
[14-Nov-2014 09:06:02] NOTICE: exiting, bye-bye!
When php-fpm is started (with debug mode):
[14-Nov-2014 09:06:28.176757] DEBUG: pid 5224, fpm_scoreboard_init_main(), line 40: got clock tick '100'
[14-Nov-2014 09:06:28.177080] DEBUG: pid 5224, fpm_event_init_main(), line 337: event module is epoll and 61 fds have been reserved
[14-Nov-2014 09:06:28.177111] NOTICE: pid 5224, fpm_init(), line 85: fpm is running, pid 5224
[14-Nov-2014 09:06:28.177121] ERROR: pid 5224, fpm_event_epoll_add(), line 177: epoll: unable to add fd 7
[14-Nov-2014 09:06:28.177126] ERROR: pid 5224, fpm_event_epoll_add(), line 177: epoll: unable to add fd 7
[14-Nov-2014 09:06:28.177131] ERROR: pid 5224, fpm_event_epoll_add(), line 177: epoll: unable to add fd 7
[14-Nov-2014 09:06:28.177136] ERROR: pid 5224, fpm_event_epoll_add(), line 177: epoll: unable to add fd 7
[14-Nov-2014 09:06:28.177141] ERROR: pid 5224, fpm_event_epoll_add(), line 177: epoll: unable to add fd 7
[14-Nov-2014 09:06:28.177147] DEBUG: pid 5224, fpm_event_loop(), line 366: 71232 bytes have been reserved in SHM
[14-Nov-2014 09:06:28.177152] NOTICE: pid 5224, fpm_event_loop(), line 367: ready to handle connections
[14-Nov-2014 09:06:28.177175] DEBUG: pid 5224, fpm_systemd_heartbeat(), line 68: have notify start to systemd
[14-Nov-2014 09:06:28.177181] NOTICE: pid 5224, fpm_systemd_heartbeat(), line 75: systemd monitor interval set to 10000ms
[14-Nov-2014 09:06:29.178228] DEBUG: pid 5224, fpm_pctl_perform_idle_server_maintenance(), line 362: [pool ex1] currently 0 active children, 0 spare children
[14-Nov-2014 09:06:29.178253] DEBUG: pid 5224, fpm_pctl_perform_idle_server_maintenance(), line 362: [pool ex2] currently 0 active children, 0 spare children
[14-Nov-2014 09:06:29.178262] DEBUG: pid 5224, fpm_pctl_perform_idle_server_maintenance(), line 362: [pool ex3] currently 0 active children, 0 spare children
[14-Nov-2014 09:06:29.178270] DEBUG: pid 5224, fpm_pctl_perform_idle_server_maintenance(), line 362: [pool ex4] currently 0 active children, 0 spare children
[14-Nov-2014 09:06:29.178275] DEBUG: pid 5224, fpm_pctl_perform_idle_server_maintenance(), line 362: [pool ex5] currently 0 active children, 0 spare children
[14-Nov-2014 09:06:29.178281] DEBUG: pid 5224, fpm_pctl_perform_idle_server_maintenance(), line 362: [pool ex6] currently 0 active children, 0 spare children
[14-Nov-2014 09:06:30.179339] DEBUG: pid 5224, fpm_pctl_perform_idle_server_maintenance(), line 362: [pool ex7] currently 0 active children, 0 spare children
[14-Nov-2014 09:06:30.179368] DEBUG: pid 5224, fpm_pctl_perform_idle_server_maintenance(), line 362: [pool ex8] currently 0 active children, 0 spare children
it continues to cycle through each of the pools etc...etc...
And then when stopped normal messages:
[14-Nov-2014 09:10:59.127531] DEBUG: pid 5224, fpm_got_signal(), line 85: received SIGTERM
[14-Nov-2014 09:10:59.127547] NOTICE: pid 5224, fpm_got_signal(), line 86: Terminating ...
[14-Nov-2014 09:10:59.127552] DEBUG: pid 5224, fpm_pctl(), line 231: switching to 'terminating' state
[14-Nov-2014 09:10:59.127557] NOTICE: pid 5224, fpm_pctl_exit(), line 70: exiting, bye-bye!
I do not think there is anything helpful in these logs, but wanted to share just in case.