Welcome to my soapbox

Tuning PHP-FPM

How it started -
I recently switched to Linode for a basic VPS (512 MB RAM, 20 GB storage). I wanted to be able to install absolutely what ever version of what ever software I wanted. Since I have become very familiar with Arch Linux (I run it on all of my home computers now) I decided to make that my server OS for the time being. Along with being lightweight, minimalistic and always up-to-date arch offers an awesome set of packages and the best package manager I have ever used. Par of Arch Linux’s appeal is the Arch Wiki. A repository of important information for installing and configuring packages for Arch. So when following the guide for nginx, I also installed PHP-FPM for PHP/CGI.

All is well until weeks later I notice my memory usage tends to balloon after a few days (some times hours).  After hours of toying, poking, prodding, and messing with WordPress settings, I figured out that PHP-FPM was the culprit. At times consuming 300+ MB of memory! Something had to be done, but what? After a good bit of digging and a few key parameter changes later PHP-FPM was well within the memory constraints of my server and website performance became noticeably faster!The Problem -

First we need to take a look at the default config file for PHP-FPM. You are going to want to find the section that looks like this:

Here, if you read the descriptions you get a basic idea of what these options are for. If we take a look at the line  pm.max_children = 60 you see that the max “children” is a nice and high 60. This means that PHP-FPM will spawn a max of 60 child processes each requiring some amount of memory.

The next line to look at is  pm.start_servers = 20 This shows us that there will be 20 server processes started.

Both  pm.min_spare_servers = 20 and  pm.max_spare_servers = 20 limit how many resources are consumed when the server is idle, you want at least a few processes available in case a spike happens but you don’t need a bunch sitting around after one dies off.

Finally the last line we need to tweak is  pm.max_requests = 500 This causes processes to be killed off and re-started every 50 requests regardless if they need to be. This is perfect for fighting memory leaks in something like a poorly written WordPress plugin.

Now these would be considered sane defaults if our server was a bit more beefy (at least 1 GB of ram) but because were in such a limited environment, way to many PHP processes are getting spawned and combined with local caching the whole thing is a bit frustrating. So what can we do to fix it?

The Solution -

Looking at the file and better understanding what the options mean is a great step but what numbers should be used? Well there really is little information on what is right because there are way to many variables to take this into account. However, lowering all these variables to more sane values will sure help a lot in the total memory consumption of PHP-FPM and the responsiveness of your server.

Currently here are the values that I am using

You want to make sure you are using pm = dynamic management. With out it, it will just use a fixed size (fine but not necessary and makes some settings pointless).

After some testing, the server seems to idle at around ~140 MB of memory (for everything) which is much improved over the ~420 MB it was using previously.

The goal here is to find values that work for you. I plan on testing my site with Apache JMeter for a better idea just how significant the savings are in terms of page load times.