Re: Library cache lock vs. pin (cool stuff - KGX mutexes)

Btw, things get more fun in 10.2, you can pin cursors without getting library 
cache pin latch, using KGX mutexes. Mutexes are new thing in 10.2 and they 
enable shared access to objects in somewhat similar manner than shared latches, 
that every successful get of particular mutex will increment its value and 
release will decrement. When the count is zero, no-one has the mutex and it is 
safe to get it in exclusive mode too. However they are more fine grained than 
kgl latches and provide better waiting mechanism as far as I understand.

So if your environment supports atomic compare and swap operation (as CMPXCHG 
on Intel), you might get away without cursor_space_for_time setting for 
ultrahigh execution rates. Otherwise the atomic mutex operations would be 
achieved using new KGX latches.

At least on my laptop this feature isn't enabled by default (from and 
OracleWorld's paper I remember that it should become default in 10.2.0.2), but 
so far you can experiment with it if you set _kks_use_mutex_pin = true and 
bounce the instance (mutex structures will be stored in shared pool, so you 
might need to increase SP size).

There are also X$MUTEX_SLEEP and X$MUTEX_SLEEP_HISTORY fixed tables which can 
show some interesting information if you generate some mutex waits into those ;)

Tanel.

  ----- Original Message ----- 
  From: Bobak, Mark 
  To: paul.baumgartel@xxxxxxxxx ; Ray Stell 
  Cc: Oracle-L 
  Sent: Friday, January 20, 2006 12:49 PM
  Subject: RE: Library cache lock vs. pin


  I think that's a pretty fair and accurate summary.

  -- 
  Mark J. Bobak 
  Senior Oracle Architect 
  ProQuest Information & Learning 

  "There are 10 types of people in the world:  Those who understand binary, and 
those who don't." 





------------------------------------------------------------------------------
  From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On 
Behalf Of Paul Baumgartel
  Sent: Friday, January 20, 2006 1:45 PM
  To: Ray Stell
  Cc: Oracle-L
  Subject: Re: Library cache lock vs. pin


  Yes--I consulted my copy and what I take from it is that a library cache lock 
is held pretty much all the time on library cache objects--in shared mode 
during parsing (when the object is also pinned), and in null mode the rest of 
the time.  When DDL invalidates an object, the lock is broken, so I interpret 
the lock as a "sentinel" of sorts that indicates whether an object requires 
reparsing or recompilation; the pin is the method by which objects are 
protected while operations on them are in progress. 

  That sound reasonable?

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