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Support TopicConfiguring Apache and PHP for stability and performance

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Configuring Apache and PHP for stabil ity and performance.

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Configuring Apache and PHP

Often people blame Apache and PHP for being slow or crashing their server when in fact both of these things can be addressed with a little bit of tuning.

In this document we are going to go through the initial things you should look at to get your server to a point where you can leave it overnight without worry it’s going to crash.

Apache MPM Modes

Prefork and Worker are two types of MPM (Multi-Processing Modules) Apache provides.

Prefork MPM is the default option and uses multiple child processes with one thread each and each process handles one connection at a time. (This is a thread safe option)

Worker MPM uses multiple child processes with many threads each. Each thread handles one connection at a time.

Which one should you choose?This decision will ultimately be made for you depending on what you select when it comes to PHP. This is a configurable you’re going to need to come back too later.

PHP as a Module or as an external resource? The traditional way to run Apache + PHP is to use Apache with ‘mod_php’. This is quick and functional and this is fine for smaller sites however if you’re trying to run a complex ecommerce site built on a large PHP framework with support for external plugins and 3rd party modules and you shouldn’t just put up with the mediocre basic settings and high resource usage that it leads too.

The first line in the PHP Installation Manual dictates that if you’re running PHP as an Apache module (mod_php) that you need to use a ThreadSafe MPM mode, which limits it to only being run in PREFORK mode.


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The main issue with mod_php and Mpm Prefork is that every time Apache forks into a new process (every new request) the entire PHP module and all its sub modules have to be loaded into memory along with all your other data which means you’re wasting a lot of memory on multiple copies of the same thing.

To get around the memory usage caused by loading php into every Apache process is to use PHP as an external resource (PHP-FPM instead of mod_php) this then also allows you to run Apache with MPM Worker which mean Apache has a master process (now smaller as it doesn’t include all the bloat that comes with PHP) that spawns a small number of child processes to deal with http requests, CSS, Images, file downloads etc allowing for faster parallel processing.

Configuring Apache to us PHP-FPM instead of mod_php

Switching from mod_php to using PHP-FPM requires a few system changes to be made. Information on installing PHP-FPM can be found here.

A brief look into the configuration files for PHP-FPM and mod_phpThe configuration files for PHP are very similar regardless of if you’re using mod_php or PHP-FPM and are stored in /etc/php/ and /etc/php/fpm

./php.ini where your normal php settings changes need to go

./PHP-FPM.conf basic settings for PHP-FPM (most things in here can be left as default)

./pool.d/www.conf This is the default FPM Pool definition, you can create more files or more entries in the same file if you need more pools however that is not something we will cover in this document.

The main file you need to look at for initial tuning is /etc/php/fpm/pool.d/www.conf

This file contains the configuration for startup and the number of processes that can be spawned while its running. By default there are a lot of variables in this file as well as some good comments explaining what everything does. However to keep things simple the file can be stripped down to just the following:


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[www]user= www-datagroup = www-datalisten = 127.0.0.1:9000

pm = dynamicpm.max_children=10pm.start_servers =5pm.min_spare_servers = 2pm.max_spare_servers = 7

chdir = /

The configuration above start PHP-FPM with 5 processes waiting for connections from Apache and will keep between 2 and 7 running at any time with a hard limit of 10. At the point it has 10 active processes all working any new requests will be queued until one is spare.

These processes will run as the www-data:www-data user, this should normally be configured to run as the same user your Apache process runs as.

For mod_php as it’s loaded into Apache as a module and you configure it within Apache’s settings.

etc/apache2/apache2.conf

or/etc/httpd/conf/httpd.conf

The variables you need to search for are in the <IfModule Prefork.c> section. They do much the same thing as with the options in PHP-FPM however the variable names are slightly different.

Looking at a default install the numbers might mean very little at the moment but Apache has a default configuration of allowing 150 MPM Workers, which means after adding mod_php for a site that uses 75mb of RAM per process (quite common) a spike in website traffic that uses all of these at the same time (often happens when a search engine crawls your site) would mean 11.25GB of RAM would be required for Apache alone.

On the other hand PHP-FPM comes with a default limit of 10 processes which is much more stable however can again be tuned to match your server.


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Tuning to stop your server crashingThe process of tuning Apache/PHP is almost the same regardless of whether you’re using mod_php or PHP-FPM however as discussed earlier the file you need to edit and the variables are slightly different.

What you’re aiming to do is to configure a hard limit on the number of PHP processes (or Apache processes with mod_php included) so that the system doesn’t run out memory and start using SWAP space while dealing with a high number of requests.

Fact FindingThings you will need to know:• How much memory the system has• What the system needs to do aside from processing PHP requests• How much memory each PHP process consumes• What you need restart for your changes to take effect

Once you can predict the maximum amount of RAM a PHP process is going to require and the amount of RAM you have in the system to use you can easily work out what the hard limit for the maximum number of processes should be set to. Note this may change over time as the code for the site changes and increases or decreases the amount

Most of the information you need can be found from the application ‘top’ which comes installed on most systems. Running top from the command line on your server will bring you into a screen like the one below. (This needs to be done once apache and PHP have been running for a while, if you do it straight away PHP will not yet have loaded all its sub modules leading to lower memory usage per process and unsustainable hard limits).


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From the screenshot in Figure 1 you can see the total system RAM highlighted in the top box, a good idea of the other high resource system processes and amount of RAM used by each PHP-FPM process in the lower box.

In this instance the server has:• 4GB Total RAM• varnishd is using 407M• mysqld is using 196MB • PHP-FPM has 6 running processes each using between 74mb and 152mb.

For reference the basic Linux system uses about 250mb of RAM.

The memory usage by PHP-FPM in this example is a little higher than I would expect to see, we normally see PHP running at around 95-100mb per process however the Apache2 processes can be seen using 15MB of RAM each.

If you look at Figure 2 you will see again this server has 4GB of RAM, MySQL is using 73mb this time, the system takes about 250mb and instead of having a PHP-FPM process we are seeing lots of Apache ‘httpd’ processes spawned instead each using ~70MB.

Figure 1


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Figure 2

Working out your limitsTo work out the hard limit for the maximum number of PHP/Apache processes that can be run you need to take the total system ram, minus system usage and other major processes, and then divide the remainder by the amount of RAM each PHP/Apache process uses.

Number of Processes =(Total System RAM – (Large processes + system usage rounded up)) / (PHP / Apache Process size)

Example 1:

(4000 – (410 +200 +250)) / (152+93+91+88+78+74 /6) = 32.708

Consequently the safe number of maximum php-fpm processes this server should be configured to run is 32. (pm.max_children=32)

Once you have made a change you will need to restart php-fpm for it to take effect, this can be done with ‘/etc/init.d/php-fpm restart’. Restarting just Apache will NOT apply these changes as PHP is now running as a separate process and as such needs restarting independently.


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Example 2(3924 – (250 +73)) / 70 = 51

which would need to be set in httpd.conf for the ServerLimit and MaxClients variables.

Having adjusted a setting in here you will need to restart Apache for this change to take effect with ‘service httpd restart’ or ‘/etc/init.d/apache2 restart’.

Conclusions and after thoughts

At this point you should have made your decision on running how you want to run PHP, looked at the configuration files and found the variables you need to adjust, worked out how much resource you have and worked out what you need to limit your system too and applied these new settings.

Some other things to consider with mod_php:Limiting your Apache to a low number of maximum workers may limit your ability to answer large numbers of requests for static content while all your processes are busy processing PHP files.

Some other things to consider with php-fpm:If your site gets enough connections to hit the hard limit in PHP you will see the following in your Apache logs:

WARNING: [pool www] server reached pm.max_children setting (10), consider raising it.

This means Apache has started queuing requests waiting for a free PHP process to pass its request to which may cause delays in page load time but is an improvement on the server crashing and being offline.

Running the server right up to its memory limit with the maximum number of PHP processes simultaneously may give you the ability to handle a large number of simultaneous requests however each one is going to be fighting for CPU time with the next so may take longer than it would if you only had one or 2 processed at a time and then moved onto the next.


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You may find by the time you have filled ½ your system RAM with PHP processes you have over saturated your CPU’s and that adding more concurrent PHP requests actually just bogs the system down further slowing down your page loads.

To see these differences however you will need to load test your site and server with different configurations in place to find out which give the best compromise between page load speed and concurrent user capacity.

What you can do if this is still not enough:

If you get to the point where your server is still not able to handle the number of requests you’re receiving here are a couple of options you should look into further:

• Full Page Caching• Getting a bigger server• Load balance across multiple backend servers

We are always happy to discuss these technical options at Wirehive so give us a call.


10T 01252.560.565E [email protected]

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This is an unpublished work the copyright in which vests in Wirehive Ltd. All rights re-served. The information contained herein is confidential and the property of Wirehive Ltd, and is supplied without liability for errors and omissions. No part may be repro-duced, disclosed or used except as authorised by contract or other written permission. The copyright and the foregoing restriction on reproduction and use extend to all media in which the information may be embodied.

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