HOW TO FIND THE SESSION HOLDING A LIBRARY CACHE LOCK

PURPOSE

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 In some situations it may happen your session is 'hanging' and is a waiting for 

 a 'Library cache lock'. This document describes how to find the session that 

 in fact has the lock you are waiting for.

SCOPE & APPLICATION

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Support analysts, dba's, ..

HOW TO FIND THE SESSION HOLDING A A LIBRARY CACHE LOCK

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 Common situations:

 * a DML operation that is hanging because the table which is accessed is currently undergoing changes (ALTER TABLE). This may take quite a long time depending on the size of the table and the type of the modification 

   (e.g. ALTER TABLE x MODIFY (col1 CHAR(200) on thousands of records). 

* The compilation of package will hang on Library Cache Lock and Library Cache Pin if some users are executing any Procedure/Function defined in the same package. 

 In the first situation the V$LOCK view will show that the session doing the 

 'ALTER TABLE' has an exclusive DML enqueue lock on the table object (LMODE=6, 

 TYPE=TM and ID1 is the OBJECT_ID of the table). The waiting session however does  not show up in V$LOCK yet so in an environment with a lot of concurrent sessions 

 the V$LOCK information is insufficient to track down the culprit blocking your 

 operation.

METHOD 1: SYSTEMSTATE ANALYSIS

------------------------------

 One way of finding the session blocking you is to analyze the system state dump.

 Using the systemstate event one can create a tracefile containing detailed 

 information on every Oracle process. This information includes all the resources

 held & requested by a specific process.

 Whilst an operation is hanging, open a new session and launch the following

 statement:

 ALTER SESSION SET EVENTS 'IMMEDIATE TRACE NAME SYSTEMSTATE LEVEL 10';

 Oracle will now create a systemstate tracefile in your USER_DUMP_DEST directory.

 Get the PID (ProcessID) of the 'hanging' session from the V$PROCESS by matching 

 PADDR from V$SESSION with ADDR from V$PROCESS:

 SELECT PID FROM V$PROCESS WHERE ADDR=

        (SELECT PADDR FROM V$SESSION WHERE SID=sid_of_hanging_session);

 The systemstate dump contains a separate section with information for each 

 process. Open the tracefile and do a search for 'PROCESS pid_from_select_stmt'.

 In the process section look up the wait event by doing a search on 'waiting for'.

 Example output:

 PROCESS 8:

   ----------------------------------------

   SO: 50050b08, type: 1, owner: 0, flag: INIT/-/-/0x00

   (process) Oracle pid=8, calls cur/top: 5007bf6c/5007bf6c, flag: (0) -

             int error: 0, call error: 0, sess error: 0, txn error 0

   (post info) last post received: 82 0 4

               last post received-location: kslpsr

               last process to post me: 5004ff08 1 2

               last post sent: 0 0 13

               last post sent-location: ksasnd

               last process posted by me: 5004ff08 1 2

     (latch info) wait_event=0 bits=0

     Process Group: DEFAULT, pseudo proc: 50058ac4

     O/S info: user: daemon, term: pts/1, ospid: 15161

     OSD pid info: 15161

     ----------------------------------------

     SO: 5005f294, type: 3, owner: 50050b08, flag: INIT/-/-/0x00

     (session) trans: 0, creator: 50050b08, flag: (41) USR/- BSY/-/-/-/-/-

               DID: 0001-0008-00000002, short-term DID: 0000-0000-00000000

               txn branch: 0

               oct: 6, prv: 0, user: 41/LC

     O/S info: user: daemon, term: pts/1, ospid: 15160, machine: goblin.forgotten.realms

               program: [e-mail] (TNS V1-V3)

     application name: SQL*Plus, hash value=3669949024

     waiting for 'library cache lock' blocking sess=0x0 seq=253 wait_time=0

!>>              handle address=5023ef9c, lock address=5019cad4, 10*mode+namespace=15

 Using the 'handle address' you can look up the process that is keeping a lock

 on your resource by doing a search on the address within the same tracefile.

 Example output: 

 PROCESS 9:

   ----------------------------------------

   SO: 50050e08, type: 1, owner: 0, flag: INIT/-/-/0x00

   (process) Oracle pid=9, calls cur/top: 5007bbac/5007bbfc, flag: (0) -

             int error: 0, call error: 0, sess error: 0, txn error 0

   <cut> ....

        ----------------------------------------

         SO: 5019d5e4, type: 34, owner: 5015f65c, flag: INIT/-/-/0x00

!>>      LIBRARY OBJECT PIN: pin=5019d5e4 handle=5023ef9c mode=X lock=0

         user=5005fad4 session=5005fad4 count=1 mask=0511 savepoint=118218 flags=[00]

 From the output we can see that the Oracle process with PID 9 has an exclusive

 lock on the object we are trying to access. Using V$PROCESS and V$SESSION we can

 retrieve the sid,user,terminal,program,... for this process. The actual statement

 that was launched by this session is also listed in the tracefile (statements and

 other library cache objects are preceded by 'name=').

METHOD 2: EXAMINE THE X$KGLLK TABLE

-----------------------------------

 The X$KGLLK table (accessible only as SYS/INTERNAL) contains all the 

 library object locks (both held & requested) for all sessions and

 is more complete than the V$LOCK view although the column names don't

 always reveal their meaning.

 You can examine the locks requested (and held) by the waiting session 

 by looking up the session address (SADDR) in V$SESSION and doing the 

 following select:

 select * from x$kgllk where KGLLKSES = 'saddr_from_v$session'

 This will show you all the library locks held by this session where

 KGLNAOBJ contains the first 80 characters of the name of the object.

 The value in KGLLKHDL corresponds with the 'handle address' of the

 object in METHOD 1.

 You will see that at least one lock for the session has KGLLKREQ > 0 

 which means this is a REQUEST for a lock (thus, the session is waiting). 

 If we now match the KGLLKHDL with the handles of other sessions in 

 X$KGLLK that should give us the address of the blocking session since

 KGLLKREQ=0 for this session, meaning it HAS the lock.

 SELECT * FROM X$KGLLK LOCK_A 

 WHERE KGLLKREQ = 0

   AND EXISTS (SELECT LOCK_B.KGLLKHDL FROM X$KGLLK LOCK_B

               WHERE KGLLKSES = 'saddr_from_v$session' /* BLOCKED SESSION */

               AND LOCK_A.KGLLKHDL = LOCK_B.KGLLKHDL

               AND KGLLKREQ > 0);

 If we look a bit further we can then again match KGLLKSES with SADDR 

 in v$session to find further information on the blocking session:

 SELECT SID,USERNAME,TERMINAL,PROGRAM FROM V$SESSION

 WHERE SADDR in 

  (SELECT KGLLKSES FROM X$KGLLK LOCK_A 

   WHERE KGLLKREQ = 0

     AND EXISTS (SELECT LOCK_B.KGLLKHDL FROM X$KGLLK LOCK_B

                 WHERE KGLLKSES = 'saddr_from_v$session' /* BLOCKED SESSION */

                 AND LOCK_A.KGLLKHDL = LOCK_B.KGLLKHDL

                 AND KGLLKREQ > 0)

  );

 In the same way we can also find all the blocked sessions:

 SELECT SID,USERNAME,TERMINAL,PROGRAM FROM V$SESSION

 WHERE SADDR in 

  (SELECT KGLLKSES FROM X$KGLLK LOCK_A 

   WHERE KGLLKREQ > 0

     AND EXISTS (SELECT LOCK_B.KGLLKHDL FROM X$KGLLK LOCK_B

                 WHERE KGLLKSES = 'saddr_from_v$session' /* BLOCKING SESSION */

                 AND LOCK_A.KGLLKHDL = LOCK_B.KGLLKHDL

                 AND KGLLKREQ = 0)

  );