Hello
Our setup:
load balancer with haproxy and memcached
2 backends ubuntu 14.04 PHP5+ Apache
Another one alone for the tests:
ubuntu 16.04 + php7-fpm
DB: mysql 5.7, plenty of RAM
NFS for moodledata
Moodle 3.1+
For many years, I've noticed that the avatars of users on the welcome page were displaying slowly, using pluginfile.php. It can be sometimes 17 sec! Today, I spent hours to understand. I put the profiling in config.php and noticed that SQL was fine 8/0 request in max 2ms. So I believe it was due to NFS and cache. I used xdebug and kcachegrind and discovered I was spending 99.3% of the time in session_start
As we use memcached It tried to change the lock parameters without success. Even reading the php memcached driver in C to understand the values. At the end I change moodle memcached.php:
/var/www/moodle.local/lib/classes/session/memcached.php
and change that:
ini_set('memcached.sess_locking', '0'); // Locking is required!
Obsviously things were going faster.. Just a few ms for all the icons. Before it was randomly an icon locked for a few sec..
I then looked at session locking to see if they were freed as fast as possible and saw many \core\session\manager::write_close(); // Unlock session during file serving. in filelib.php which is called by pluginfile.php
I don't understand why it is not running well.. Somewhere, there should be a piece of code locking sessions for too long. I hesitate to get rid of session locking in production..
Any advice?
