Random 502 bad gateway with php-fpm, why?

[justin]

My PHP application went down for a few hours with 502 bad gateway. In the
nginx error log all I see is:

2013/11/14 10:02:16 [error] 1466#0: *57964 recv() failed (104: Connection
reset by peer) while reading response header from upstream

I fixed it by restarting php-fpm. However, what caused this? I don't see
anything interesting logged in the php-fpm or php logs. No indication of why
happened, and why simply restarting php-fpm fixed the issue. I thought that
php-fpm auto-restarted processes if they fail?

Any insights and place to determine the root cause? Thanks.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,244669,244669#msg-244669