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Display Latches in Oracle

There have been volumes written about Oracle latches.  But, in case you do not know what they are, latches protect the many memory structures in Oracle's SGA.  They ensure that one and only one process at a time can run or modify any memory structure at the same instant.  Much more restrictive than locks (which at least allow for some collective user interaction), latches have no queuing mechanism, so either you get it or you don't, and you are forced to continually retry if you don’t.

Common indicators of latch contention are a latch miss ratio, which records willing-to-wait mode latch requests, and latch immediate miss ratio, which records no-wait mode latch requests. 

These statistics reflect how often latch requests were made and satisfied without waiting.  If either of these exceeds 1%, then you should drill down further into latching details to identify what latches are responsible for the contention. 

To drill down into latch miss details, you can use the latchdet.sql script:

See Code depot for complete script

 

 
select
       name,
       gets,
. . .
       sleeps
from
       sys.v$latch
where
. . .
order by
      2 desc;
 
Figure 5.11 – Output showing latch details

The above is an excerpt from Oracle Performance Troubleshooting by Robin Schumacher.

It's only $19.95 and you can order it and get instant access to the Oracle scripts here:

http://www.rampant-books.com/book_2003_1_perf.htm

 

 

 

 
 
 

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