Apache vs. PHP-FPM Bottleneck with Child Processes

Overview

Apache servers that use the prefork MPM with fewer server instances than the maximum number of PHP-FPM child processes may experience performance issues. This document explains how to resolve these dependency issues.

The issue

In the following scenario, the PHP-FPM server’s configuration allows a maximum of 20 PHP-FPM child processes and the Apache server’s configuration allows five server instances.

The following example represents the PHP-FPM server’s configuration file:

1
2
3

_is_present: 1
pm_max_children: 20
pm_max_requests: 20

The following example represents the Apache server’s configuration file:

1
2
3
4
5

StartServers: 5
<IfModule prefork.c>
MinSpareServers: 10
MaxSpareServers: 10
</IfModule>

If the Apache server receives 20 requests, it immediately passes ten of those requests to the available PHP-FPM child processes. After PHP-FPM processes those requests, Apache will pass another ten requests to PHP-FPM.

The solution

To solve this issue, you must configure Apache to allow enough server instances to handle the maximum number of PHP-FPM child processes. To do this, configure the following options in WHM’s Global Configuration interface (WHM >> Home >> Service Configuration >> Global Configuration):

• Minimum Spare Servers

• Maximum Spare Servers

• Max Request Workers

Set these options to a value greater than or equal to the pm_max_children setting in the PHP-FPM server’s configuration file.