Websites on PHP-FPM are unavailable or loading slowly: server reached max_children setting (*), consider raising it

Symptoms

• Websites that have PHP handler set as FPM application served by Apache or FPM application served by nginx at Domains > example.com > PHP Settings experience one of the following issues:

  • load slowly

  • not accessible (permanently or periodically) with a 50x error in a web-browser:

    502 Bad Gateway

    ---

    503 Service Temporarily Unavailable

    ---

    504 Gateway Time-out

• The following message appears in the PHP-FPM error logfile
/var/log/plesk-phpXX-fpm/error.log (where XX is a PHP version used by the websites):

  WARNING: [pool example.com] server reached max_children setting (*), consider raising it

  • The message above is usually accompanied with this error:

    unable to read what child say: Bad file descriptor (9)

• If nginx is enabled on the server, one of the following error messages may appear in domain's error logfile /var/www/vhosts/system/example.com/logs/proxy_error_log:

  [error] 11456#0: *118348 connect() to unix:///var/www/vhosts/system/example.com/php-fpm.sock failed (11: Resource temporarily unavailable) while connecting to upstream, client: 203.0.113.2, server: example.com, request: "GET / HTTP/1.1", upstream: "fastcgi://unix:///var/www/vhosts/system/example.com/php-fpm.sock:", host: "example.com"

  ---

  [error] 20058#0: *4725139 upstream prematurely closed connection while reading response header from upstream, client: 203.0.113.2, server: example.com, request: "POST /path/to/some/file HTTP/2.0", upstream:

Cause

The PHP-FPM pm.max_children limit has been reached by the website(s).

Resolution

Increase the pm.max_children limit for the affected domains:

1. Connect to the Plesk server via SSH

2. Find affected domains using the command below:

   Note: The number on the left indicates how many times the pm.max_children limit has been reached by the website.
   
   If a domain is using a dedicated FPM application and has a custom vhosts location, change the path /var/www/vhosts/system/ in the command below:

   # grep -r "server reached max_children setting" /var/log/*php*-fpm/error.log | cut -d' ' -f5 | tr -d ']' | sort | uniq -c | sort -nr
   
   100 example.com
   50 example.net
   25 example.org
   <...>

3. Log into Plesk

4. Go to Domains > example.com > PHP Settings > PHP-FPM Settings.

5. Increase the pm.max_children value for the domain and apply the changes. Increase this value for all domains listed in step 2.

   Note: In this example, we are setting pm.max_children to 20. To fine tune this setting, visit this KB article.

   

Other options to increase the pm.max_children limit: