metalink关于latch free的介绍.doc

文档 ID: 注释:34576.1 主题: WAITEVENT: latch free Reference Note 类型: REFERENCE 状态: PUBLISHED 内容类型: TEXT/X-HTML 创建日期: 27-FEB-1996 上次修订日期: 11-NOV-2002 latch free Reference Note This is a reference note for the wait event latch free which includes the following subsections: Brief definition Individual wait details (eg: For waits seen in View:V$SESSION_WAIT) Systemwide wait details (eg: For waits seen in View:V$SYSTEM_EVENT) Reducing waits / wait times See Note:61998.1 for an introduction to Wait Events. Definition: Versions:7.0 - 9.2 Documentation: 9.0 Latches are like short duration locks that protect critical bits of code. This wait indicates that the process is waiting for a latch that is currently busy (held by another process). Individual Waits: ?Parameters: P1 = Latch address P2 = Latch number P3 = Tries Latch address The address of the latch that the process is waiting for. The hexadecimal value of P1 (P1RAW) can be used to determine which latch is waited for thus: SELECT name, Child ||child#, gets, misses, sleeps FROM v$latch_children WHERE addr=P1RAW UNION SELECT name, null, gets, misses, sleeps FROM v$latch WHERE addr=P1RAW ; This will show Child N in the second column if the latch is a child latch. Latch number This is the latch number that indexes the V$LATCHNAME view: SELECT * FROM v$latchname WHERE latch# = P2 ; Note that the latch number for a given set of latches can vary between releases / platforms so it is best to avoid hard coding P2 or LATCH# in SQL scripts. Tries This is basically a counter that counts the number of times we tried to get the latch (slow with spinning) and the process had to sleep. See the Wait Time notes below. ?Wait Time: When a session waits on latch free it effectively sleeps for a short time then re-tests the latch to see if it is free . If it still cannot be acquired then P3 is incremented and the session waits again. The wait time increases exponentially and does not in