Oracle High Performance Tuning for 9i and 10g
This text covers the three parts of tuning an Oracle database: data modeling, SQL code tuning, and physical database configuration, and explains both problem detection and resolution.
TABLE OF CONTENTS Oracle High Performance Tuning for 9i and 10g
Chapter 20: Latches
Wait events were introduced in the previous chapter. A wait event is recorded when one thing is waiting for another. There are many different types of wait events. High latch free wait event rates indicate latch misses, showing possible problems with SGA memory buffers. High quantities of latch misses can seriously affect overall database performance; what is required is not being acquired.
20.1 What is a Latch?
Latches protect memory buffers much like locks protect table rows. When a latch is already held on an area of a buffer a latch free wait event occurs if another process attempts to access the same type of data and fails to do so. There are numerous types of latches for all the different types of memory buffer areas. Latches apply to items such as data in the database buffer cache, preventing multiple simultaneous changes to the same data.
| Tip | A latch cannot be tuned. Latch problems are indicative of other problems and can help to focus where significant performance problems may be occurring. |
| Note | Oracle Database 10 Grid Oracle9 i Database Release 2 (9.2) has 239 latches. Oracle Database 10 g has 316 latches. |
20.1.1 Latch Misses, Spins, and Sleeps
In the previous chapter we saw that a latch free wait event is generated when a latch is missed.
| Note | Oracle Database 10 Grid There are many more latch wait events where there were only two in Oracle9 i Database. |
What is a missed latch? When a...
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