I have my Linode 360 set up to run Nginx and PHP FastCGI. I have around a dozen low-traffic Drupal sites and the total memory usage usually sits at around 160MB.

For the most part it's very fast and responsive. However, around every 10th page load or so I seem to be getting a 504 timeout. Since there's not much load on the server I have to imagine I have something less-than-optimal in my configuration, like maybe too many/not enough processes or some buffer size setting being out of order.

Here's my Nginx config:

user www-data;
worker_processes  2;

error_log  /var/log/nginx/error.log;
pid        /var/run/nginx.pid;

events {
    worker_connections  1024;
}

http {
    include       /etc/nginx/mime.types;
    default_type  application/octet-stream;
    server_names_hash_bucket_size 128;

    access_log  /var/log/nginx/access.log;

    sendfile        on;
    tcp_nopush     on;

    #keepalive_timeout  0;
    keepalive_timeout  65;
    tcp_nodelay        on;

    gzip  on;
    gzip_comp_level 5;
    gzip_http_version 1.0;
    gzip_min_length 0;
    gzip_types text/plain text/html text/css image/x-icon application/x-javascript;
    gzip_vary on;

    include /etc/nginx/conf.d/*.conf;
    include /etc/nginx/sites-enabled/*;
}

A typical site config for Nginx is like this:

server {
        listen 80;
        server_name www.findmud.com;
        rewrite ^/(.*) http://findmud.com/$1 permanent;
        }

server {
        listen 80;
        server_name findmud.com;
        access_log /home/zeta/public_html/findmud.com/log/access.log;
        error_log /home/zeta/public_html/findmud.com/log/error.log;

        location / {
            root /home/zeta/public_html/findmud.com/public;
            index index.php index.html index.htm;
            if (!-e $request_filename) {
                rewrite ^/(.*)$ /index.php?q=$1 last;
            }
        }

        location ~ .php$ {
                fastcgi_pass 127.0.0.1:9000;
                fastcgi_index index.php;
                fastcgi_param SCRIPT_FILENAME /home/zeta/public_html/findmud.com/public$fastcgi_script_name;
                include /etc/nginx/fastcgi.conf;
                fastcgi_param QUERY_STRING $query_string;
                fastcgi_param REQUEST_METHOD $request_method;
                fastcgi_param CONTENT_TYPE $content_type;
                fastcgi_param CONTENT_LENGTH $content_length;
        }

        location ~* ^.(jpg|jpeg|gif|css|png|js|ico|pdf|zip|exe)$ {
            access_log off;
            expires 30d;
        }

}

Here's my PHP FastCGI config:

START=yes
EXEC_AS_USER=(not shown)
FCGI_HOST=localhost
FCGI_PORT=9000
PHP_FCGI_CHILDREN=1
PHP_FCGI_MAX_REQUESTS=1000

PHP memory limit is set at 48M, but I never really get below 180M free on the server so I'm obviously not running out of RAM.

What can I do to reduce the likelihood of a gateway timeout? If there are any other config files I should post please let me know.

you only have one php process to serve php pages. i'd try at least as many as you have nginx processes (= 2)
see if that helps

Thank you for the tip, I knew it'd be something obvious to someone who wasn't me.

As a general rule of thumb should children be equal the the number of Nginx-es, or is it OK to go higher (i.e. 4 or 8 if I have the extra RAM lying around)?

i'd run more nginx processes than php processes as not every http request needs php processing.
i run about 10 nginx worker instances and maybe 6 or 8 php instances, works pretty well.

You are thinking in terms of Apache prefork MPM, where each process can only serve one request at a time.

Nginx uses async-based IO and can handle a lot more connections on a single process. I have a Linode 720 running a moderate sized Drupal site (around 2.5 million page views/70 million HTTP requests a month). With HTTP 1.1 keep alive at 15 second, currently it has 189 connections hanging onto it. It runs on 5x PHP/FastCGI process and... 1 single Nginx worker process.

Since Nginx is not blocking on IO, you don't need to have more than one process unless it's exhausting one single CPU core.