if you are getting slammed or being DDoS attacked, you can move your ONLINE and TOPIC_VIEWS temporary tables from heap (memory) to disk.

--- Set temp data table engines: disk ---
SQL Query
alter table ubbt_ONLINE ENGINE=MyISAM
SQL Query
alter table ubbt_TOPIC_VIEWS ENGINE=MyISAM


DEFAULT SETTINGS used since 2007 (UBB.threads from version 7.1.0 through 7.7.4)
--- Set temp data table engines: heap (memory) ---
SQL Query
alter table ubbt_ONLINE ENGINE=heap
SQL Query
alter table ubbt_TOPIC_VIEWS ENGINE=heap