【MOS】 Troubleshooting waits for enq: TX - allocate ITL entry(1472175.1)

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【MOS】 Troubleshooting waits for enq: TX - allocate ITL entry(1472175.1)

小麦苗 2016-09-05 20:06:21 浏览534
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Troubleshooting waits for 'enq: TX - allocate ITL entry' (文档 ID 1472175.1)



In this Document

Symptoms
Cause
Solution
  Increase INITRANS
  Increase PCTFREE
  A Combination of increasing both INITRANS and PCTFREE
References


APPLIES TO:

Oracle Database - Enterprise Edition - Version 10.2.0.1 and later
Information in this document applies to any platform.

SYMPTOMS

Observe high waits for event enq: TX - allocate ITL entry

Top 5 Timed Foreground Events

Event                           Waits  Time(s)  Avg wait (ms)  % DB time  Wait Class
enq: TX - allocate ITL entry    1,200   3,129           2607       85.22  Configuration
DB CPU                                                   323        8.79
gc buffer busy acquire         17,261      50              3        1.37  Cluster
gc cr block 2-way             143,108      48              0        1.32  Cluster
gc current block busy          10,631      46              4        1.24  Cluster

CAUSE

By default INITRANS value for table is 1 and for index is 2. This defines an internal block structure called the Interested Transaction List (ITL). In order to modify data in a block, a process needs to use an empty ITL slot to record that the transaction is interested in modifying some of the data in the block. If there are insufficient free ITL slots then new ones will be taken in the free space reserved in the block. If this runs out and too many concurrent DML transactions are competing for the same data block we observe contention against the following wait event - "enq: TX - allocate ITL entry".

You can see candidates for re-organisation due to ITL problems in the "Segments by ITL Waits"  section of an Automatic Workload Repository (AWR) report:

Segments by ITL Waits

  * % of Capture shows % of ITL waits for each top segment compared
  * with total ITL waits for all segments captured by the Snapshot

Owner Tablespace Name Object Name Subobject Name Obj. Type       ITL  Waits % of Capture
PIN   BRM_TABLES      SERVICE_T                  TABLE           188               84.30
PIN   BRM_TABLES      BILLINFO_T  P_R_06202012   TABLE PARTITION  35               15.70


SOLUTION

The main solution to this issue is to increase the ITL capability of the table or index by re-creating it and altering the INITRANS or PCTFREE parameter to be able to handle more concurrent transactions. This in turn will help to reduce "enq: TX - allocate ITL entry" wait events.

To reduce enq: TX - allocate ITL entry" wait events, We need to follow the steps below:

Increase INITRANS

A)

1) Depending on the number of transactions in the table we need to alter the value of INITRANS. here it has been changed to 50:

alter tableINITRANS 50;


2) Then re-organize the table using move (alter table move;)

3) Then rebuild all the indexes of this table as below

alter index rebuild INITRANS 50;


Increase PCTFREE

If the issue is not resolved by increasing INITRANS then try increasing PCTFREE. Increasing PCTFREE holds more space back and so spreads the same number of rows over more blocks. This means that there are more ITL slots available overall :
B)

1) Spreading rows into more number of blocks will also helps to reduce this wait event.

alter table
  PCTFREE 40;

2) Then re-organize the table using move (alter table service_T move;)

3) Rebuild index

alter index index_name  rebuild PCTFREE 40;


A Combination of increasing both INITRANS and PCTFREE


1) Set INITRANS to 50 and  pct_free to 40

alter table PCTFREE 40  INITRANS 50;


2) Re-organize the table using move (alter table move;)

3) Then rebuild all the indexes of the table as below

alter index  rebuild PCTFREE 40 INITRANS 50;


NOTE: The table/index can be altered to set the new value for INITRANS. But the altered value takes effect for new blocks only. You need to rebuild the objects so that the blocks are initialized again.

For an index this means the index needs to be rebuild or recreated.

For a table this can be achieved through:
  • exp/imp
  • alter table move
  • dbms_redefenition

You can find information relating to other 'enq: TX - ...' variants in the following articles:

Document 1476298.1 Resolving Issues Where 'enq: TX - row lock contention' Waits are Occurring 
Document 873243.1 Troubleshooting 'enq: TX - index contention' Waits in a RAC Environment. 
Document 1946502.1 Resolving Issues Where 'enq: TX - contention' Waits are Occurring





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