Our site is currently on a linux VPS with DreamHost and using nginx 0.8.53 as the web server and php-fcgi version 5.2.15. On occasion, our php-fastcgi processes just suddenly crash, and our users are left with a "502 Bad Gateway" error from nginx. The server is configured to use unix sockets and not a TCP port.

Our php-fastcgi settings are the following:
PHP_FCGI_CHILDREN = 8 (should be plenty for our traffic)
PHP_FCGI_MAX_REQUESTS = 400 (used to be 1000, we tried lowering to see if it would help, and nada.)

The http error logs show these sort of error messages:

>> *9 connect() to unix:/home/***/.php.sock failed (111: Connection refused) while connecting to upstream, client: *****, server: ****, request: "GET ****, upstream: "fastcgi://unix:/home/***/.php.sock:"
>> *28898 connect() to unix:/home/****/.php.sock failed (11: Resource temporarily unavailable) while connecting to upstream, client: **, server: **, request: "GET **", upstream: "fastcgi://unix:/home/***/.php.sock:"

We wrote a daemon that restarts the nginx web server when it detects the processes crash, or these error codes pop up, but sometimes restarting the web server fails to bring back the php5.cgi processes, and our users are left without any access to our site material for a while. This approach is also pretty sloppy and doesn't work sometimes.

Why would php lock up and crash like this, and why isn't nginx automatically reload the php5.cgi processes? Is there another way to serve php to our users that doesn't use php-fastcgi? (I heard people mention php-fpm, but I'm not too familar with using it or its advantages). We think that it might be the php5.cgi processes timing out waiting for MySQL queries to finish processing, since we can sometimes crash php5.cgi simply by running long queries on the web site. Any suggestions on settings we can tweak?

Thanks to anyone who can help.