I recently switched to nginx + php-fpm on my server. There are about 60 domains on the server, most low traffic, but some mid-to-high traffic. All seemed to be working fine for the first 12 hours, but then I started getting a 502 Bad Gateway error on one of the high traffic domains. The nginx error log is full of messages like: Code: 2017/02/10 00:32:42 [error] 46165#0: *119100 connect() to unix:///var/www/vhosts/system/mysite.com/php-fpm.sock failed (11: Resource temporarily unavailable) while connecting to upstream, client: 18.104.22.168, server: mysite.com, request: "GET /some-page.php HTTP/1.1", upstream: "fastcgi://unix:///var/www/vhosts/system/mysite.com/php-fpm.sock:", host: "www.mysite.com" I started googling, and this seems to be quite a common problem, but nothing I have tried so far has helped. I am guessing, because only 1 domain is affected, and that is the highest traffic one, that it is an issue of php-fpm being overwhelmed. I create a file /var/www/vhosts/system/mysite.com/conf/php.ini where I tried playing around with the pool settings, gradually increasing it. Currently, I have the following settings, and I'm still getting the error: Code: [php-fpm-pool-settings] ; By default use ondemand spawning (this requires php-fpm >= 5.3.9) pm = dynamic pm.max_children = 1500 pm.process_idle_timeout = 10s ; Following pm.* options are used only when 'pm = dynamic' pm.start_servers = 20 pm.min_spare_servers = 20 pm.max_spare_servers = 50 According to what I've read, the max_children value is certainly too high for my specs! However, I tried every value in between including 5, 40, 100, 300, 1000. I have read that I should try switching from unix to tcp/ip socket, but I'm not sure of a safe way to do this on Plesk. The config file for the domain in /opt/plesk/php/7.0/etc/php-fpm.d/ says: Code: ; Don't override following options, they are relied upon by Plesk internally The socket is listed just below this. In short, I am not sure what I can do next. Any advice? Edit: It seems that the other domains on the server, which are lower traffic, have also started throwing this error. So perhaps it is not a traffic issue? Anyway, I read somewhere that the error message "pool seems busy" is posted to the error log when it is a load issue... Edit 2: Ok, this is very strange. If I set php-fpm as the PHP handler for 1 service plan (which contains 5 domains, but these are by far the PHP-using domains with the highest traffic), everything seems to work fine. As soon as I start changing the settings for other service plans, I begin getting errors!