Server A has 30.7 GB RAM and there were 5 php-fpm processes consuming 17.6 up to 22.7 percent each. So you could say 20% of 30.7 GB = 6.14 GB.
I got the advice to monitor the problematic processes using "strace". I noticed in the output that php files are listed very often. So I used grep to get all the lines containing "\.php". In the resulting file, a lot of lines containing the word "unfinished" appeared, but the unfinished processes were never resumed. So I assume that this could cause my problem.
The system calls that are never finished are:
access
lstat
open
readlink
stat
on php files.
Some example lines are:
14686 09:12:21.497129 lstat("/var/www/vhosts/shoplink/vendor/autoload.php", <unfinished ...>
14686 09:12:21.497367 lstat("/var/www/vhosts/shoplink/vendor/autoload.php", <unfinished ...>
14686 09:12:21.497788 access("vendor/autoload.php", F_OK <unfinished ...>
14686 09:12:21.497837 lstat("/var/www/vhosts/shoplink/autoload.php", <unfinished ...>
14686 09:12:21.498051 lstat("/var/www/vhosts/shoplink/autoload.php", <unfinished ...>
14686 09:12:21.498263 lstat("/var/www/vhosts/shoplink/autoload.php", <unfinished ...>
14686 09:12:21.498485 lstat("/var/www/vhosts/shoplink/autoload.php", <unfinished ...>
14686 09:12:21.498841 open("/var/www/vhosts/shoplink/autoload.php", O_RDONLY <unfinished ...>
14686 09:12:21.499113 lstat("/var/www/vhosts/shoplink/vendor/autoload.php", <unfinished ...>
14686 09:12:21.499381 lstat("/var/www/vhosts/shoplink/vendor/autoload.php", <unfinished ...>
14686 09:12:21.499621 lstat("/var/www/vhosts/shoplink/vendor/autoload.php", <unfinished ...>
14686 09:12:21.499889 lstat("/var/www/vhosts/shoplink/vendor/autoload.php", <unfinished ...>
14686 09:12:21.500329 open("/var/www/vhosts/shoplink/vendor/autoload.php", O_RDONLY <unfinished ...>
Does someone have any idea why the calls remain unfinished? What could cause this behaviour in general?