Mutexes And Changes in Library Cache - HrOUG

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DbProf. com Mutex Benefits (1) • Smaller and faster – faster to get in comparison with latches and use less memory • Less potential for false collision – Latches usually protect multiple objects and are therefore candidates for false contention – it is contention for the latch rather than the shared resource they protect. – Mutexes are smaller and are usually part of the structure they protect or there may be several mutexes to protect one structure. Therefore false contention is less likely. • Use more flexible wait strategy – wait can be: blocking wait, sleep or CPU yield • They can be held in shared or exclusive mode. www.dbprof.com - © 2012 Jože Senegačnik Oracle ACE Director 18
DbProf. com Mutex Benefits (2) • Re-executing SQL statements already loaded in the library cache. • When Library cache pin is used: – Cursor must be pinned by each session prior execution. – Library cache pin must be acquired when cursor is left opened for reexecution and CURSOR_SPACE_FOR_TIME=FALSE. – Library cache pin is not required when cursor is left opened for reexecution and can be rebound and CURSOR_SPACE_FOR_TIME=TRUE. • Mutexes instead of a “library cache pin” – When mutexes are used instead of library cache pin replacements, CURSOR_SPACE_FOR_TIME does not need to be set. The session executing shared cursor will only need a mutex pin – it will increment the ref count when re-executing the cursor. – Parent examination: The parent cursor is examined when finding a child cursor to execute. Mutex is used for the parent examination. – Building a new cursor under a parent – is cheaper, however building many cursors under a parent cursor is not recommended. www.dbprof.com - © 2012 Jože Senegačnik Oracle ACE Director 19
Page 1 and 2: DbProf. com Mutexes And Changes in
Page 3 and 4: Latches, Hash Buckets & Data Blocks
Page 5 and 6: DbProf. com Structure of the Child
Page 7 and 8: DbProf. com Memory Allocations •
Page 9 and 10: DbProf. com Multiple Child Cursors
Page 11 and 12: Efficiency of setting SESSION_CACHE
Page 13 and 14: DbProf. com Parsing Latches (Prior
Page 15 and 16: DbProf. com Mutexes www.dbprof.com
Page 17: DbProf. com Mutex Holding Modes •
Page 21 and 22: DbProf. com Mutex Creation and Usag
Page 23 and 24: DbProf. com Library Cache Mutexes M
Page 25 and 26: DbProf. com Mutex Locations select
Page 27 and 28: DbProf. com The Mystery of Numbers
Page 29 and 30: DbProf. com Mutex Usage - “Cursor
Page 31 and 32: DbProf. com V$MUTEX_SLEEP SQL> sele
Page 33 and 34: DbProf. com Blocker Session Id •
Page 35 and 36: DbProf. com High Version Count and
Page 37 and 38: DbProf. com Cardinality Feedback Tu
Page 39: DbProf. com Thank you for your inte

Mutexes And Changes in Library Cache - HrOUG

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