Do not force PHP-FPM restart until there is 9+ connections with 499 Nginx response in the last 60 seconds.

"499 is logged by nginx if client closes connection before nginx
was able to send anything to client.

Large number of 499 may indicate that something is wrong on your
server, e.g. response generation takes too long. Some number is
usually ok: you are expected to have some users closing
connections prematurely due to their internal reasons (e.g. bad
connectivity, closing browser while loading page and so on)."

http://forum.nginx.org/read.php?2,213789,213793#msg-213793