Default fpm settings easily lead to out of memory events or OOMkiller being active

[xrow]

Container platform

No response

Version

Any with FPM.

OS version of the container image

RHEL 8

Bugzilla, Jira

No response

Description

Problem: There is currently no automatic or dynamic way to controll the memory usage of php-fpm threads.

I am noticing many OOMKiller events / pod restarts in kubernetes. The reason for this is that my php pod has a max pod memory limit of 3 GB. Sooner or later it will hit 3GB because it will consume for memory over time. Currently I can`t tell if the reason is that the threads are just to many or if I have some threads with too much memory. In any case I think there is a smart way needed to controll the common memory footprint of the php-fpm, but currently there is none. I currently have no proper advise yet i can give to make it work in all the cases.

With the default settings in my scenario of a symfony application. I can do the following calculation: One php thread at 400 MB * 50 threads are 20 GB per POD plus maybe 1 GB Apache. It is obvious that a desired common footprint should be something like 1-4 GB per default.

bash-5.1$ grep -i "max_children" /etc/php-fpm.d/www.conf 
;   static  - a fixed number (pm.max_children) of child processes;
;             pm.max_children      - the maximum number of children that can
;             pm.max_children           - the maximum number of children that
pm.max_children = 50

bash-5.1$ ps -ylC php-fpm --sort:rss
S   UID     PID    PPID  C PRI  NI   RSS    SZ WCHAN  TTY          TIME CMD
S  1001      63       1  0  80   0 15376 294786 ep_pol ?       00:00:00 php-fpm
S  1001      66      63  0  80   0 202108 318985 skb_wa ?      00:00:17 php-fpm
R  1001     501      63  1  80   0 266220 338824 -    ?        00:00:05 php-fpm
S  1001     367      63  0  80   0 317184 349248 skb_wa ?      00:00:22 php-fpm
S  1001      65      63  3  80   0 321776 349415 skb_wa ?      00:03:00 php-fpm
S  1001      64      63  0  80   0 331004 350440 skb_wa ?      00:00:32 php-fpm
S  1001     292      63  1  80   0 333932 352134 skb_wa ?      00:01:49 php-fpm
S  1001     365      63  0  80   0 341204 390597 skb_wa ?      00:00:28 php-fpm
S  1001      68      63  0  80   0 345152 352108 skb_wa ?      00:00:24 php-fpm
S  1001      67      63  0  80   0 406312 406638 skb_wa ?      00:00:33 php-fpm

What do you think?

Reproducer

No response

[xrow]

Maybe one solution could look similar to this:

export_vars=$(cgroup-limits); export $export_vars
export PHP_MEMORY_PER_CHILD=${PHP_MEMORY_PER_CHILD:-500}
max_childs_computed=$((MEMORY_LIMIT_IN_BYTES/1024/1024/$PHP_MEMORY_PER_CHILD))
# The pm.max_children should never be lower than pm.min_spare_servers, which is set to 5.
[[ $max_childs_computed -le 5 ]] && max_childs_computed=5
export PHP_MAX_CHILDREN=${PHP_MAX_CHILDREN:-$max_childs_computed}
echo "-> pm.max_children and pm.max_spare_servers is set to $PHP_MAX_CHILDREN, using PHP_MEMORY_PER_CHILD=${PHP_MEMORY_PER_CHILD}"
sed -i "s/.*pm.max_children.*=.*/pm.max_children = $PHP_MAX_CHILDREN/g" /etc/php-fpm.d/www.conf
sed -i "s/.*pm.max_spare_servers.*=.*/pm.max_spare_servers = $PHP_MAX_CHILDREN/g" /etc/php-fpm.d/www.conf