1. lck processes to pick up extra or missed 'posts' (messages).in all cases that can be identified the wait time is zero,although on most ports this is changed to 1/100th second inorder to force the process to be rescheduled.

2. multiple db writers.two cases:- when the master has sent i/o requests to the slaves, itwaits on this event for up to 6 seconds until the slavessignal that the i/o is complete.- the slaves wait on this event for up to 3 seconds whilstwaiting for requests from the master db writer.目前已经被证实的会涉及到null event 的等待事件还有有sql.net message to client、 db file scattered (or sequential) read