Oracle Wait Events

General Information
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Purpose Wait events are events measured by the database where the user/application must wait for an activity to complete
Related Dynamic Performance Views
View Name View Description
GV$SESSION_EVENT One row for each session-specific wait event since session startup (gv$session logon_time)
GV$SESSION_WAIT One row for each session listed in v$session (gv$session.sid)
V$EVENT_NAME One row for each named Oracle kernel wait event
V$SYSTEM_EVENT One row for each instance-wide wait event since instance startup (v$instance.startup_time)
Buffer Cache Busy Waits
Description Buffer busy waits happens when a session tries to access a block in the buffer cache but it cannot because the buffer is busy, i.e. another session is modifying the block and the contents of the block are in flux. To guarantee that the reader has a coherent image of the block with either all of the changes or none of the changes, the session modifying the block marks the block header with a flag letting other users know a change is taking place and to wait until the complete change is applied.

The two main cases where this wait occurs are:
  • Another session is reading the block into the buffer - this specific case has been split out into a "read by other session" wait event in 10g and higher
  • Another session holds the buffer in an incompatible mode to our request
While the block is being changed, the block is marked as unreadable by others. The changes that are being made should last under a few hundredths of a second, e.g. a disk read should be under 20 milliseconds and a block modification should be under one millisecond. Therefore it will take a lot of buffer busy waits to cause a problem, but some examples of this are:
  • Hot block issue, such as the first block on the free list of a table, with high concurrent inserts. All users will insert into that block at the same time, until it fills up, then users start inserting into the next free block on the list, and so on
  • Multiple users running an inefficient SQL statement performing a full table scan on the same large table at the same time. One user will read the block off disk, and the other users will wait on buffer busy waits (or read by other session in 10g and higher) for the physical I/O to complete
Gather information SELECT s.sql_hash_value, sw.p1 file#, sw.p2 block#, sw.p3 reason
FROM gv$session_wait sw, gv$session s
WHERE sw.event = 'buffer busy waits'
AND sw.sid = s.sid;
Identify the object of a wait event SELECT owner , segment_name , segment_type
FROM dba_extents
WHERE file_id = &FileNo
AND &BlockNo BETWEEN block_id AND (block_id + blocks-1);
Top 10 buffer busy wait events col owner format a10
col object_name format a20
col tsname format a10
col value format 99999

  SELECT owner, object_name, subobject_name, object_type, tablespace_name TSNAME, value
  FROM gv$segment_statistics
  WHERE statistic_name='buffer busy waits'
  ORDER BY value DESC)
Issue Resolution Considerations
  • Increase extent size (are extents added too frequently?)
  • Reduce rows per block (is there hot block contention?)
  • Increase undo retention (by altering size or retention time)
  • Tune queries
TKPROF Output Parameters
Parameter Number Description
P1 File number of the data file containing the block
P2 Block number within the datafile
P3 Reason code
Control File Waits
Description The three different wait events of 'control file sequential read', 'control file single write', and 'control file parallel write' all contribute to the amount of time Oracle takes to keep the control file current.

Oracle maintains a record of the consistency of the database's physical structures and operational state through a set of control files. The Oracle control file is essential to the database operation and ability to recover from an outage. In fact, if you lose the control file(s) associated with an instance you may not be able to recover completely. It is the Oracle control file(s) that records information about the consistency of a database's physical structures and operational statuses. The database state changes through activities such as adding data files, altering the size or location of datafiles, redo being generated, archive logs being created, backups being taken, SCN numbers changing, or checkpoints being taken.

Through normal operation the control file is continuously hammered with reads and writes as it is being updated.
Why Control File Waits Occur

The performance around reads and writes against control files is often an indication of misplaced control files that share the same I/O access path or are on devices that are heavily used. It is interesting to note that Oracle has always defaulted the creation of control files in a single directory. You can check where your control files reside on disk with this simple query.
View wait events col event format a30
col wait_class format a20

SELECT inst_id, event, total_waits, total_timeouts, time_waited, average_wait, wait_class
FROM gv$system_event
WHERE event LIKE '%control%';
View sessions impacted by control file wait events SELECT event, wait_time, p1, p2, p3
FROM v$session_wait WHERE event LIKE '%control%';
Issue Resolution Considerations
  • Relocate files for less contention
  • Reduce the frequency of commits and log switches
TKPROF Output Parameters (Control File Parallel Write)
Parameter Number Description
P1 Number of control files being written to
P2 Number of blocks written
P3 Number of I/O requests
TKPROF Output Parameters (Control File Sequential Read)
Parameter Number Description
P1 Control containing the block
P2 Bock number within the control file
P3 Number of blocks read
DB File Waits
TKPROF Output Parameters (db file parallel read)
Parameter Number Description
P1 Number of files the session is reading
P2 Total number of blocks read
P3 Total number of I/O requests
TKPROF Output Parameters (db file parallel write)
Parameter Number Description
P1 Total number of I/O requests
P3 Timeout value in Hundredths of a second to wait for the I/O completion
TKPROF Output Parameters (db file scattered read)
Parameter Number Description
P1 File number
P2 Block number
P3 Number of blocks that the session is trying to read from the file# starting at block#
TKPROF Output Parameters (db file sequential read)
Parameter Number Description
P1 File number
P2 Block number
P3 Number of blocks that the session is trying to read (should be 1)
TKPROF Output Parameters (db file single write)
Parameter Number Description
P1 Number of control files being written to
P2 Number of blocks written
P3 Number of I/O requests
Idle Events
The table to the right is not complete but is a listing of the events most commonly seen idle events
Event Name Description
client message  
dispatcher time  
gcs for action  
gcs remote message  
ges remote message  
i/o slave wait  
jobq slave wait  
lock manager wait for remote message  
null event  
parallel query dequeue  
pipe get  
PL/SQL lock timer  
pmon timer  
PX Deq Credit: need buffer  
PX Deq Credit: send blkd  
PX Deq: Execute Reply  
PX Deq: Execution Msg  
PX Deq: Signal ACK  
PX Deq: Table Q Normal  
PX Deque wait  
PX Idle Wait  
queue messages  
rdbms ipc message  
slave wait  
smon timer  
SQL*Net message from client  
SQL*Net message to client  
SQL*Net more data from client  
virtual circuit status  
wakeup time manager  
Log Buffer Space
Wait for space in the SGA redo buffer
Parameter Number Description
P1 Not used
P2 Not used
P3 Not used
Log File Switch
Database switches redo logs. The previously current log file becomes active and is archived (or) waiting for checkpoint to complete because all redo logs are full (or) waiting for the switch to complete
Parameter Number Description
P1 Not used
P2 Not used
P3 Not used
Log File Sync
Wait for redo flush upon commit or rollback
Parameter Number Description
P1 buffer# in log buffer that needs to be flushed
P2 Not used
P3 Not used
Session Wait Events
DFS Lock Handle SELECT CHR(TO_CHAR(BITAND(p1,-16777216))/16777215) ||
       CHR(TO_CHAR(BITAND(p1, 16711680))/65535) "Lock",
       TO_CHAR(BITAND(p1, 65535)) "Mode"
FROM v$session_wait
WHERE event = 'DFS lock handle'
Unspecified Wait Events
This query is one way to identify so-called unspecified wait events by object SELECT current_obj#, SUM(time_waited)/1000000, COUNT(*)
FROM gv$active_session_history
WHERE event = 'unspecified wait event'
GROUP BY current_obj#
Wait Event Tracing
Active tracing in the current session ALTER SESSION SET EVENTS '10046 trace name context forever, level 12';
Active tracing in a different session SELECT schemaname, sid, serial#
FROM gv$session;

exec dbms_monitor.session_trace_enable(144, 253, TRUE, FALSE);

Related Topics
Built-in Functions
Built-in Packages
Dynamic Performance Views
What's New In 12cR1
What's New In 12cR2

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