Craig Shallahamer's Blog
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The text below is an except from the book,
Oracle Performance Firefighting, written by
Craig Shallahamer of
OraPub, Inc.
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©2009, 2010 by Craig Shallahamer. This is copyrighted material.
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their familes, we ask you to respect the copyright both in intent and deed. Thank you.
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<p>There are actually a number of mutex-related wait events, as listed in Table 7-1. While I would like all mutex-related wait events to start with mutex, Oracle has taken a different path. The mutexes associated with the library cache all start with the word cursor. It makes sense, since the library cache is full of cursors, but it makes discovering new mutex usage more difficult for the performance analyst.
</p><p>A session posts this event when requesting a mutex in exclusive mode, cannot get it by spinning, and therefore sleeps. It takes only one session holding the mutex in shared mode to prevent an exclusive acquisition. Building a child cursor, capturing SQL bind data, and build or updated cursor-related statistics require a mutex exclusive hold.
</p><p>A session posts this event when requesting a mutex in shared mode, cannot get it by spinning, and therefore sleeps. Multiple sessions can hold a mutex in shared mode. A mutex cannot be held in shared mode if another session holds the mutex exclusively. A session holds a mutex in shared mode, not exclusive mode, when changing the reference count (refer to Chapter 3 for details). So another session may be in the middle of changing the reference count. When this occurs, the mutex is said to be "in flux." Seeing this event is extremely unlikely, as changing the reference count is spectacularly fast (so I'm told and the algorithm suggests). So while multiple sessions can hold the mutex in shared mode, changing the reference count is truly a serial operation.
</p>
©2009, 2010 by Craig Shallahamer. This is copyrighted material.
Please—Out of respect for those involved in the creation of the book and also for
their familes, we ask you to respect the copyright both in intent and deed. Thank you.
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