hi all -- i hav 11g RAC environment, some messages in alert log are beyond comprehension like;

1. Wed Jul 08 22:11:50 2009
found dead shared server 'S054', pid = (100, 15) --though it doesnt affect any service

2. ORA-07445: exception encountered: core dump [kzvd_objprivCheck()+1264] [SIGSEGV] [ADDR:0x18] [PC:0x103AB43F0] [Address not mapped to object] []
Incident details in: /u01/app/oracle/diag/rdbms/grid3/grid31/incident/incdir_388228/grid31_s031_8059_i388228.trc

(how to read this .trc file, its in some pro-cosmic lang )

3. Wed Jul 08 22:42:59 2009
Sweep Incident[387936]: completed --whats a sweep incident?

these r commonly seen messages. in general, plz tell me how to make best use of alert log? what to see and how to react?
thanks in advance.