Session garbage collection configuration

src/_includes/config/locate-session.md

@@ -27,3 +27,24 @@ The preceding example stores session files in `/var/www/session`
 ### `php.ini` example {#session-where-phpini}
 
 As a user with `root` privileges, open your `php.ini` file and search for the value of `session.save_path`. This identifies where sessions are stored.
+
+## Garbage collection configuration {#session-gc}
+
+To clean up expired sessions Magento application calls `gc` (_garbage collection_) handler randomly according to probability which is calculated by `gc_probability / gc_divisor`. For example, if we set these directives to `1/100` respectively, it means a probability of `1%` (_probability of one call of garbage collection per 100 requests_).
+
+The garbage collection handler uses the `gc_maxlifetime` directive - the number of seconds after which the sessions will be seen as _garbage_ and potentially cleaned up.
+
+On some operating systems (Debian/Ubuntu) `session.gc_probability` set to `0` by default, and the garbage collection handler will never be executed.
+
+`session.gc_` directives from the `php.ini` file can be overwritten in `<magento_root>/app/etc/env.php`:
+
+```php
+ 'session' =>
+    array (
+      'save' => 'db',
+      'gc_probability' => 1,
+      'gc_divisor' => 1000,
+      'gc_maxlifetime' => 1440
+ ),
+```
+The configuration depends on the traffic and specific needs of a certain Magento application.