nginx on high load (upstream prematurely closed connection while reading response header from upstream)

[muzi]

Dear Francis Daly , thankyou for your response, upstream is php-fpm server
(127.0.0.1) and its also setup to handle huge load, during above errors, i
see the site is opening fine. but don't know where to catch it. PM is also
set on demand and maximum childerns set to 90000 to handle the load, and no
logs appears in php-fpm. Can you please suggest how to got the issue ?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,266978,266995#msg-266995