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Wait events are events measured by the database where the user/application must wait for an activity to complete
Related Dynamic Performance Views
One row for each session-specific wait event since session startup (gv$session logon_time)
One row for each session listed in v$session (gv$session.sid)
One row for each named Oracle kernel wait event
One row for each instance-wide wait event since instance startup (v$instance.startup_time)
Buffer Cache Busy Waits
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
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
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
WHERE statistic_name='buffer busy waits'
ORDER BY value DESC)
WHERE ROWNUM < 11;
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)
TKPROF Output Parameters
File number of the data file containing the block
Block number within the datafile
Control File Waits
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
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%';
Number of blocks that the session is trying to read (should be 1)
TKPROF Output Parameters (db file single write)
Number of control files being written to
Number of blocks written
Number of I/O requests
The table to the right is not complete but is a listing of the events most commonly seen idle events
gcs for action
gcs remote message
ges remote message
i/o slave wait
jobq slave wait
lock manager wait for remote message
parallel query dequeue
PL/SQL lock 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
rdbms ipc message
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
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
Log File Sync
Wait for redo flush upon commit or rollback
buffer# in log buffer that needs to be flushed
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"
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(*)
WHERE event = 'unspecified wait event'
GROUP BY current_obj#
ORDER BY 3 DESC;
Wait Event Tracing
Active tracing in the current session
ALTER SESSION SET EVENTS '10046 trace name context forever, level 12';