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PHP-FPM Limit Tuning Basics for Max Children

Austin Lowery
  • Updated

Overview

This resource provides information about tuning the Max Children limit for PHP FPM.

If you would  prefer not to tune PHP-FPM's Max Children limit, you may consider switching to the LSAPI handler:

How to switch from PHP-FPM to the LSAPI handler

The max_children directive defines the maximum number of PHP-FPM processes allowed to spawn. In the cPanel context, max_children is applied on a per-domain basis. A max_children setting of 5 means only five processes can spawn for that particular domain.
The purpose of the max_children directive is to provide a way to limit the number of resources used by PHP-FPM so that it cannot overwhelm the server. When the limit is hit, only a single domain will become unavailable. Without the limit, traffic from a single domain could crash the entire server in certain circumstances.

Adjusting Max Children via WHM

The most straightforward approach to adjusting max_children is to increase the value by increments of 5 when the domain hits the limit. While doing this, you should pay close attention to the server's overall CPU, memory, and IO resource usage.

  1. Login to WHM as the root user
  2. Navigate to: "Home / Software / MultiPHP Manager"
  3. Click on the "User Domain Settings" tab
  4. In the table, click on the "PHP-FPM Settings" button at the far right of your domain's row
  5. Update the "Max Children" value to your desired value
  6. Click the blue "Update" button at the bottom of the page

Checking the PHP-FPM Logs for Max Children Limit Hits

  1. Access your server as the root user via Terminal or SSH
  2. Run the following command to find the FPM logs: 
    find /opt/cpanel/ea-php*/root/usr/var/log/php-fpm/ -type f -name error.log
  3. Run the following command on the log corresponding to the version of PHP your domain is using. PHP 8 is used in this example: 
    grep max_children /opt/cpanel/ea-php80/root/usr/var/log/php-fpm/error.log

A Domain is Constantly Hitting Max Children

If your domain constantly hits the max_children limit when you raise the value and your server runs out of resources, you should check to see if the server is experiencing a DoS attack.

How can I tell if Apache is experiencing a DDoS attack?

If you confirm that the traffic is legitimate and is not due to a DoS attack, you may consider a few options:

  • Migrate the site to a different server with more resources
  • Upgrade the resources of the existing server
  • Consult with a systems administrator to optimize the site so that it uses less resources

Apache Max Request Workers in Relation to Max Children

Apache's Max Request Workers configuration is a very similar concept to Max Children for PHP-FPM.

For each request that comes in, Apache will assign a worker to that request. The Apache workers then send the request to a PHP-FPM child process.

If the combined total of the max_children values for all domains on the server is larger than the Apache Max Request Workers value, the Max Request Workers configuration can create a bottleneck. You may need to adjust Apache's Max Request workers after adjusting PHP-FPM's max_children.

A detailed explanation of the Max Request Workers configuration for Apache is out of the scope of this guide, but another more in-depth guide can be found here:
Tuning MaxRequestWorkers for Apache

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