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)?
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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).