Forgive me, I’m about to ramble here.
For the past several months now I’ve been dealing with trying to get my VPS configured in such a way that it was stable and used as few resources (mostly RAM) as possible. During this process I had considered switching the web server from Apache2 to one of the lighter replacements. More and more I’ve been reading about the preference for Nginx (pronounced engine-x), along with PHP-FPM, as the defacto standard for high performance PHP sites.
Time to investigate.
The Nginx Stack
I swapped Apache 2 and mod_php out on my dev machine with Nginx and php-fpm a couple of days ago. Mostly to make sure everything would go smoothly if I decided to move my VPS over and figure out what rule changes I’d have to make to get Nginx running.
To start with Nginx doesn’t use the 1-process per connection model Apache does, instead it uses async IO. This addresses one of the biggest Apache problems I’ve had to contend with, a sudden spike in traffic spawning off a 10s of new processes, is no longer an issue at all.
Nginx’s memory foot print is comparatively tiny too. I’m seeing about 10MB total for the 2 workers + the master process, instead of 4-8MB or more per process.
Couple that with a fixed number of cgi processes on the back end (either with fastcgi or php-fpm) and you can account for most if not all of resources that will be used under any load conditions.
PHP
With Apache gone, so to goes mod_php and mod_fcgid. Neither are ideal solutions to running PHP sites, but those are the breaks (devsrv was running mod_php because it’s was what Ubuntu setup back when I installed it, and mod_fcgid is what Dreamhost uses).
Nginx does things a bit differently. PHP is run as a stand alone CGI “server” that Nginx proxies requests to. I find there are a couple of really nice advantages to this, especially if you can run php-fpm.
For example, you can pool cgi processes based on actual more broadly defined considerations rather than Apache’s process class. Say you have 3 vhosts, each running Wordpress, they can be served by a single pool of php processes that can share resources like a php-apc cache.
Ultimately, this means you can still control the number of backend processes that used for PHP, but can do so while still sharing resources where it makes the most sense.
Figuring and Managing Resources
With a VPS like Dreamhost’s where there are hard limits on memory usage, and since there’s no swap space you can’t really deal with overages when they occur other than to have the watchdog kill your VPS. In short, you really want to be able to account for resource usage and deal with transient spikes in a way that doesn’t result in spiraling resource usage.
Apache has always made this a fun exercise. Yes it can be done, but like I said, it’s fun trying to tune a fuzzy system optimally when there are hard limits.
With Nginx, I can count on the server’s staying exactly, or very nearly exactly, where they were at initialization. 2 worker processes + a controller process nets me between 8 and 12MB of RAM used, and it’ll be that much regardless of whether I have 1 connection or 100 connections. The CGI upstream servers are likewise manageable. With php-fpm you can let it spawn more processes, but you can just as easily limit it to 1 or 2 if resources are scarce. Simply put, it’s much easier to account for how much resources you actually need.
Performance
The whole point of Nginx is performance, but I’m not use to seeing 2 processes handle a lot of requests and that takes some getting use to.
I ran Apache Bench (ab) against my dev server to see just how it performed against Apache setup similarly to how it is in production and the results were certainly impressive to me.
Serving static content (cached HTML files, images, css, js, etc.) 2 Nginx processes could handle more than 2000 connections per second (over 1000mbit network). This is about 2x more than Apache2 was able to handle in about 1/30th the memory foot print.
Serving dynamic content is of course considerably slower, ~3 connections/second can be handled going though the full Wordpress PHP + MySQL stack with 4 php-fpm workers and xcache running.
It’s real hard to completely quantify all the variables, especially when I’m deliberately trying not to.
Conclusions
Okay I admit this post was sparse on details, I’ll try and rectify that in the near future. For now, let me just say, if you can switch to Nginx with your php application you should see better performance and lower resource usage than the same thing running under Apache. I certainly have.