(...) Modern Linux distro's, say Ubuntu (8.10) comes with a default PHP configuration of session.gc_probability = 0. This means that in general, session GC will never run(!). There is a unix cron job that runs every half hour and checks for old sessions to delete (see more at /etc/cron.d/php5) but this is done only if you're using the standard "files" as your session handler (check out the directive of session.save_handler on your machine to learn what php default sessions handler is but make sure to check that your application doesn't overwrite this), so for Drupal users using plain Debian/Ubuntu, stock LAMP packages and stock Drupal applications, this means that the PHP garbage collection, despite your best intentions, will also never run. (...)
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