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LOB HWM CONTENTION,Using AWR to Identify Problem; Confirm,Verify Fix-837883.1

原创 Linux操作系统 作者:rongshiyuan 时间:2013-09-27 11:15:06 0 删除 编辑
LOB HWM CONTENTION :Using AWR Reports to Identify the Problem; Confirm and Verify the Fix (文档 ID 837883.1)

In this Document

Applies to:

Oracle Server- Enterprise Edition - Version to [Release 10.2]
Information in this document applies to any platform.
***Checked for relevance on 30-Apr-2012***


LOB performance issues have multiple sources: This note is not designed to be a One-fix for all HWM issues:

One of the most prevalent problems seen with LOB performance is when the High WaterMark (HWM) enqueue has notable contention.

  • This Document will use examples from the Automatic Workload Repository (AWR) and Active Session History (ASH) reports to confirm if you are encountering a problem with HWM contention.
  • We will provide one of the possible fixes when you see this HWM enqueue contention on LOBs if using ASSM
  • We will then provide you with examples of AWR and ASH reports showing the HWM enqueue before and after the application of the Fix
  • Last: we will provide Tips and Techniques on how to diagnose other LOB performance problems using the AWR and ASH reports

COMMENT: There are SEVERAL conditions that can lead to HWM + LOBs during updates or delete/inserts. IF you are on RAC you should also review LOB HW enqueue contention in RAC environments with ASSM space management

Setting the LOB to use Pctversion 100% may be one of the quickest methods to check if the contention can easily be reduced for the LOB

Reducing the segment size is perhaps the strongest recommendation when attempting to improve LOB performance with BASICFILEs. This is usually best accomplished by creating Multiple partitions (powers of 2) to reduce LOB index size and contention. Freepools are also important for RAC configurations.

Securefiles which are available as of 11.2 will relieve most performance issues found with Basicfiles.


We will focus on "HW enqueue contention for ASSM LOB segments" and applying the fix:

“…HW enqueue contention can occur for LOB segments which are ASSM managed as space allocation only acquires one block at a time. With this fix ASSM lobs get a minimum number of chunks based on the value of event 44951 (up to 1024) which should help reduce or eliminate HWM enqueue contention.

This is the most important LOB HWM Enqueue bug encountered with the following prerequisites
  a) Automatic Segment Space Mangement (ASSM) is used with the tablespace storing the LOB
  b) Inserts and updates are slow (vs read)
  c) High Load and/or concurrency exists
  d) HW: enqueue is seen in the top 5 wait events of the AWR report
  e) Not using RAC or Securefiles



1) The fix is already included in and but not implemented until the event is enabled
 - for Download and apply the patch for :

2)  Set EVENT 44951 up to 1024 (which changes Chunks allocated from 1 on ASSM to 1024)

NOTE: The fix for this problem introduces a *mandatory event* which must be set in order for the ASSM architectural change leading to more efficient allocation of LOB chunks during high load and transaction contention when Extent allocation is needed. The event is *not* set even after you apply the patch.


 - or set directly in the pfile/init.ora -


 3) If using the patch then Shutdown and Restart the database

 COMMENT: MONITOR the impact of this event as it may introduce performance degradation in some cases: As the event is dynamic once the fix is in place you can enable at various levels or disable using level 0. Common levels include 128,512 or 1024.


  1. Confirm you are hitting the LOB + enq: HWM - Contention Problem
  2. Confirm if the Patch + EVENT set resolved the problem
  3. Confirm if the problem did not help HWM contention  

The following three Symptoms will help you identify if you have encountered the problem

Symptom #1


*  "enq: HW - contention" will be in the Top 5 Wait events 
** "SQL*Net more data from client" is usually in the Top 5  Wait events also 

Top 5 Timed Events
                                                     wait     Total  Call Wait
Event                              Waits    Time(s)  (ms)     Time%  Class
------------------------------ ------------ ----------- ------ ------ ---------
db file sequential read        2,580,474    35,544    14       77.4  User I/O
SQL*Net more data from client    659,140     5,513     8 **    12.0  Network
CPU time                                     4,540   9.9
enq: HW - contention              88,910     2,890    33 *     6.3   Configuration
log file sync                    777,146     1,688     2       3.7   Commit


Symptom #2

Review the TOP Enqueue Activity:

  • The HWM enqueue will be among the top 5
  • Transaction Contention may also be in the top few enqueues
  • High Total and Average Wait times for Segment High Water Mark and TX row-lock contention
  • High Transaction Waits in fractional or complete seconds (user timeouts or long commits

Enqueue Activity Snaps: 1234-1235
-> only enqueues with waits are shown
-> Enqueue stats gathered prior to 10g should not be compared with 10g data
-> ordered by Wait Time desc, Waits desc

Enqueue Type                      Requests     Succ Gets Failed Gets Waits  Wt.Time(s) Avg Wt Time(ms)
---------------------------------------------- -------- ----------   -----  --------- ----------------

HW-Segment High Water Mark          93,860      93,862      0        88,226   2,961        33.56 *
TX-Transaction(row lock contention)    272         272      0           209     570     2,729.44 ** 
TX-Transaction (index contention)    4,564       4,564      0         4,144      34         8.16
TX-Transaction                     793,989     794,042      0            97       0         4.08

-- enq: TX - row lock contention hit 2.73 seconds Per Transaction !

Symptom #3

AWR WAIT EVENTS: More evidence

  • enq: HW - contentionSQL
  • Net more data from client **
  • enq: TX - row lock contention ***
  • High Average Wait in ms (20ms or higher) *
  • Transaction Time in fractional (or complete) seconds * 

-> s -second                                                                                                                  
-> ms - millisecond - 1000th of a second                                                                          
-> ordered by wait time desc, waits desc (idle events last)                                                  
                                                     %Time   TotalWait       wait     Waits                            
Event                                   Waits        -outs   Time (s)        (ms)     /txn                               
----------------------------        --------------  ------  ----------- ------- ---------                             
db file sequential read              2,580,474        .0      35,544           14      3.3                           
SQL*Net more data from client  **      659,140        .0       5,513            8      0.9                            
enq: HW - contention                    88,910        .0       2,890          33*      0.1                           
log file sync                          777,146        .0       1,688            2      1.0                            
read by other session                  103,140        .0         929            9      0.1                            
SQL*Net break/reset to client          114,782        .0         813            7      0.1                            
enq: TX - row lock contention ***          380      43.4***      557        1466*      0.0                            
log file parallel write                552,663        .0         394            1      0.7                            
latch: cache buffers chains             55,203        .0         382            7      0.1                            

-- enq: TX - row lock contention hit 1.46 seconds Per Transaction !



TOP 5 TIMED EVENTS : you should notice the following changes

  • Enq HWM - contention             -- this event is usually missing from the top 5 post patch or should show as less than ~6ms avg/read if this bug is the HWM problem! 
  • SQL*Net more data from client -- this events time will usually decrease dramatically or drop from the top 5
    ------ ------ ---------

TOP ENQUEUE ACTIVITY: you should notice the following changes

  • The HWM enqueue will be move far down the list and may drop off measurable enqueue activity
  • Transaction Contention should no longer be in fractional or higher seconds
  • Total and Average Wait times for Segment High Water Mark and TX row-lock contention should be a fraction of previous assessed times
  • AWR WAIT EVENTS to Monitor after the PATCH has been applied, the EVENT is set and the Instance has been bounced include: 

Top 5 Timed Events
RAC example

                                              Total Wait    Avg Wait    Total%
Event                           Waits          Time(s)      Time(ms)     Time      Class
------------------------------ ------------   -----------  ------       ------   ---------
db file sequential read        140,990         3,529         25          19.1      User I/O
log file sync                   62,033         3,313         53          17.9      Commit
gc buffer busy                  44,012         2,442         55          13.2      Cluster
CPU time                                       2,268                     12.3                      
log file parallel write         66,681         1,993         30          10.8      System I/O


AWR WAIT EVENTS to Monitor after the Patch, Event and Bounce has occurred:

Missing or notably decreased from the WAIT EVENTS section of the AWR report after the patch:

Event Description              New Outcome Post Patching

enq HWM                        HWM Contention Drops substantially down the Enqueue List

enq HWM                        Average HWM wait times will drop from 20-30+ ms to below ~6 ms
SQL*Net more data from client  Total Time will usually decrease by more than 75%
enq: TX - row lock contention  Will decrease during LOB load/updates (barring application issues




The fix for BUG 6376915 requires:

  1. Patch    (already built into, and or higher)
  2. EVENT 44951 set in the spfile of init.ora
  3. LEVEL for the event: this value can be set up to 1024


Many times you may believe that this event is already set in the database or for the instance: However, experience has shown that this is not always the case.
The easiest method to determine if the event has been set is to look at the most recent startup in the ALERT.LOG which reports Non-Default values.


Shutting down archive processes
Archiving is disabled
Archive process shutdown avoided: 0 active
Fri Dec 11 12:46:27 2009
Starting ORACLE instance (normal)
Picked latch-free SCN scheme 2
Using LOG_ARCHIVE_DEST_1 parameter default value as D:\oracle\product\10.2.0\db_1\RDBMS
Autotune of undo retention is turned on.
ILAT =18
SYS auditing is disabled
ksdpec: called for event 13740 prior to event group initialization
Starting up ORACLE RDBMS Version:
System parameters with non-default values:
processes = 150
event = 44951 TRACE NAME CONTEXT FOREVER, LEVEL 1024     
                ^^^ < this event will show as a non-default values in the ALERT.LOG
__shared_pool_size = 104857600
__large_pool_size = 4194304
__java_pool_size = 4194304




There are several different sources for performance issues with LOBs. This event is only one of many potential problem source/solutions.

Under some circumstances this event will increase HWM contention so it is important to qualify and test the fix. The AWR reports can be used to provide feedback on any changes made.



NOTE:9801919.8 - Bug 9801919 - "enq: HW - contention" against segments that add an extent frequently during high concurrency




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