Adjusting child processes for PHP-FPM (Nginx)

When setting these options consider the following:

How long is your average request?
What is the maximum number of simultaneous visitors the site(s) get?
How much memory on average does each child process consume?

Determine if the max_children limit has been reached.

sudo grep max_children /var/log/php8.2-fpm.log.1 /var/log/php8.2-fpm.log

Replace 8.2 with the exact version number of php in use on your web server. Any output indicates error log lines that relate to this setting.

Determine system RAM and average pool size memory.

free -h
All fpm processes: ps -ylC php-fpm8.2 --sort:rss
Average memory: ps --no-headers -o "rss,cmd" -C php-fpm7.0 | awk '{ sum+=$1 } END { printf ("%d%s\n", sum/NR/1024,"M") }'
All fpm processes memory: ps -eo size,pid,user,command --sort -size | awk '{ hr=$1/1024 ; printf("%13.2f Mb ",hr) } { for ( x=4 ; x<=NF ; x++ ) { printf("%s ",$x) } print "" }' | grep php-fpm

Calculate max_children

Based on RAM

pm.max_children = Total RAM dedicated to the web server / Max child process size
System RAM: 2GB
Average Pool size: 85Mb
pm.max_children = 1500MB / 85MB = 17

Based on average script execution time

max_children = (average PHP script execution time) * (PHP requests per second)
visitors = max_children * (seconds between page views) / (avg. execution time)

Configure

sudo vim /etc/php/7.0/fpm/pool.d/www.conf

pm.max_children = 17
pm.start_servers = 2
pm.min_spare_servers = 1
pm.max_spare_servers = 3
pm.max_request = 1000

; Choose how the process manager will control the number of child processes.
; Possible Values:
;   static  - a fixed number (pm.max_children) of child processes;
;   dynamic - the number of child processes are set dynamically based on the
;             following directives:
;             pm.max_children      - the maximum number of children that can
;                                    be alive at the same time.
;
;             pm.start_servers     - the number of children created on startup.
;                                    this value must not be less than min_spare_servers 
;                                    and not greater than max_spare_servers.
;
;             pm.min_spare_servers - the minimum number of children in 'idle'
;                                    state (waiting to process). If the number
;                                    of 'idle' processes is less than this
;                                    number then some children will be created.
;
;             pm.max_spare_servers - the maximum number of children in 'idle'
;                                    state (waiting to process). If the number
;                                    of 'idle' processes is greater than this
;                                    number then some children will be killed.
; Note: This value is mandatory.

jcanepa/php-pools.md