PHP 7.2 fails to start when pm.max_children > 506

on Debian 10 when I set pm.max_children = 507 or more and try to start php7.2-fpm I get following error:

-- The unit php7.2-fpm.service has entered the 'failed' state with result 'protocol'.
Sep 26 12:07:13 server.local systemd[1]: Failed to start The PHP 7.2 FastCGI Process Manager.
-- Subject: A start job for unit php7.2-fpm.service has failed
-- Defined-By: systemd
-- Support: https://www.debian.org/support
--
-- A start job for unit php7.2-fpm.service has finished with a failure.

When I set pm.max_children = 506 or less all is working fine.

My www.pool config:

[www]

user = www-data
group = www-data

listen = /var/run/php7-fpm.sock
listen.owner = www-data
listen.group = www-data


pm = static
pm.max_children = 506
pm.max_requests = 1000

catch_workers_output = yes

My server config:

• 10-core CPU
• 256 GB RAM

What can cause that? Is there some hard limit on how much php-fpm children can be started? Or is this some system limit? Can it be increased somehow (I have lot of free memory)?

Answer

Chances are that most of those 500+ php-fpm children will be going unused in the long term, but if you did want to increase the quantity, there are other limits being set by the master-FPM process and the operating system.

Open File Descriptor Limit Set open file descriptor rlimit for the master process. Default value: the rlimit for the master process (from php-fpm.conf).

# in php-fpm.conf and pool.d/*.conf
rlimit_files = 131072
# the default value may be as low as 1024, and since each child will typically 
# open 2 files (stdin & stdout), and there are additional files being used, 
# 506*2 = 1012 +12 others seems very likely to be an issue.

You may need to also enable php-fpm (or its user) to be able to use more file handles by editing the /etc/security/limits.conf file, with the hard & soft limits for the user (www-data).