Friday, May 25, 2012

V$SYSTEM_EVENT

This was tested on Oracle 11.2. V$SYSTEM_EVENT shows what an instance has been waiting for since it was last started. You can see the top 10 events (by total time waited) as follows. The times are in hundredths of a second:

SQL> l
  1  select * from
  2  (select event, time_waited
  3   from v$system_event
  4   where wait_class != 'Idle'
  5   order by 2 desc)
  6* where rownum <= 10
SQL> /
 
EVENT                               TIME_WAITED
----------------------------------- -----------
control file sequential read             992637
db file sequential read                  569680
control file parallel write              543468
os thread startup                        515992
log file parallel write                  499896
db file parallel write                   426726
db file scattered read                   202302
log file sync                            159840
Disk file operations I/O                  45765
ADR block file read                       29553
 
10 rows selected.
 
SQL>
 
If you include Idle events, you see the wait events where the database wasn’t doing anything. The SQL*Net message from client event, for example, records how long the database spent waiting for its next instruction. This may not be what you want to see:

SQL> l
  1  select * from
  2  (select event, time_waited
  3   from v$system_event
  4   order by 2 desc)
  5* where rownum <= 10
SQL> /
 
EVENT                                    TIME_WAITED
---------------------------------------- -----------
rdbms ipc message                         1956293654
SQL*Net message from client                637129764
DIAG idle wait                             326433989
dispatcher timer                           163322456
shared server idle wait                    163320698
Streams AQ: qmn coordinator idle wait      163318553
Streams AQ: qmn slave idle wait            163315578
smon timer                                 163264804
pmon timer                                 163252749
Space Manager: slave idle wait             163171812
 
10 rows selected.
 
SQL>
 
The TIME_WAITED for SQL*Net message from client works out at almost 74 days. However, the database has been open for less than 20 days:
 
SQL> l
  1  select startup_time, sysdate
  2* from v$instance, dual
SQL> /
 
STARTUP_TIME SYSDATE
------------ ---------
04-MAY-12    23-MAY-12
 
SQL>
 
That’s because the TIME_WAITED values are the sum of all the TIME_WAITED values for all the sessions which have logged in since the database was opened.
 
If you combine these figures with the CPU time used, you start to get an idea of what your database has been doing:
 
SQL> l
  1  select a.name, b.value
  2  from v$statname a, v$sysstat b
  3  where a.statistic# = b.statistic#
  4* and a.name = 'CPU used by this session'
SQL> /
 
NAME                                     VALUE
----------------------------------- ----------
CPU used by this session               1854863
 
SQL>

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