php-fpm,

How to determine the correct number of max children processes for PHP-FPM

Jul 28, 2021 · 2 mins read · Post a comment
How to determine the correct number of max children processes for PHP-FPM

Most of the PHP developers are using Nginx with php-fpm as the most efficient approach. Most likely nginx will throw php errors, that the server reached the pm.max_children number. It means that probably you haven’t set up the number correctly or you are using the default value. In this tutorial, I will explain how to determine the correct number of max children processes.

Prerequisites

  • php and php-fpm installed
  • nginx

Determine the correct number of child processes for php-fpm

Step 1. Firstly you will have to find the correct path of the php-fpm configuration file depending on your OS. On Debian-based distro you can find it at /etc/php/fpm/pool.d/www.conf, open the file and find the pm.max_children line.

Step 2. The following command will show you the average memory usage in kilobytes per process in the RSS column:

ps -ylC php-fpm --sort:rss

Output:

S   UID   PID  PPID  C PRI  NI   RSS    SZ WCHAN  TTY          TIME CMD
S     0  8550  8452  0  80   0 31800 142192 SyS_ep ?       00:00:02 php-fpm
S     0 14648 14548  0  80   0 31868 142196 SyS_ep ?       00:00:29 php-fpm
S     0 32356 32258  0  80   0 32040 142196 SyS_ep ?       00:00:25 php-fpm
S  1000 28358  8550  0  80   0 36400 143952 inet_c ?       00:00:00 php-fpm
S  1000 28359  8550  0  80   0 36456 143935 inet_c ?       00:00:00 php-fpm
S  1000 28153  8550  0  80   0 36652 144003 inet_c ?       00:00:00 php-fpm
S  1000 27850  8550  0  80   0 36940 144071 inet_c ?       00:00:00 php-fpm
S  1000 27948 14648  0  80   0 43356 144993 inet_c ?       00:00:03 php-fpm
S  1000 27933 32356  0  80   0 47588 145732 inet_c ?       00:00:03 php-fpm
S     0 30312 30212  0  80   0 48228 193672 SyS_ep ?       00:01:11 php-fpm
S  1000 27932 32356  0  80   0 50800 146100 inet_c ?       00:00:03 php-fpm
S  1000 27930 32356  0  80   0 53124 145818 inet_c ?       00:00:03 php-fpm

Step 3. The next command will calculate the average memory usage for all above fpm processes in MB:

ps --no-headers -o "rss,cmd" -C php-fpm | awk '{ sum+=$1 } END { printf ("%d%s\n", sum/NR/1024,"Mb") }'

Output:

37Mb

Step 4. So the approach is max_children = System_RAM / Average FPM Pool size. Let’s say if your server has 4GB RAM and you decide to give 2GB memory for the FPM processes the result will be:

max_children = 2048 MB / 37 MB = 55.351 ~ (50)

Approximately you should set the pm.max_children number to 50.

Conclusion

This tutorial shows you how to determine the correct number of max children processes for php-fpm. Feel free to leave a comment below and if you find this tutorial useful, follow our official channel on telegram.