public function execute() {
        global $CFG;
        require_once($CFG->dirroot . '/repository/lib.php');

        $fs = get_file_storage();
        // Find all generated thumbnails and group them in array by itemid (itemid == repository instance id).
        $allfiles = array_merge(
            $fs->get_area_files(SYSCONTEXTID, 'repository_filesystem', 'thumb'),
            $fs->get_area_files(SYSCONTEXTID, 'repository_filesystem', 'icon')
        );
        $filesbyitem = array();
        foreach ($allfiles as $file) {
            if (!isset($filesbyitem[$file->get_itemid()])) {
                $filesbyitem[$file->get_itemid()] = array();
            }
            $filesbyitem[$file->get_itemid()][] = $file;
        }
        // Find all instances of repository_filesystem.
        $instances = \repository::get_instances(array('type' => 'filesystem'));
        // Loop through all itemids of generated thumbnails.
        foreach ($filesbyitem as $itemid => $files) {
            if (!isset($instances[$itemid]) || !($instances[$itemid] instanceof repository_filesystem)) {
                // Instance was deleted.
                $fs->delete_area_files(SYSCONTEXTID, 'repository_filesystem', 'thumb', $itemid);
                $fs->delete_area_files(SYSCONTEXTID, 'repository_filesystem', 'icon', $itemid);
                mtrace(" instance $itemid does not exist: deleted all thumbnails");
            } else {
                // Instance has some generated thumbnails, check that they are not outdated.
                $instances[$itemid]->remove_obsolete_thumbnails($files);
            }
        }
    }