Shared Pool Waits. #.2 Copyright 2006 Kyle Hailey Shared Pool Waits Library Cache Latch Shared...

Shared Pool Waits

Copyright 2006 Kyle Hailey

#.2

Shared Pool Waits

Library Cache Latch Shared Pool Latch Library Cache Pin Library Cache Lock Library Cache Load Lock Row Cache Lock


#.3

Hash Table

handlehandle

handle handlehandle

handle handlehandle

handle

handlehandle

handle

handle

Shared Pool Structure


#.4

Shared Pool Latch

Protects Space Allocation in the Shared Pool Make sure two users don’t get same chunk of

memory


#.5

Bucket sizes 0 < 80 bytes1 < 1442 < 2723 < 5284 < 10405 < 20646 < 41127 < 82088 < 164009 < 3278410 bigger

Shared Pool Latch 8.1.6

Shared Pool pre 8.1.6Memory Chunk Buckets


#.6

Shared Pool Free Space

Shared Pool Latch covers changes in the lists of free memory chunks

Shared Pool Latch


#.7

•Get library cache latch

•Get shared pool latch

•Search right bucket

•Find best fit

=> If lists get long, search gets long

Shared Pool Latch


#.8

Shared Pool Free Space 8.1.6+

Shared Pool Latch

Shared Pool Latch


#.9

Shared Pool

Memory Chunk Buckets 8.1.6 and up0 16 bytes1 20 bytes … (0-198 only have one chunk size in bucket)198 808 bytes199 812 to 872…. (199-248 only have 16 possible chunk sizes per bucket)248 3948 - 4008249 4012 - 4104250 4108 - 8204251 8204 - 16392252 16396 - 32776253 32780 - 65544254 bigger

Shared Pool Latch


#.10

library cache pin and locks

Locks control access, protects handle Pins guarentees coherency, protects heaps To Access to a cursor

Lock handle Locking is the way of locating

Pin Pinning loads any necessary heaps Guarenteed to stay in memory until pin is released

handle handle handle

pin

pin

lock

lock

Heap 1

Child cursor 1Heap 6

Heap 0


#.11

library cache lock and pins

Contention when Sessions try to load/compile same SQL at same time

Locks and Pins are usually in share mode unless modifications are being made


#.12

library cache lock


pin

pin

lock

lock

#20 P1 = address of object P2 = address of lock P3 = mode | namespace See x$kgllk


#.13

library cache pin #13 P1 = address of object P2 = address of lock P3 = Mode | Namespace See x$kglpn


pin

pin

lock

lock


#.14

Solutions

Have only one Session compile a the same cursor at a time

Waits – find “competing” Sessions


#.15

SQL: Lib Cache Lock Find the waiters and who blocks them

column wevent format a20column bevent format a20select waiter.sid waiter, waiter.p1raw wlockp1, waiter.event wevent, blocker_event.sid blocker, blocker_event.event beventfrom x$kglpn p, gv$session blocker_session, gv$session_wait waiter, gv$session_wait blocker_eventwhere p.kglpnuse=blocker_session.saddr and p.kglpnhdl=waiter.p1raw and (waiter.event like 'library cache lock' ) and blocker_event.sid=blocker_session.sidorder by waiter.p1raw,waiter.sid/


#.16

library cache load lock

#23 Waiting For a Reload by another Session


#.17

Library Cache Latch

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


#.18

Hash Table

handle

pin

pin

lock

lock

Library Cache

handle

Find and LockFind and Lock

Pin (and Load)Pin (and Load)


#.19

Child cursor 3

Hash Table


Cursor (0)

pin

pin

lock

lock

Heap 1

Child cursor 1

Child cursor 2

Heap 6

pin

pin

lock

lock

Heap 0

pin

pin

lock

lock

waiters

holders

Handle

Cursor(0)

flags

Library Cache Structures

Library Cache Latch


#.20

Library Cache Latch Contention

Excesive Hard ParsingNot Sharing SQL – use of Literal ValuesShared Pool too smallToo many invalidations

Excessive Soft Parsing


#.21

Sharing SQL & Literals

SQL> @dups

PLAN_HASH_VALUE CNT--------------- ---------- 272002086 520

SQL_TEXT---------------------------------------------------SELECT * FROM dual WHERE dummy=-634891633SELECT * FROM dual WHERE dummy=1987751014SELECT * FROM dual WHERE dummy=25965276SELECT * FROM dual WHERE dummy=32449789SELECT * FROM dual WHERE dummy=-364632215SELECT * FROM dual WHERE dummy=-34273351SELECT * FROM dual WHERE dummy=-699712683SELECT * FROM dual WHERE dummy=1752437199SELECT * FROM dual WHERE dummy=-1081512404

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)

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)

select sql_text from v$sql where plan_hash_value = 272002086 and rownum < 10;

select sql_text from v$sql where plan_hash_value = 272002086 and rownum < 10;


#.22

Cursor Sharing

Bind VariablesSelect * from dual where dummy = :var;

Cursor_SharingCursor_sharing = Force

Oracle replaces variables with bind variables

Defaults to Exact


#.23

Shared Pool too Small

Reloads means Cursor heaps were kicked out implying shared_pool too small

SQL> select namespace, reloadsSQL> select namespace, reloads

from v$librarycache;from v$librarycache;

NAMESPACE RELOADS NAMESPACE RELOADS

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

SQL AREA 367SQL AREA 367

TABLE/PROCEDURE 592TABLE/PROCEDURE 592

SQL> select namespace, reloadsSQL> select namespace, reloads

from v$librarycache;from v$librarycache;

NAMESPACE RELOADS NAMESPACE RELOADS

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

SQL AREA 367SQL AREA 367

TABLE/PROCEDURE 592TABLE/PROCEDURE 592


#.24

Invalidations

Changes in dependent objects invalidate cursorFOR i IN 1..3000 LOOP

l_cursor:=dbms_sql.open_cursor;

dbms_sql.parse(l_cursor,

'SELECT * FROM toto',dbms_sql.native);

execute immediate 'analyze table toto compute statistics';

dbms_sql.close_cursor(l_cursor);

END LOOP;

SQL> select namespace,SQL> select namespace,

invalidations invalidations

from v$librarycache;from v$librarycache;NAMESPACE INVALIDATIONSNAMESPACE INVALIDATIONS--------------- ---------------------------- -------------SQL AREA 6065SQL AREA 6065

SQL> select namespace,SQL> select namespace,

invalidations invalidations

from v$librarycache;from v$librarycache;NAMESPACE INVALIDATIONSNAMESPACE INVALIDATIONS--------------- ---------------------------- -------------SQL AREA 6065SQL AREA 6065


#.25

Re-Executing a Cursor Re-Executing a Cursor

1.1. Libray Cache latch Libray Cache latch 2.2. LocksLocks3.3. PinsPins

pin pin pin pin pin

lock

Execute 1 Execute 2 Execute 3 Execute 4 Execute 5

Cursor MemoryCursor Memory

locklock lock lock

= Latch= Latch

Soft Parsing


#.26

Session Cached Cursors



lock

= Latch= Latch

pin pin pin pin pin

Session_cached_cursor:Session_cached_cursor:

If Opening/Closing keeps locked in MemoryIf Opening/Closing keeps locked in Memory


#.27

Session Cached Cursors

FOR i IN 1..30000 LOOPFOR i 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;

FOR i IN 1..30000 LOOPFOR i 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=0Session_cached_cursors=0

Latch Gets Latch Gets ----- ---- ----- ---- library cache lock 120,028 library cache lock 120,028 library cache 180,074 library cache 180,074 library cache pin 60,048library cache pin 60,048

Session_cached_cursors=20Session_cached_cursors=20

library cache lock 4 library cache lock 4 library cache 60,061 library cache 60,061 library cache pin 60,048library cache pin 60,048


#.28

Cursor Space for Time



Cursor_space_for_time=true :Cursor_space_for_time=true :

if open and re-executing – keeps cursor pinnedif open and re-executing – keeps cursor pinned

(Cursor already locked because cursor is kept open)(Cursor already locked because cursor is kept open)

= Latch= Latch

pin

lock

OpenOpenCursorCursor

CloseCloseCursorCursor


#.29

Cursor Space For Time

FOR i IN 1..30000 LOOPFOR i IN 1..30000 LOOP rc:=dbms_sql.execute(l_cursor);rc:=dbms_sql.execute(l_cursor); IF DBMS_SQL.FETCH_ROWS (l_cursor) < 0 THENIF 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;

FOR i IN 1..30000 LOOPFOR i IN 1..30000 LOOP rc:=dbms_sql.execute(l_cursor);rc:=dbms_sql.execute(l_cursor); IF DBMS_SQL.FETCH_ROWS (l_cursor) < 0 THENIF 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;

Cursor_space_for_time=falseCursor_space_for_time=false

Latch Gets Latch Gets ----- ---- ----- ---- library cache lock 35library cache lock 35library cache 60,096library cache 60,096library cache pin 60,044library cache pin 60,044

Cursor_space_for_time=trueCursor_space_for_time=true

library cache lock 30library cache lock 30library cache 85library cache 85library cache pin 42library cache pin 42


#.30

Efficient Lock and Pinning

Reduce use of latches Improve throughput Improve Concurrency ***


#.31

row cache lock : args

P1 = cache# P2 = Lock Mode Held P3 = Lock Mode Requested

select parameter as “name”

from v$rowcache

where cache# = P1;

select parameter as “name”

from v$rowcache

where cache# = P1;


#.32

Summary

Shared Pool LatchShard pool too small or too much hard pasing

Loading Same CursorLibrary Cache PinLibrary Cache LockLibrary Cache Load Lock

Library Cache LatchToo much hard or soft parsing

Row Cache Lock


#.33

Library Cache Latch Solutions Share Cursors

Use bind variablesUser cursor_sharing=force

Avoid invalidations and reloadsSize shared_pool large enoughAvoid changing dependent objects

Soft ParsingSession_cached_cursors =20 : keep across open/closeCursor_space_for_time=true : keep pinned across executeshold_cursor=true : used in precompilers

Shared Pool Waits. #.2 Copyright 2006 Kyle Hailey Shared Pool Waits Library Cache Latch Shared...

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