Most of the PHP developers are using Nginx with PHP-FPM as the most efficient approach. Most likely Nginx will throw PHP errors as 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
- 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 PHP-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 = 2048MB / 37MB = 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.