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Don Burleson Blog 







Oracle Deadlock cursor pin wait on x in 11g

Oracle Database Tips by Donald BurlesonJune 7, 2015

Question:  I recently migrated from Oracle 10g to Oracle 11g and I now see excessive deadlocks in Oracle 11g that did not appear in 10g.  

What has changed in 11g that causes additional deadlocks?

Answer:  Oracle 10g release 2 and beyond replaced some latch mechanisms with the mutex approach, claiming that they are faster and more efficient than traditional locking mechanisms.  To improve cursor execution speed and hard pare time within the library cache, mutexes replace library cache latches and library ache pins.  

Oracle claims that mutexes are faster and have improved concurrency over library cache latches (and therefore less deadlocks) because a mutex has a shorter code path.  Oracle also say that a mutex uses less CPU, which is important for CPU-bound database where large data buffers remove I/O as a primary source of contention.   

The mutex memory structure is quite different from the older latch structure and all 131,072 library cache buckets in 11g are now protected by an exclusive mutex.  

However, some shops complain that their 11g systems are more sensitive to deadlocks than they were in Oracle 10g.     

Beware, there are some early release issues with mutexes causing excessive 'cursor pin s wait on x' wait events.  

The cursor pin S wait on X wait event is mostly related to mutex and hard parse and it happens when a session waits for this event when it is requesting a shared mutex pin and another session is holding an exclusive mutex pin on the same cursor object.  

These are the parameters associated with this mutex wait event:

  •  P1 Hash value of cursor
  •  P2 Mutex value (top 2 bytes contains SID holding mutex in exclusive mode, and bottom two bytes usually hold the value 0)
  •  P3 Mutex where (an internal code locator) OR'd with Mutex Sleeps  

Beware, Oracle bug 5184776 can affect 11g databases that deploy the mutex latching, causing excessive 'cursor pin s wait on x' wait events.

If you suspect that you have a bug, you can revert to the pre-mutex method with these commands that bounce your database into the older latching mode, which may result in less deadlocks:  

alter system set "_kks_use_mutex_pin"=false scope=spfile;
shutdown immediate

This hidden parameter should only be used with the consent of Oracle support, and this parameter should NOT ever be set in Oracle 12c.

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