如何診斷等待事件 enq: HW - contention

如何診斷等待事件

SQL>select event,p1,p2,p3 from v$session_wait;
EVENT P1 P2 P3
---------------------- --------   ------- ----------
enq: HW - contention 1213661190  4 17005691

透過P3進行DBMS_UTILITY轉換可以獲知發生爭用的檔案和block
SQL> select DBMS_UTILITY.DATA_BLOCK_ADDRESS_FILE(17005691) FILE#,
  2  DBMS_UTILITY.DATA_BLOCK_ADDRESS_BLOCK(17005691) BLOCK#
  3  from dual;

     FILE#     BLOCK#
---------- ----------
         4     228475
        
進而透過file#和block#定位物件

SQL> select owner, segment_type, segment_name
  2  from dba_extents
  3  where file_id = 4
  4  and 228475 between block_id and block_id + blocks - 1;


OWNER SEGMENT_TYPE SEGMENT_NAME
--------------- --------------- ------------------------------
SCOTT LOBSEGMENT EMP_DATA_LOB

我們知道enqueue鎖的p2,p3值與v$lock的id1，id2值相同，同樣透過id2，也可以知道發生爭用的檔案和block
SQL> select DBMS_UTILITY.DATA_BLOCK_ADDRESS_FILE(ID2) FILE#,
  2  DBMS_UTILITY.DATA_BLOCK_ADDRESS_BLOCK(ID2) BLOCK#
  3  from v$lock
  4  where type = 'HW';
      FILE#     BLOCK#
---------- ----------
         4     228475

透過p1值可以知道鎖型別和模式

SQL> select chr(bitand(1213661190,-16777216)/16777215)||
  2  chr(bitand(1213661190,16711680)/65535) "Lock",to_char( bitand(1213661190, 65535) ) "Mode" from dual;
Lock       Mode
---------- ----------
HW         6
1. When using Automatic Segment Space Management (ASSM)

   a) As temporary workaround, manually add extra space to the LOB segment
      ALTER TABLE
      MODIFY LOB () (allocate extent (size ));
OR
   b) It may related Bug 6376915.
   Refer to Note 6376915.8 "Bug 6376915 HW enqueue contention for ASSM LOB segments"
   In 10.2.0.4 or above, this fix has been included, and can be enabled by setting event 44951 to a value
   between 1 and 1024.  A higher value would be more beneficial in reducing contention.
   EVENT="44951 TRACE NAME CONTEXT FOREVER, LEVEL < 1 - 1024 >"
OR
  c) Consider partitioning the LOB  in a manner that will evenly distribute concurrent DML across multiple partitions 

2. When using Manual Segment Space Management(MSSM)

a) As temporary workaround, manually add extra space to the LOB segment
    ALTER TABLE    
    MODIFY LOB () (allocate extent (size ));
OR
     b) Consider partitioning the LOB in a manner that will evenly distribute concurrent DML across multiple partitions