The document discusses various waits that can occur in the shared pool of an Oracle database. It describes latches like the shared pool latch and library cache latch that protect structures in the shared pool. It also introduces mutexes as an alternative locking mechanism in Oracle 10g. Specific waits are explained like the shared pool latch wait and library cache latch contention. Solutions to reduce waits involve optimizing SQL sharing, bind variables, invalidations and tuning the shared pool size.

1. Shared Pool Waits
Kyle Hailey
http://perfvision.com/ftp/emea2010
Kyle.hailey@embarcadero.com

2. Shared Pool Waits
1. Latches
 Latch: Library Cache
 Latch: Shared Pool Latch
1. Mutexes
2. Library Cache locks and pins
3. Row Cache Lock
Copyright 2006 Kyle Hailey

3. Library Cache
Lib
Cache
Copyright 2006 Kyle Hailey

4. Shared Pool Structure
Hash
Table
SQL statements are hashed
handle
On their text. The resulting
handle handle Hash is used to find the appropriate
bucket, which is searched for the
Compiled SQL. If it’s not there,
then we parse it.
handle handle
handle
handle handle handle
handle handle handle
handle
Copyright 2006 Kyle Hailey

5. Shared Pool Latch
 Cause: Hard Parses by Concurrent Sessions
 Contention can arise when too many sessions are hard
parsing and looking for space in the shared pool.
 Protects Space Allocation in the Shared Pool
 Protects the structure containing memory chunks
 Ensures two users don’t get same chunk of
memory
Copyright 2006 Kyle Hailey

6. Shared Pool Latch
Shared Pool Latch covers changes in the lists
of free memory chunks
Shared Pool Free Space
Copyright 2006 Kyle Hailey

7. Shared Pool Latch 8.1.6
Bucket sizes
0 < 80 bytes
1 < 144
2 < 272
3 < 528 Shared Pool pre 8.1.6
4 < 1040 Memory Chunk Buckets
5 < 2064
6 < 4112
7 < 8208
8 < 16400
9 < 32784
10 bigger
Copyright 2006 Kyle Hailey

8. Shared Pool Latch
Shared Pool Latch
Shared Pool Free Space 8.1.6+
Before 8.1.6, oversizing the shared pool could be
a problem, after 8.1.6 should be fine
Copyright 2006 Kyle Hailey

9. Shared Pool Latch
Shared Pool
Memory Chunk Buckets 8.1.6 and up
0 16 bytes
1 20 bytes
… (0-198 only have one chunk size in bucket)
198 808 bytes
199 812 to 872
…. (199-248 only have 16 possible chunk sizes per bucket)
248 3948 - 4008
249 4012 - 4104
250 4108 - 8204
251 8204 - 16392
252 16396 - 32776
253 32780 - 65544
254 bigger
Copyright 2006 Kyle Hailey

10. Library Cache Latches
 Protects changes in Library Cache
 Library Locks are not atomic
 Thus need library cache latch
 Broken out into
 library cache pin allocation
 library cache lock allocation
 library cache lock
 library cache
 library cache pin
 library cache load lock
Copyright 2006 Kyle Hailey

11. Library Cache
Hash Table
pin lock
pin lock
handle handle
Find and Lock
Pin (and Load)
Copyright 2006 Kyle Hailey

12. Library Cache Structures
Hash Table waiters
pin lock
pin lock
Library Cache Latches
pin lock
holders
pin lock
Handle
handle handle handle
Cursor(0)
flags
Cursor (0)
Heap 1
pin lock
Heap 0
Heap 6
pin lock
Child cursor 1
Child cursor
2
Child cursor 3
Copyright 2006 Kyle Hailey

13. Library Cache Latch Contention
Library Cache Latch Contention because of these
possibilities:
 Excessive Hard Parsing
 Not Sharing SQL – use of Literal Values
 Shared Pool too small
 Too many invalidations
 Excessive Soft Parsing
Copyright 2006 Kyle Hailey

14. Sharing SQL & Literals
select
select
plan_hash_value,
plan_hash_value,
count(plan_hash_value)
count(plan_hash_value)
from
from
v$sql
v$sql
group by plan_hash_value
group by plan_hash_value
order by count(plan_hash_value)
order by count(plan_hash_value)
SQL> @dups
SQL> @dups
PLAN_HASH_VALUE
PLAN_HASH_VALUE CNT
CNT
--------------- ----------
--------------- ----------
272002086
272002086 520
520
Copyright 2006 Kyle Hailey

15. Sharing SQL & Literals
SQL> @dups
select sql_text
select sql_text SQL> @dups
from v$sql
from v$sql PLAN_HASH_VALUE CNT
PLAN_HASH_VALUE CNT
where
where
--------------- ----
--------------- ----
272002086 520
plan_hash_value = 272002086
plan_hash_value = 272002086 272002086 520
and rownum < 10;
and rownum < 10;
SQL_TEXT
SQL_TEXT
-----------------------------------------------
-----------------------------------------------
SELECT * FROM dual WHERE dummy=-634891633
SELECT * FROM dual WHERE dummy=-634891633
SELECT * FROM dual WHERE dummy=1987751014
SELECT * FROM dual WHERE dummy=1987751014
SELECT * FROM dual WHERE dummy=25965276
SELECT * FROM dual WHERE dummy=25965276
SELECT * FROM dual WHERE dummy=32449789
SELECT * FROM dual WHERE dummy=32449789
SELECT * FROM dual WHERE dummy=-364632215
SELECT * FROM dual WHERE dummy=-364632215
SELECT * FROM dual WHERE dummy=-34273351
SELECT * FROM dual WHERE dummy=-34273351
SELECT * FROM dual WHERE dummy=-699712683
SELECT * FROM dual WHERE dummy=-699712683
SELECT * FROM dual WHERE dummy=1752437199
SELECT * FROM dual WHERE dummy=1752437199
SELECT * FROM dual WHEREKyledummy=-1081512404
SELECT * FROM dual WHERE dummy=-1081512404
Copyright 2006 Hailey

16. Cursor Sharing
 Bind Variables
 Select * from dual where dummy = :var;
 Select sum(money) from orders where
odate > :odate;
 Cursor_Sharing
 Cursor_sharing = Force
Oracle replaces variables with bind variables
 Defaults to Exact
Copyright 2006 Kyle Hailey

17. Shared Pool too Small
SQL> select namespace, reloads
SQL> select namespace, reloads
from v$librarycache;
from v$librarycache;
NAMESPACE
NAMESPACE RELOADS
RELOADS
--------------- ----------
--------------- ----------
SQL AREA
SQL AREA 367
367
TABLE/PROCEDURE
TABLE/PROCEDURE 592
592
 Reloads means Cursor heaps were kicked
out implying shared_pool too small
Copyright 2006 Kyle Hailey

18. Invalidations
SQL> select namespace,
SQL> select namespace,
invalidations
invalidations
from v$librarycache;
from v$librarycache;
NAMESPACE
NAMESPACE INVALIDATIONS
INVALIDATIONS
--------------- -------------
--------------- -------------
SQL AREA
SQL AREA 6065
6065
 Changes in dependent objects invalidate
cursor
FOR i ININ 1..3000 LOOP
FOR i 1..3000 LOOP
l_cursor:=dbms_sql.open_cursor;
l_cursor:=dbms_sql.open_cursor;
dbms_sql.parse(l_cursor,
dbms_sql.parse(l_cursor,
'SELECT * FROM toto',dbms_sql.native);
'SELECT * FROM toto',dbms_sql.native);
execute immediate 'analyze table toto compute statistics';
execute immediate 'analyze table toto compute statistics';
dbms_sql.close_cursor(l_cursor);
dbms_sql.close_cursor(l_cursor);
END LOOP;
END LOOP;
Copyright 2006 Kyle Hailey

19. Soft Parsing
Cursor Memory
= Latch
lock lock lock lock lock
pin pin pin pin pin
Execute 1 Execute 2 Execute 3 Execute 4 Execute 5
Re-Executing a Cursor
1. Libray Cache latch
2. Locks
3. Pins
Copyright 2006 Kyle Hailey

20. Session Cached Cursors
= Latch Cursor Memory
lock
pin pin pin pin pin
Execute 1 Execute 2 Execute 3 Execute 4 Execute 5
Session_cached_cursor:
If Opening/Closing keeps locked in Memory
Copyright 2006 Kyle Hailey

21. Session Cached Cursors
FOR i iIN 1..30000 LOOP
FOR IN 1..30000 LOOP
l_cursor:=dbms_sql.open_cursor;
l_cursor:=dbms_sql.open_cursor;
dbms_sql.parse(l_cursor,'SELECT **FROM dual’,dbms_sql.native);
dbms_sql.parse(l_cursor,'SELECT FROM dual’,dbms_sql.native);
dbms_sql.close_cursor(l_cursor);
dbms_sql.close_cursor(l_cursor);
END LOOP;
END LOOP;
Session_cached_cursors=0
Latch Gets
----- ----
library cache lock 120,028
library cache 180,074
library cache pin 60,048
Session_cached_cursors=20
library cache lock 4
library cache 60,061
library cache pin 2006 Kyle Hailey
Copyright
60,048

22. Cursor Space for Time
= Latch Cursor Memory
Open
lock
Close
Cursor pin
Cursor
Execute 1 Execute 2 Execute 3 Execute 4 Execute 5
Cursor_space_for_time=true :
if open and re-executing – keeps cursor pinned
(Cursor already locked because cursor is kept open)
Copyright 2006 Kyle Hailey

23. Cursor Space For Time
FOR i iIN 1..30000 LOOP
FOR IN 1..30000 LOOP
rc:=dbms_sql.execute(l_cursor);
rc:=dbms_sql.execute(l_cursor);
IF DBMS_SQL.FETCH_ROWS (l_cursor) < 0 THEN
IF DBMS_SQL.FETCH_ROWS (l_cursor) < 0 THEN
DBMS_SQL.COLUMN_VALUE (l_cursor, 1, cnt);
DBMS_SQL.COLUMN_VALUE (l_cursor, 1, cnt);
end if;
end if;
End loop;
End loop;
Cursor_space_for_time=false
Latch Gets
----- ----
library cache lock 35
library cache 60,096
library cache pin 60,044
Cursor_space_for_time=true
library cache lock 30
library cache 85
library cache pin 42
Copyright 2006 Kyle Hailey

24. Cursor Sharing
pin lock
pin lock
Handle
handle
handle handle handle
Cursor(0)
flags
Cursor (0)
select * from (
select sql_id, count(*) cnt
Child cursor 0
from V$SQL_SHARED_CURSOR
group by sql_id )
where cnt > 5
Child cursor 1 order by cnt;
Child cursor 2
Copyright 2006 Kyle Hailey

25. V$SQL_SHARED_CURSOR
 10gR2, 53 reasons why cursors aren’t shared
 If using “cursor_sharing=similar” might not
work – bugs
 Examples
 OPTIMIZER_MODE_MISMATCH , see V$SQL_OPTIMIZER_ENV
 STATS_ROW_MISMATCH, could be sql trace
 AUTH_CHECK_MISMATCH
TRANSLATION_MISMATCH – different object in SQL stmt
 BIND_MISMATCH – bind variable different sizes
 LANGUAGE_MISMATCH – NLS Language
http://www.juliandyke.com/Presentations/Presentations.html#LibraryCacheInternals
Copyright 2006 Kyle Hailey

26. V$SQL_SHARED_CURSOR
UNBOUND_CURSOR USER_BIND_PEEK_MISMATCH
SQL_TYPE_MISMATCH
OPTIMIZER_MISMATCH TYPCHK_DEP_MISMATCH
OUTLINE_MISMATCH NO_TRIGGER_MISMATCH
STATS_ROW_MISMATCH FLASHBACK_CURSOR
LITERAL_MISMATCH ANYDATA_TRANSFORMATION
SEC_DEPTH_MISMATCH INCOMPLETE_CURSOR
EXPLAIN_PLAN_CURSOR TOP_LEVEL_RPI_CURSOR
BUFFERED_DML_MISMATCH DIFFERENT_LONG_LENGTH
PDML_ENV_MISMATCH LOGICAL_STANDBY_APPLY
INST_DRTLD_MISMATCH DIFF_CALL_DURN
SLAVE_QC_MISMATCH BIND_UACS_DIFF
TYPECHECK_MISMATCH PLSQL_CMP_SWITCHS_DIFF
AUTH_CHECK_MISMATCH CURSOR_PARTS_MISMATCH
BIND_MISMATCH STB_OBJECT_MISMATCH
DESCRIBE_MISMATCH ROW_SHIP_MISMATCH
LANGUAGE_MISMATCH PQ_SLAVE_MISMATCH
TRANSLATION_MISMATCH TOP_LEVEL_DDL_MISMATCH
ROW_LEVEL_SEC_MISMATCH MULTI_PX_MISMATCH
INSUFF_PRIVS BIND_PEEKED_PQ_MISMATCH
INSUFF_PRIVS_REM MV_REWRITE_MISMATCH
REMOTE_TRANS_MISMATCH ROLL_INVALID_MISMATCH
LOGMINER_SESSION_MISMATCH OPTIMIZER_MODE_MISMATCH
INCOMP_LTRL_MISMATCH PX_MISMATCH
OVERLAP_TIME_MISMATCH MV_STALEOBJ_MISMATCH
SQL_REDIRECT_MISMATCH FLASHBACK_TABLE_MISMATCH
MV_QUERY_GEN_MISMATCH LITREP_COMP_MISMATCH
Copyright 2006 Kyle Hailey

27. 10g : Mutex
A new semaphore method which can replace latches
 Mutex
 Mutual exclusion object
 Similar to a latch, prevents
 Deallocation while someone is using it
 Read/write while someone else is modifying
 Different from latch
 Every object can have it’s own mutex
 A mutex can cover multiple objects
 Usually dynamically allocated along with structure they
protect
 Can be stored in the structure, thus destroying structure
deletes the mutex
Copyright 2006 Kyle Hailey

28. Mutex Views and Stats
 Views
 V$mutex_sleep
 Cursor:pin S  V$mutex_sleep_history
 Pin cursor for execute, and cursor is currently being
examined by another Session Instead of “library cache pin”
 Bug 6968152 cursor_space_for_time
 Cursor:pin S wait on X not needed
 Pinning a cursor for execute
 Bug on 10.2.0.3 typically with DBMS_STATS
 Metalink Note:401435.1, Note:5907779.8, bug 5907779
Instead of latching for execute pin we use a shared mutex
If can’t get the mutex spin Fixed 10.2.0.4 ?
Turn off with
_kks_use_mutex_pin=FALSE"
Copyright 2006 Kyle Hailey

29. Debugging Pin Waits
NAME P1 P2 P3
------------------------- ----- ------- ---------------
cursor: mutex X idn value where|sleeps
cursor: mutex S idn value where|sleeps
cursor: pin S wait on X idn value where|sleeps
cursor: pin X idn value where|sleeps
cursor: pin S idn value where|sleeps
select p1, p2raw, count(*)
from v$session
where event = ‘cursor: pin S wait on X’
and wait_time = 0
Group by p1, p2; Images from Marcin Przepiorowski
P1 P2RAW COUNT(*)
---------- ---------------- --------
2700259466 0000139700000000 9
<Mutex Id> < SId><RefCnt>
select to_number(1397,'XXXX') from dual;
TO_NUMBER(1397,'XXXX')
----------------------
5015
Select serial# from v$session where sid=5015;
SERIAL#
--------
82
Alter system kill session ‘5015,82’;
Copyright 2006 Kyle Hailey

30. lock and pins
1. Library Cache Pin
2. Library Cache Lock
3. Library Cache Load Lock
 Contention when Sessions try to
 load/compile same SQL
 Compile package others are running
 Locks and Pins are usually in share
mode unless modifications are being
made
Copyright 2006 Kyle Hailey

31. Lib Cache Lock : blockers and waiters
select
select
waiter.sid waiter,
waiter.sid waiter,
waiter.event wevent,
waiter.event wevent,
to_char(blocker_event.sid)||','||to_char(blocker_session.serial#) blocker,
to_char(blocker_event.sid)||','||to_char(blocker_session.serial#) blocker,
substr(decode(blocker_event.wait_time,
substr(decode(blocker_event.wait_time,
0, blocker_event.event,
0, blocker_event.event,
'ON CPU'),1,30) bevent
'ON CPU'),1,30) bevent
from
from WAITER WLOCKP1 WEVENT BLOCKER BEVENT
x$kglpn p, WAITER WLOCKP1 WEVENT BLOCKER BEVENT
x$kglpn p, ------- ---------------- ----------------- --------- -----------------
------- ---------------- ----------------- --------- -----------------
gv$session
gv$session blocker_session,
blocker_session, 129 00000003B76AB620 library cache pin 135,15534 PL/SQL lock
gv$session_wait waiter, timer 00000003B76AB620 library cache pin 135,15534 PL/SQL lock
129
gv$session_wait waiter, timer
gv$session_wait blocker_event
gv$session_wait blocker_event
where
where
p.kglpnuse=blocker_session.saddr
p.kglpnuse=blocker_session.saddr
and p.kglpnhdl=waiter.p1raw
and p.kglpnhdl=waiter.p1raw
and waiter.event in ( ('library cache pin' , ,
and waiter.event in 'library cache pin'
'library cache lock' , ,
'library cache lock'
'library cache load lock')
'library cache load lock')
and blocker_event.sid=blocker_session.sid
and blocker_event.sid=blocker_session.sid
and waiter.sid != blocker_event.sid
and waiter.sid != blocker_event.sid
order by
order by
waiter.p1raw,waiter.sid;
waiter.p1raw,waiter.sid;
Copyright 2006 Kyle Hailey

32. Solutions
1. Library Cache Pin
2. Library Cache Lock
3. Library Cache Load Lock
 Have only one Session compile the same cursor
at a time
 Avoid compiling while executing
 Waits – find “competing” Sessions
Copyright 2006 Kyle Hailey

33. row cache lock : args
 P1 = cache#
 P2 = Lock Mode Held
 P3 = Lock Mode Requested
select parameter as “name”
select parameter as “name”
from v$rowcache
from v$rowcache
where cache# = P1;
where cache# = P1;
Copyright 2006 Kyle Hailey

34. Row Cache Lock - ASH
select
ash.session_id sid,
ash.blocking_session bsid,
nvl(o.object_name,to_char(CURRENT_OBJ#)) obj,
o.object_type otype,
CURRENT_FILE# filen,
CURRENT_BLOCK# blockn,
ash.SQL_ID,
nvl(rc.name,to_char(ash.p3)) row_cache
from v$active_session_history ash,
( select cache#, parameter name from v$rowcache ) rc,
all_objects o
where event='row cache lock'
and rc.cache#(+)=ash.p1
and o.object_id (+)= ash.CURRENT_OBJ#
and ash.session_state='WAITING'
and ash.sample_time > sysdate - &minutes/(60*24)
Order by sample_time

35. Row Cache Lock - ASH
select
ash.session_id sid,
ash.blocking_session bsid,
nvl(o.object_name,to_char(CURRENT_OBJ#)) obj,
o.object_type otype,
CURRENT_FILE# filen,
CURRENT_BLOCK# blockn,
ash.SQL_ID,
nvl(rc.name,to_char(ash.p3)) row_cache
from v$active_session_history ash,
( select cache#, parameter name from v$rowcache ) rc,
all_objects o BSID OBJ
SID OTYPE FILEN BLOCKN SQL_ID ROW_CACHE
where event='row cache lock'
143 131 -1 0 0 41y8w0sfqb61m dc_sequences
and rc.cache#(+)=ash.p1
134 131 -1 0 0 dc_sequences
and o.object_id (+)= ash.CURRENT_OBJ#
151 -1 0 0 dc_sequences
and ash.session_state='WAITING'
134 151 -1 0 0 dc_sequences
and ash.sample_time > sysdate - &minutes/(60*24)
131 151 -1 0 0 dc_sequences
Order by sample_time
151 -1 0 0 dc_sequences

36. Row Cache Lock
 Select seq.next_val
 Sequence cache set to 1
 Default sequence cache is 20
SQL> @sqltext
Enter value for 1: 41y8w0sfqb61m
SQL_FULLTEXT
SELECT TOTO_SEQ.NEXTVAL FROM DUAL
Copyright 2006 Kyle Hailey

37. Library Cache Latch Solutions
 Share Cursors
 Use bind variables
 User cursor_sharing=force
 Avoid invalidations and reloads
 Size shared_pool large enough
 Avoid changing dependent objects
 Soft Parsing
 Session_cached_cursors =20 : keep across open/close
 Cursor_space_for_time=true : keep pinned across executes
 hold_cursor=true : used in precompilers
Copyright 2006 Kyle Hailey

38. Summary
 Shared Pool Latch
 Shared pool too small or too much hard parsing
 Loading Same Cursor
 Library Cache Pin
 Library Cache Lock
 Library Cache Load Lock
 Row Cache Lock
 Depends on the cache
Copyright 2006 Kyle Hailey