I have Tomcat5.5 running on virtual server(Debian). Tomcat hangs when I make to two/more concurrent heavy-db-using http-request with browser. By hanging I mean that tomcat accepts new http-requests but does not reply to them anymore. Db-query that tomcat executes is select that returns many rows(just to make sure they executes concurrently) but otherwise it is very simple.
When tomcat hangs one tomcat(java) process start slowly increasing cpu-usage(eventually to 99%). Tomcat cannot be terminated by "kill -3" signal.
I have tested this with Postgresql and mysql, direct JDBC connection/hibernate, pooling/no pooling. Pure java-application(no Tomcat) works fine with many concurrent queries.