Apache, PHP Fastcgi and PHP_FCGI_CHILDREN

We had long suspected that something wasn't quite acting as advertised with our PHP Fastcgi set up. Not that we had any issues, but like many tutorials recommend, we using a wrapper script to run the fastcgi enabled version of php. In that wrapper script we set PHP_FCGI_CHILDREN variable and exec'd php. Here is the snippet from our config:

Apache:

ScriptAlias /php-fastcgi/ /path/to/users/home/php-fastcgi/
AddType application/x-httpd-fastphp .php
Action application/x-httpd-fastphp /php-fastcgi/php5-fcgi


php5-fcgi:

#!/bin/sh
PHPRC="/etc/php4/cgi"
export PHPRC
PHP_FCGI_CHILDREN=4
export PHP_FCGI_CHILDREN
PHP_FCGI_MAX_REQUESTS=5000
export PHP_FCGI_MAX_REQUESTS
exec /usr/lib/cgi-bin/php4


So for every site, an instance of php would be launched to talk to apache via factcgi and that php instance would, in turn, launch 4 children processes to actually handle the requests, giving 5 processes in total.

Now there were a couple of things with this set up. The first, which was a minor headache, was that often an apache reload would kill the instance of php it was talking to, but the 4 children would be left behind. This was annoying, but would only take up a bit of memory and we wrote a script to clean them up.

The other thing with this set up is that it didn't seem to make any difference to the sites what PHP_FCGI_CHILDREN was set to. It turns out there was a reason for this. After receiving several alerts for the number of processes on our web servers, we thought we'd find out what was going on.

This thread was very informative on the topic:
http://www.mail-archive.com/mod-fcgid-users@lists.sourceforge.net/msg002....

Apache with fcgid will launch one fastcgi child (in this case php5-fcgi) for each request that needs to be serviced. This means that if apache receives 2 concurrent php requests for the same site, 2 php fastcgi processes will be launched, with each in turn launching 4 children. This means that only one of the php children will ever be used (which we verified using strace). So, since the php processes are being managed by mod_fcgid, there is no point in also configuring php to manage processes. PHP_FCGI_CHILDREN is only useful when the php instance is a stand alone fcgi server as in nginx or lighttpd.

To confirm this behaviour, we reloaded apache and confirmed that there were no php processes running for the test site. We then ran the following script:

#!/bin/sh
wget http://testsite/test.php -q &
wget http://testsite/test.php -q &
wget http://testsite/test.php -q &
wget http://testsite/test.php -q &
wget http://testsite/test.php -q &
wait

Where test.php is the following:

<?php
sleep(5);
echo 'OK';
?>


So the code will produce 5 concurrent requests. Running this resulted in 25 php5-fcgi processes, which means 5 copies of the script were launched each with 5 processes (1 talking to mod_fcgid and 4 children). So it appears that setting PHP_FCGI_CHILDREN is pointless under mod_fcgi/apache as each group will only ever handle 1 request at a time. We have therefore removed the PHP_FCGI_CHILDREN value from our php5-fcgi scripts to give the following:

#!/bin/sh
PHPRC="/etc/php4/cgi"
export PHPRC
PHP_FCGI_MAX_REQUESTS=5000
export PHP_FCGI_MAX_REQUESTS
exec /usr/lib/cgi-bin/php4


This has had no adverse consequences and we now have about a fifth of the processes running on the web servers and no problems with lost children.