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8/14/2019 ASDASVRRSDASD

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Enqueue Waits : Locks


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#.2

Copyright 2006 Kyle Hailey

Locks

REDO Lib

Cache

Buffer

Cache

IO

Locks

Network


3/64

#.3


Enqueues = Locks

Waits

Disk I/O

Library Cache

Enqueue

Undo

TX - row lock contention

TX - allocate ITL entry

HW

Redo

Buffer Cache

SQL*Net

TM

STTS

TX - index contention

US

CI

SQ

Locks

Come in

Many

Types


4/64

#.4


Locks Covered in this Section

Part I

TMtable modification

TXTransaction locks

ULuser lock

Part II

CICross Instance

CUCursor Bind

HWHigh Water ROReusable Object

STSpace Transaction

TSTemporary Space


5/64

#.5


Locks Can be Intractable

Lock seems easy compared to latches orother esoteric waits, but they can befrustratingly intractable

Statspack & AWR lacks enough data to solve

To Solve, you need eitherReal time

ASH Insolvable: No dependable way to find the

blocking SQL


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#.6


Solving Enqueues

Need SQL and Object/row

Statspack fails

V$active_session_history succeeds

In real time can also use

v$lockv$session

dba_blockers

dba_waiters

?/rdbms/admin/utllockt.sql

http://www.evdbt.com/enqwaits.sql


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#.7


Statspack 9iTop 5 Timed Events Avg %Total

~~~~~~~~~~~~~~~~~~ wait Call

Event Waits Time (s) (ms) Time

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

Enqueue 42 126 3000 96.5CPU time 4 2.8

db file sequential read 165 1 4 .4

control file sequential read 214 0 1 .1

log file switch completion 2 0 40 .1

Statspack is INSUFFICIENT, missing

Who is blocked

Who Blocks

What blocks

How they are blocked


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#.8

9i Investigation

Select * from v$event_name wherename=enqueue;


Parameter1 parameter2 parameter3---------- ---------- ----------

Name|mode id1 id2


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#.9


9i Enqueue P1: Type and Mode

Type: 5458

P1 P1RAW

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

1415053318 54580006

Hex Decimal ASCII

54 = 84 = T

58 = 88 = X

Lock = TX 6

Mode: 0006


10/64

#.10

9i Translating Lock and Mode

SELECT

chr(bitand(p1,-16777216)/16777215)||

chr(bitand(p1, 16711680)/65535) Type",mod(p1,16) as Mode"

from v$session_wait

Where event=enqueue/

SELECTchr(bitand(p1,-16777216)/16777215)||

chr(bitand(p1, 16711680)/65535) Type,

mod(p1,16) lmode

from v$session_waitwhere event=enqueue;

TY LMODE-- ----------

TX 6


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#.11


10g Translates all the Enquques

9iOne Wait : enqueue

10g

208 enqueue waits

Specific to each type of enqueueenq: HW - contention Configurationenq: SQ - contention Configurationenq: SS - contention Configurationenq: ST - contention Configuration

enq: TM - contention Applicationenq: TW - contention Administrativeenq: TX - allocate ITL entry Configurationenq: TX - index contention Concurrencyenq: TX - row lock contention Applicationenq: TX contention Application


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#.13


Common Lock Types

Wait Event Parameter2 Parameter3enq: CI - contention opcode typeenq: CF - contention 0 operationenq: CU - contention handle handleenq: HW - contention table space # blockenq: IM - contention for blr pool # 0

enq: RO - contention 2 0enq: RO - fast object reuse 2 0enq: SQ - contention object # 0enq: ST - contention 0 0enq: TM - contention object # table/partitionenq: TS - contention tablespace ID dbaenq: TX - allocate ITL entry usn


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#.14


Statspack 10g5 Timed Events Avg %Total

~~~~~~~~~~~~~~~~~~ wait CallEvent Waits Time (s) (ms) Time----------------------------- ------ -------- ------ ------

enq: TX - row lock contention 42 126 3000 96.5

CPU time 4 2.8

db file sequential read 165 1 4 .4

control file sequential read 214 0 1 .1

log file switch completion 2 0 40 .1

10g Better, tells what kind of lock

But still notwho is blocked,

who blocks them

what they are blocked on


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#.15


V$active_session_history

Succeeds Who is waiting

SESSION_ID

SESSION_SERIAL#

USER_ID On what object

CURRENT_OBJ#

CURRENT_FILE#

CURRENT_BLOCK#

With what SQL SQL_ID

Who is the blocker BLOCKING_SESSION

BLOCKING_SESSION_STATUS

BLOCKING_SESSION_SERIAL#

What is the Blocker SQL not reliably possible

Maybe by dumping REDO

OEM externalize all of this


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#.16


OEM 10g

if P1 = 1415053318then mode = 6

Then it is a data block

row lock


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#.17


10g: find the lock modeP1 (parameter1) same for all locks

select distinct parameter1 from v$event_name

where name like 'enq:%'

PARAMETER1----------

name|mode

select distinct event, p1, to_char(p1,'XXXXXXXX') p1raw

from v$active_session_history where event like 'enq:%;

EVENT P1 P1RAW------------------------------ ---------- ---------enq: TX - row lock contention 1415053316 54580004

enq: TX - row lock contention 1415053318 54580006

enq: TX - allocate ITL entry 1415053316 54580004


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#.18


P1 = name | modeP1 (parameter1) same for all locks

select distinct parameter1 from v$event_namewhere name like 'enq:%'

select

distinct event,p1,mod(p1,16) as "mode"

from v$active_session_historywhere event like 'enq:%;

EVENT P1 mode----------------------------- ---------- ----enq: TX - allocate ITL entry 1415053316 4enq: TX - row lock contention 1415053318 6enq: TX - row lock contention 1415053316 4

PARAMETER1----------name|mode


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#.19


Mode Cheat Sheet

TXif P1 = 1415053316

then mode = 4

if P1 = 1415053318then mode = 6


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#.20


Lock Modes

# Type Name

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

1 Null Null

2 SS Sub share

3 SX Sub exclusive

4 S Share

5 SSX Share/sub exclusive

6 X Exclusive


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#.21


Transaction Locks (TX)

TX = Transaction = Wait on UNDOMode 6

modification of a row lock

Mode 4 Index block spiltUnique Index Key enforcementForeign key enforcement ITL space waits

Bitmap chunk conflictsAlter tablespace read only;Free Lists slot waitsPossible with two phase commit


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#.23


TX Lock

update toto set name =SMITH where id = ;

Undo

Segment

Session a

Data BlockHeader

Transaction 1

Data Block

Header

Row 1

Table Toto

Undo

SegmentHeader

Session B

update toto set name =ADAMS where id = ;

Wait for TxTo commit

Delete from toto where id = ;Delete from toto where id = 9;


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#.24


enq: TX - row lock contention

User 1

SQL> delete from toto

where id = 1;

User 2

SQL> delete from toto

where id =1;--hang

Mode 6, row in data blockonly one reason

SQL> Delete from titi

Where id = 1;


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#.25



if P1 = 1415053318

then mode = 6

Then it is a data blockrow lock


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#.26



Table

ValueID

fooSession 1: delete where id = 1;no commit

Session 2: delete where id = 1 ;

Enqueue TX 6

1

TX Mode 6


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#.27


TXMode 4

if P1 = 1415053316

then mode = 4

Not same data but conflicts


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#.28


Mode 4, happens for 3 reasons

1. Unique key contention

2. Foreign Key contention3. Bitmap index contention

(others?)



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#.29


1. enq: TX - row lock contention

User 1

create table p(n number);

create unique index p_i

on p(n);

insert into p values(3);

User 2

insert into p values(3);

--hang

Mode 4 , unique index


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# 31


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#.31



User 8

create table parent (id number primary key);

create table child (id number references parent,name varchar2(20));

insert into parent values (2);

User 10

insert into child values (2,88);

-- hang

Mode 4, foreign key

# 32


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#.32



Parent Child

ID ID NameValueID

PK

Session 1: Insert key into parentno commit

2 ? 2 ? 2

Foreign Key

Session 2: Insert into child same key :Enqueue TX 4

TX 4 - Foreign Key

# 33


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#.33



Mode 4

Bitmaps are compressed

Changes to the same bitmap cause locks

Value StartRowid

EndRowid

Bitmap

1000.000.0000 000.000.000

01010000111000011100001100

2000.000.0000 000.000.000

01010000111000011100001100

# 34


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#.34



update t1 set n1 = 2

where n2 = 13;

Session 2

Different rows butsame key value

create table t1 (n1 number(4),n2 number(4));

insert into t1select 1, rownum

from all_objectswhere rownum


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#.35



Bitmaps are compressed

Changes to the same bitmap chunk causelocks

Value StartRowid

EndRowid

Bitmap

1200.0 204.7

01010000111000011100001100

1205.0 210.3

01010000111000011100001100

2200.0 205.6

01010000111000011100001100

block row

# 36


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#.36


Value StartRowid

EndRowid

Bitmap

1200.0 204.7

01010000111000011100001100

2205.0 210.3

01010000111000011100001100

3200.0 205.6

01010000111000011100001100


Session 1 Session 2Update id=800set value 2

Update id=3

set value 2

# 37


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#.37

Summary: TX 4 from ASHuniq index

ST EVENT SID LM P2 P3 OBJ OTYPE FN BLOCKN SQL_ID BSID----- ---------------------- --- --- ------ ---- ----- ----- --- ------ --

10:39 enq: TX - row lock c 141 4 655406 6672 -1 0 0 bjvx94vnxtxgv 158




FK (10.2.0.3)

ST EVENT SID LM P2 P3 OBJ OTYPE FN BLOCKN SQL_ID BSID----- ---------------------- --- --- ------ ---- ----- ----- --- ------ --

10:41 enq: TX - row lock c 144 4 179681 7074 CHILD TABLE 1 60954 ahm7c9rupbz9r 1



bitmap

ST EVENT SID LM P2 P3 OBJ OTYPE FN BLOCKN SQL_ID BSID

----- ---------------------- --- --- ------ ---- ----- ----- --- ------ --10:41 enq: TX - row lock c 143 4 966081 4598 I1 INDEX 0 0 azav296xxqcjx 144

10:41 enq: TX - row lock c 143 4 966081 4598 I1 INDEX 0 0 azav296xxqcjx 144




# 38


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#.38


enq: TX - allocate ITL entry

Data Block

Header

ITL

Data

Data Block

Header

Transaction 1 Info

Transaction 2 Info

# 39


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#.39



Data Block

Header

Transaction 1

Data

Data Block

Header

Transaction 2

Row 1

Row 2

Row 3

Transaction 3

# 40


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#.40



create table itl (id number,

data varchar2(20)

)

pctfree 0initrans 1;

insert into itl select rownum,'a' from all_objects

where rownum < 2000;

commit;

session 1: update itl set data=data where id=1;session 2: update itl set data=data where id=2;

session 3: update itl set data=data where id=3;



# 41


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#.41


enq: TX - contention

Example

Session 1start transaction, dont commit

Session 2alter tablespace read only

ExampleData File Extensionwaiter waitng for another session to extend file

Index Block Splitwaiter waiting for another session to split the block

Others ?

Free Lists Non-ASSM

Sessions freeing block

If no txs free lists available (min 16, grow up depending on block size) , pick one and wait TX4

2PCtwo phase commit First does a prepare the commit

Any read or write access in the intervening time waits TX 4

# 42


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#.42


TX Further Investigation

EVENT SQL_ID OBJ FILE# BLOCK#------------------------------ ------------- --------- --- ------enq: TX - row lock contention ahm7c9rupbz9r 53363 FOO 1 123586

enq: TX - row lock contention bjvx94vnxtxgv 53363 FOO 1 123586

enq: TX - row lock contention ahm7c9rupbz9r 53363 FOO 1 123586

enq: TX - row lock contention bjvx94vnxtxgv 53363 FOO 1 123586

enq: TX - row lock contention ahm7c9rupbz9r 53363 FOO 1 123586

select event, sql_id,CURRENT_OBJ# || ' ' || name obj

,CURRENT_FILE# file#,CURRENT_BLOCK# block#

from v$active_session_history ash,obj$ o

whereevent like 'enq: TX%'

and o.obj# (+)= ash.current_obj#order by sample_time

# 43


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#.43


TX Further Investigation

V$active_session_history :

BLOCKING_SESSIONBLOCKING_SESSION_STATUSBLOCKING_SESSION_SERIAL#

Who is the blocker:

No Guarentee of finding blocker SQL

# 44


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#.44


enq: TM - contention

TX locks have a corresponding TM lock

TM locks the structure from change

LOCK Parmeter1 Parmeter2(ID1) Parameter3(ID2)

------- --------- ------------- ---------------enq: TM name|mode object # table/partition

Parameter1 = object id

# 45


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#.45


enq: TM - contention

User 1

create table parent (id number primary key);

create table child (id number references parent,name varchar2(20));

insert into parent values (1);

insert into parent values (2);commit;

delete from parent where id=2;

User 2

insert into child values (1,2);

Exclusive Row Level Lock

#.46


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#.46


enq: TM - contentionParent

ChildID ID NameValueID

PK

Delete from Parent where ID=2 :

1 1 1

Insert into Child ID=1

Enqueue TM 4

Session 2 doesnt know the value Session 1 inserted

Session 2 only knows there is an outstanding change

X X

Session 1

Session 2

2 2

#.47


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#.47


enq: TMcontentionSolution

Parent Child

ID ID NameValueID

PK

Session 1: Insert into Child ID=1

Session 2: Delete from Parent ID=2

OKcan verify quickly in the child index

ID

Index

Foreign Key

121

#.48

TM F th I ti ti


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# 8


TM Further Investigation

EVENT SQL_ID mode OBJ

------------------------------ ------------- ---- ---------------enq: TM - contention 8zw36yw3fq4yy 4 53372 CHILDenq: TM - contention 8zw36yw3fq4yy 4 53372 CHILDenq: TM - contention 8zw36yw3fq4yy 4 53372 CHILDenq: TM - contention 8zw36yw3fq4yy 4 53372 CHILDenq: TM - contention 8zw36yw3fq4yy 4 53372 CHILDenq: TM - contention 8zw36yw3fq4yy 4 53372 CHILD

select

event,

sql_id,mod(p1,16) as "mode",p2|| ' ' || o.name obj


whereevent like 'enq: TM%'and o.obj# (+)= ash.p2

order by sample_time;

#.49


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UL Locks

User-defined Locks

dbms_lock

Wait Event Parameter2 Parameter3enq: UL - contention id 0

dbms_lock.allocate_unique(v_lockname, v_lockhandle);

dbms_lock.request(v_lockhandle, p_ltype);dbms_lock.release(v_lockhandle);

#.50

I t l L k


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Internal Locks

CICross Instance

CUCursor Bind

HWHigh Water

ROReuse Object

STSpace Transaction

#.51


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enq: CI - contention

Cross Instance

not OPS lock.

invoke actions in background processes checkpoints

log switches

instance is shut down

#.52


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CICross InstanceId2 Meaning (parameter3)1 Pass in Parameters

2 Invoke the call in background process

3 Foreground has not returned yet

4 Used to allocate the CI call

5 Used to queue up interested clients

Id1 Meaning (parameter2)

0 Flush buffers for reuse as new class

1 LGWR checkpointing and Hot Backup

2 DBWR synchronization of SGA with control file

3 Log file add/drop/rename notification

4 Write buffer for CR read

5 Test Call

6 Invalidate KCK cache in all instances

7 Alter rollback segment optimal

8 Signal Query Servers/coordinator

9 Create Remote Parallel Query Server

10 Set Global Partitions

11 Stop Disk Writes

12 Drop Sort Segments

13 Release unused space from Sort Segments

14 Instance Recovery for Parallel operation Group

15 Validate parallel slave Lock Value

16 Check Transaction State Objects

#.53

CI L k


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CI Locks

select

substr(sql_text,0,20) sql_text,p2,p3,CURRENT_OBJ# || ' ' || name obj,CURRENT_FILE# file#,CURRENT_BLOCK# block#

from v$active_session_history ash,

obj$ o,v$sqltext sql

whereevent like 'enq: CI%'

and o.obj# (+)= ash.current_obj#and sql.sql_id (+)= ash.sql_id


SQL_TEXT P2 P3 OBJ FILE# BLOCK#-------------------- ----- ------- ------------ ------ -------

INSERT INTO TOTO1 VA 1 5 54225 TOTO1 6 682721

#.54

CI L k


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CI Locks

SQL_TEXT P2 P3---------------------------------------- --- --alter table XXXXX drop partition YYYYY 1 5

ID1 = 2 " LGWR checkpointing and Hot Backup "

ID2 = 5. "Used to queue up interested clients"

If p2=1 and p3=5, then contention on blocks being

checkpointed, try raising fast_start_mttr_target

alter system set fast_start_mttr_target=600 scope=both;

#.55


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enq: CU - contention

cursor bind

From Jonathan Lewis:

Protects the variable definitions while thecursor is being optimized.

8i one CU per bind

9i one CU per cursor

#.56enq: HW - contention


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enq: HW contention

Data

Empty

High Water Mark

Header

Table

Wait Event Parameter2 Parameter3

enq: HW - contention table space # block

Session 1

Session 2

Session 3

#.57

HW


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HW

Use Freelists Cause multiple jumps in High Water Mark

Pre-Allocate ExtentsAlter table XXXX allocate extent;

Hidden Parameter_bump_highwater_mark_count

alter session set "_bump_highwater_mark_count"=100;

Not supported

ASSM

Automatic segment space management

Partition Object

#.58HW Further Investigation


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HW Further Investigation

EVENT SQL_ID OBJ FILE# BLOCK#-------------------- ------------- ----------- ----- ------enq: HW - contention 49ch3jfkncnsp 53410 T1_I1 13 29734

select

event,

sql_id,CURRENT_OBJ# || ' ' || name obj,CURRENT_FILE# file#,CURRENT_BLOCK# block#


whereevent like 'enq: HW%'

and o.obj# (+)= ash.current_obj#


#.59


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enq: KO - fast object checkpoint

Used when checking the cache for blocksfrom a table for PQO direct read

#.60

enq: RO fast object reuse


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enq: RO - fast object reuse

Drop or Truncate a tableWait for DBWR to clean cache

Solution

Smaller cache

Tune DBWR using smaller MTTRUse GTT

Truncate/drop less often

SELECT TARGET_MTTR, ESTIMATED_MTTR,

CKPT_BLOCK_WRITES

FROM V$INSTANCE_RECOVERY

alter system set fast_start_mttr_target=17 scope=both;

#.61


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enq: ST - contention

Space Transaction management lock

Space Transaction Lock

Allocating/deallocating extentsDictionary Managed Tablespaces

Id1, Id2 Combinations:Always 0, Always 0.

SolutionGot to Locally Managed Tablespaces

#.62

Other Resources


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Other Resources

@?/rdbms/admin/utllockt

WAITING_SESSION LOCK_TYPE MODE_REQUESTED MODE_HELD LOCK_ID1 LOCK_ID2

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

144 None

139 Transaction Share Exclusive 131113 7507

#.63

Blocking Sessions


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Blocking Sessions


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