If you are getting a bunch of lines in your /var/log/messages that looks like this: 

Oct  7 09:40:48 localhost -- MARK --
Oct  7 10:00:48 localhost -- MARK --
Oct  7 10:20:48 localhost -- MARK --
Oct  7 10:40:49 localhost -- MARK --
Oct  7 11:00:49 localhost -- MARK --
Oct  7 11:20:49 localhost -- MARK --

And you're wondering what they are then here is an explanation. It's nothing more than syslog placing a 'mark' every 20 minutes(by default) just to let you know that syslog is running properly. You can however change this default time of 20 minutes to something different, or even disable it completely. I personally choose to disable it completely.

If you want to change the default from 20 minutes to once every hour, then you need to open up your /etc/init.d/sysklogd file. For this example I will use kwrite which is a GUI editor. You can however use whatever editor you want. If it's a GUI editor, prefix it with 'kdesu' and if it's a CLI editor, then simply use 'sudo'

Please notice that the file name is actually /etc/init.d/sysklogd Many users think that is a typo and that the correct file is /etc/init.d/syslog when in fact that is not the case. The correct file is sysklogd 

kdesu kwrite /etc/init.d/sysklogd

Then you need to look for the following line: 

SYSLOGD="-u syslog"

And replace it with this one: 

SYSLOGD="-m 60 -u syslog"

Notice the -m 60 switch we added in there. That tells it to add that --MARK-- every 60 minutes.

If you want to disable it completely, change the above line to this: 

SYSLOGD="-m 0 -u syslog"

Notice the -m 0 switch we added this time. This tells it to never add that --MARK-- into the log files.

Once you have edited the file to your desire then you need to restart syslog with the following command: 

sudo /etc/init.d/sysklogd restart

Then you should have it setup the way you want.