Which cronjob empties the session mysql table?
 

Because it happens to me in vbulletin 4.2.5 that the session table is full and vbulletin reports mysql error via e-mail. So please which cron task does it and how to run that task manually and see exact output of it to verify it really empty the table?

footer template contains {vb:raw cronimage}

I think they are hourly cleanup and hourly cleanup2

those seems enabled.
session table was full again, but i think only temporarily and then vbulletin solved it possibly, 8107 lines in table.
Should i run any cronjob more frequently? how frequently and which? Or how to create new job that will empty just session table? Or how to increase its size?

cronjob will run when someone visits your site, no visits = no cronjob.
You may be able to add these cronjob to server/cpanel etc..

If you get that error when the session table contains 8107 lines as you said then your host has extreme low limitations applied to your MySQL server. Generally this issue only happens when you have a significant number of visitors on your site (or HTTP flood/DDoS/DoS)

Thank you Dave, i contacted my provider and he told me "I've increased our MEMORY table limit (it was at default, nothing to do with us restricting)."
Also my session timeout (AdminCP > Settings > Options > "Cookies and HTTP header options") was set to 36 hours.. now i decreased it to 1800 seconds as suggested + emptied the session mysql table. Let's see what will happen.. If it will not work, i will be interested on how to run the table emptying sql query "truncate table session;" via cron..