WAITEVENT: "buffer busy waits" Reference Note (文档 ID 34405.1)

"buffer busy waits" Reference Note

This is a reference note for the wait event  "buffer busy waits" which includes the following subsections:

• Brief definition
• Individual wait details (eg: For waits seen in <>)
• Systemwide wait details (eg: For waits seen in <>)
• Reducing waits / wait times

See  Note:61998.1 for an introduction to Wait Events.

Definition:

• Versions:7.0 - 10.2 Documentation: 9.0
• This wait happens when a session wants to access a database block in the buffer cache but it cannot as the buffer is "busy". The two main cases where this can occur are:
  1. Another session is reading the block into the buffer
  2. Another session holds the buffer in an incompatible mode to our request

Individual Waits:

  Parameters:

• P1 = file# (Absolute File# in Oracle8 onwards)
• P2 = block#
• P3 = id (Reason Code)/Block Class# in 10g

• file# (Absolute File# in Oracle8 onwards)

  This is the file number of the data file that contains the block that the waiting session wants.

• block#

  This is the block number in the above  file# that the waiting session wants access to.
  See  Note:181306.1 to determine the tablespace, filename and object for this file#,block# pair.

• id (Reason Code)

  The buffer busy wait event is called from different places in the Oracle code. Each place in the code uses a different "Reason Code" . These codes can differ between versions thus:

  Versions	Values used
  7.1 - 8.0.6	Uses one set of ID codes (mostly >1000)
  8.1.5	8.1.5+ does not include a value for P3 when waiting
  8.1.6 - 9.2	Uses a different set of ID codes (100-300)
  10.1+	Uses the block class

  Buffer Busy Waits ID's and Meanings

  Reason Code (Id)			Reason
  <=8.0.6	8.1.6-9.2	>=10.1
  0	0	n/a	A block is being read
  1003	100	n/a	We want to NEW the block but the block is currently being read by another session (most likely for undo).
  1007	200	n/a	We want to NEW the block but someone else has is using the current copy so we have to wait for them to finish.
  1010	230	n/a	Trying to get a buffer in CR/CRX mode , but a modification has started on the buffer that has not yet been completed.
  1012	-	n/a	A modification is happening on a SCUR or XCUR buffer, but has not yet completed
  1012 (dup.)	231	n/a	CR/CRX scan found the CURRENT block, but a modification has started on the buffer that has not yet been completed.
  1013	130	n/a	Block is being read by another session and no other suitable block image was found e.g. CR version, so we wait until the read is completed. This may also occur after a buffer cache assumed deadlock. The kernel can't get a buffer in a certain amount of time and assumes a deadlock. Therefore it will read the CR version of the block. This should not have a negative impact on performance, and basically replaces a read from disk with a wait for another process to read it from disk, as the block needs to be read one way or another.
  1014	110	n/a	We want the CURRENT block either shared or exclusive but the Block is being read into cache by another session, so we have to wait until their read() is completed.
  1014 (duplicate)	120	n/a	We want to get the block in current mode but someone else is currently reading it into the cache. Wait for them to complete the read. This occurs during buffer lookup.
  1016	210	n/a	The session wants the block in SCUR or XCUR mode. If this is a buffer exchange or the session is in discrete TX mode, the session waits for the first time and the second time escalates the block as a deadlock and so does not show up as waiting very long. In this case the statistic: "exchange deadlocks" is incremented and we yield the CPU for the "buffer deadlock" wait event.
  1016 (duplicate)	220	n/a	During buffer lookup for a CURRENT copy of a buffer we have found the buffer but someone holds it in an incompatible mode so we have to wait.

  Wait Time:

Normal wait time is 1 second. If the session has been waiting for an exclusive buffer during the last wait then it waits 3 seconds this wait. The session will keep timing-out/waiting until it acquires the buffer.

  Finding Blockers:

Finding the blocking process can be quite difficult as the information required is not externalised. If  P3 (Reason Code) shows that the "buffer busy wait" is waiting for a block read to complete then the blocking session is likely to be waiting on an IO wait (eg: "db file sequential read" or "db file scattered read") for the same  file# and  block#.

If the wait is due to the buffer being held in an incompatible mode then it should be freed very soon. If not then it is advisable to contact Oracle Support and get 3 SYSTEMSTATE dumps at one minute intervals as the blocking session may be spinning. (Look for ACTIVE sessions with high CPU utilisation).

Systemwide Waits:

If the TIME spent waiting for buffers is significant then it is best to determine which segment/s is/are suffering from contention. The "Buffer busy wait statistics" section of the Bstat/estat or STATSPACK reports shows which block type/s are seeing the most contention. This information is derived from <> which can be queried in isolation:

  SELECT time, count, class 
    FROM V$WAITSTAT 
   ORDER BY time,count
  ;

This shows the  class of block with the most waits at the BOTTOM of the list.

Oracle Support may also request that the following query be run to show where the block is held from when a wait occurs:

  SELECT kcbwhdes, why0+why1+why2 "Gets", "OTHER_WAIT"
    FROM x$kcbsw s, x$kcbwh w
   WHERE s.indx=w.indx
     and s."OTHER_WAIT">0
   ORDER BY 3
  ;
  Note: "OTHER_WAIT" is "OTHER WAIT" in Oracle8i (a space rather than an underscore)

Additional information regarding which files contain the blocks being waited for can be obtained from the internal <> thus:

  SELECT count, file#, name
    FROM x$kcbfwait, v$datafile
   WHERE indx + 1 = file#
   ORDER BY count
  ;

This shows the file/s with the most waits (at the BOTTOM of the list) so by combining the above of information we know what block type/s in which file/s are causing waits. The segments in each file can be seen using a query like:

  SELECT distinct owner, segment_name, segment_type
    FROM dba_extents
   WHERE file_id= &FILE_ID
  ;

If there are a large number of segments of the type listed then monitoring <> may help isolate which object is causing the waits.
Eg: Repeatedly run the following statement and collect the output. After a period of time sort the results to see which file & blocks are showing contention:

  SELECT p1 "File", p2 "Block", p3 "Reason"
    FROM v$session_wait 
   WHERE event='buffer busy waits'
  ;

Note:
In the above query there is no reference to WAIT_TIME as you are not interested in whether a session is currently waiting or not, just what buffers are causing waits.

If a particular block or range of blocks keep showing waits you can try to isolate the object using the queries in Note:181306.1.

One can also look at:

• Capturing session trace and noting the "buffer busy waits" may help - See Note:62160.1.

Reducing Waits / Wait times:

As buffer busy waits are due to contention for particular blocks then you cannot take any action until you know which blocks are being competed for and why. Eliminating the cause of the contention is the best option. Note that "buffer busy waits" for data blocks are often due to several processes repeatedly reading the same blocks (eg: if lots of people scan the same index) - the first session processes the blocks that are in the buffer cache quickly but then a block has to be read from disk - the other sessions (scanning the same index) quickly 'catch up' and want the block which is currently being read from disk - they wait for the buffer as someone is already reading the block in.

The following hints may be useful for particular types of contention - these are things that MAY reduce contention for particular situations:

Block Type	Possible Actions
data blocks	Eliminate HOT blocks from the application. Check for repeatedly scanned / unselective indexes. Change PCTFREE and/or PCTUSED. Check for 'right- hand-indexes' (indexes that get inserted into at the same point by many processes). Increase INITRANS. Reduce the number of rows per block.
segment header	Increase of number of FREELISTs. Use FREELIST GROUPs (even in single instance this can make a difference).
freelist blocks	Add more FREELISTS. In case of Parallel Server make sure that each instance has its own FREELIST GROUP(s).
undo header	Add more rollback segments.

Related:

Bug can cause "buffer busy waits" and latch contention in 817/901  Note:176129.1
Tracing User sessions  Note:62160.1 

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