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一、今天在查看awr报告中，发现Top 5 Timed Foreground Events发现enq: HW - contention的等待事件；
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二、enq: HW - contention的官方说明：
The HW enqueue is used to serialize the allocation of space beyond the high water mark of a segment.
?V$SESSION_WAIT.P2 / V$LOCK.ID1 is the tablespace number.
?V$SESSION_WAIT.P3 / V$LOCK.ID2 is the relative data block address (dba) of segment header of the object for which space is being allocated.
If this is a point of contention for an object, then manual allocation of extents solves the problem.
三、等待事件解释
为防止多个进程同时修改HWM而提供的锁称为HW锁。想要移动HWM的进程必须获得HW锁。若在获取HW锁过程中发生争用，则等待enq: HW - contention事件。HW锁争用大部分是因大量执行insert所引发的，偶尔也会因大量执行update在回滚段中发生HW锁争用现象。若是update，表中段的扩展的大小虽然不多，但在创建回滚数据的过程中，需要回滚段的急速扩张。HW锁争用是在急速空间扩张时普遍出现的等待现象，有时也会引发严重的性能下降。
SQL> select event#,name,parameter1,parameter2,parameter3 from v$event_name where name = 'enq: HW - contention';
EVENT# NAME PARAMETER1 PARAMETER2 PARAMETER3
---------- ---------------------------------------- -------------------- -------------------- --------------------
250 enq: HW - contention name|mode table space # block
众所周知，Oracle高水位线标志着该线以下的block均被Oracle格式过，通俗一点讲就是该高水位线以下的block都被Oracle使用过。 通常在执行insert操作时，当高水位线以下block不够用时，Oracle将会推进高水位线。更进一步讲，当有多个进程在同时进行insert操作时，比较容易引起高水位线争用，主要表现为enq: HW - contention。
四、如何找到事件：'enq: HW - contention' 热点对象：
1、查看v$session_wait，应该会有如下等待事件：
SQL> select p1, p2, p3 from v$session_wait where event = 'enq: HW - contention';
P1 P2 P3
---------- ---------- ----------
1213661190 7 140003563
1213661190 7 140003563
1213661190 7 140003563
1213661190 7 140003563
1213661190 7 140003563
1213661190 7 140003563
1213661190 7 140003563
2、通过P3进行DBMS_UTILITY转换可以获知发生争用的文件和block：
SQL> select dbms_utility.data_block_address_block(140003563),dbms_utility.data_block_address_file(140003563) from dual;
DBMS_UTILITY.DATA_BLOCK_ADDRESS_BLOCK(140003563) DBMS_UTILITY.DATA_BLOCK_ADDRESS_FILE(140003563)
------------------------------------------------ -----------------------------------------------
1591531 33
3、进而通过file#和block#定位对象：
SQL> select owner, segment_type, segment_name
from dba_extents
where file_id = 33 and 1591531 between block_id and block_id + blocks - 1;
五、减少HW锁争用的方法如下：
If this is a point of contention for an object, then manual allocation of extents solves the problem.
脚本：alter table
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本文作者：JOHN
ORACLE技术博客：ORACLE 猎人笔记 数据库技术群：367875324 （请备注ORACLE管理 ）
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