[syslog-ng] mark-freq() and mark-mode() understanding

Hello. I need an explanation of mark-freq() and mark-mode() works. === a part of syslog-ng.conf === options { ... mark-freq(300); ... } destination mesg { file("/var/log/syslog/messages" mark-mode(periodical)); }; === === a part of /var/log/syslog/messages === Jun 24 08:26:35 -- MARK -- Jun 24 08:30:01 crond[10512]: (root) CMD ( /usr/bin/freshclam --quiet --daemon-notify) Jun 24 08:31:30 -- MARK -- Jun 24 09:01:01 crond[10544]: (root) CMD (run-parts /etc/cron.hourly) Jun 24 09:25:30 syslog-ng[9983]: Log statistics ... === "MARK" label absent between 08:31:30 and 09:01:01 for example. Does it turns out that "idle" is checked for all file() globally ? Can I setup it for one file() ? -- Regards, Sergey.