I found the problem: This is because I use log: KnowledgeRuntimeLogger logger = KnowledgeRuntimeLoggerFactory.newFileLogger(session, "test"); If I use Stateful session, The rule engine will print many log in XML file. But if i use stateless session, it will not print any log. my code is like this one: for(int i = 0; i < 1000000; i ++){ session.insert(new Object())// session.fireAllrules(); } session.dispose(); If I send 1,000,000 message, it will generate hundreds xml format log files. -- View this message in context: http://drools.46999.n3.nabble.com/Performance-StatefulKnowledgeSession-di... Sent from the Drools: User forum mailing list archive at Nabble.com.