Buffer Cache Waits

In This Section

1. latch: cache buffers chains

2. latch: cache buffers lru chain 3. Free Buffer Wait

4. Buffer Busy Wait

5. Write Complete Wait

6. Buffer Exterminate

REDO Log FilesREDO Log Files Data FilesData Files

DBWRDBWRLGWRLGWR

User3User3 User2User2User1User1

Log BufferLog Buffer

Buffer CacheBuffer CacheLog Log BufferBuffer

Buffer Buffer CacheCache

SGASGALibrary Library CacheCache

Oracle Memory Structures

RAM :

Machine

Memory

Buffer Cache Access

Buffer Cache ManagementLocating Free blocksFinding data blocksManaging LRU listsCleaning Dirty Blocks

Buffer Cache management can cause contentionDifferent from IO ( reading blocks of disk )

Is Block in cache?

Now you have a file# and block#

How do you know if a block is cached?

ShadowShadow

ProcessProcess ??Do you search all the blocks?

Could be 1000s of blocks to search.

Buffer caches are in the multi Gig

Buffer Cache

Find a block by:

1) Hash of

Data file #

Block#

2) Result = Bucket #

3) Search linked list for that bucket #

What is a hash value

What are Buckets

What is the linked list?

Double Linked Lists

03C39000 03C39478

03C38F60 03C39000

03C3900003C38F60 03C39478Address

Next

Previous

Hashing Function Simple hash could be a Mod function

1 mod 4 = 1 2 mod 4 = 2 3 mod 4 = 3 4 mod 4 = 0 5 mod 4 = 1 6 mod 4 = 2 7 mod 4 = 3 8 mod 4 = 0

Using “mod 4” as a hash function creates 4 “buckets” to store things

Hash Bucket Fill

Data Block Header

11

22

00

33

Hash Block’s file# block #’s

Result in a bucket#Put Block in bucket

?

?

?

?

Hash Block’s 1 file# 437 block

#’s

(1+437) mod 4 = 2After a while the buckets become populated with blocksAfter a while the buckets become populated with blocks

Latches Protect Bucket Contents

BufferHeaders

Data BlocksHashbucket

latches

Buffer Headers contents described by X$BH

X$bh Describes Contents of Buffer Headers

SQL> desc x$bh Name Type ------------ -------- ADDR RAW(4) DBARFIL NUMBER DBABLK NUMBER OBJ NUMBER HLADDR RAW(4) NXT_HASH RAW(4) PRV_HASH RAW(4)… much more

ADDR DBARFIL DBABLK OBJ HLADDR NXT_HASH PRV_HASH…

A each buffer header contains Information about the data block It points to and the previous and next Buffer header in a linked list

X$BH describes Headers

BufferHeaders

Data BlocksHashbucket

latches

HLADDR

NXT_HASH

PRV_HASH

ADDR ADDR

DBARFIL

DBABLKOBJ

x$bh ADDR DBARFIL DBABLK OBJ HLADDR NXT_HASH PRV_HASH

To Find a Block

1. Hash the block address

2. Get Bucket latch

3. Look for header

4. Found, read block in cache

5. Not Found Read block off disk

ShadowShadowProcessProcess

BufferHeaders

Data BlocksHashbucket

22 33 5544

latches

1122334455

Cache Buffers ChainsHash Buckets

s5s5

s4s4

s3s3

s2s2

s1s1

SessionsSessions

Contention if too many accesses on a bucketContention if too many accesses on a bucket

latches BlockHeaders Data

Blocks

Cache Buffer Chain

Examples

S1 S2 S3 S4

1. Look up Table

2. Nested Loops

Select t1.val, t2.val

from t1, t2

where t1.c1 = {value}

and t2.id = t1.id;

t1Index_t2

t2

CBC Solutions

Find SQL ( Why is application hitting the block so hard? ) Nested loops, possibly

Hash Partition Uses Hash Join Hash clusters

Look up tables (“select language from lang_table where ...”) Change application

Use plsql function Spread data out to reduce contention (pctfree=99)

Select from dual Possibly use x$dual

How do you find the SQL?

Which SQL

select count(*), sql_idfrom v$active_session_history ashwhere event like 'latch: cache buffers chains'group by sql_idorder by count(*)/

CNT SQL_ID

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

84 a09r4dwjpv01q

CBC: OEM

CBC: ADDM

Problem

SQL Statement

Solution?

Finding the Hot Block:

X$BH.TCH

Updated when block read Updated by no more than 1 every 3 seconds Can be used to find “hot” blocks

Note: set back to zero when block cycles through the buffer cache

CBC – Further Investigation

select * from v$event_name

where name = 'latch: cache buffers chains'

EVENT# NAME

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

58 latch: cache buffers chains

PARAMETER1 PARAMETER2 PARAMETER3

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

address number tries

NOTE: _db_block_hash_buckets = # of hash buckets

_db_block_hash_latches= # of hash latches

CBC – Real Timeselect

count(*),

lpad(replace(to_char(p1,'XXXXXXXXX'),' ','0'),16,0) laddr

from v$active_session_history

where event='latch: cache buffers chains'

group by p1;

select o.name, bh.dbarfil, bh.dbablk, bh.tch

from x$bh bh, obj$ o

where tch > 100

and hladdr='00000004D8108330'

and o.obj#=bh.obj

order by tch

COUNT(*) LADDR

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

4933 00000004D8108330

NAME DBARFIL DBABLK TCH

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

EMP_CLUSTER 4 394 120

Putting into one Query

select name, file#, dbablk, obj, tch, hladdr from x$bh bh , obj$ o where o.obj#(+)=bh.obj and hladdr in ( select ltrim(to_char(p1,'XXXXXXXXXX') ) from v$active_session_history where event like 'latch: cache%' group by p1 having count(*) > 5) and tch > 5order by tch

NAME FILE# DBABLK OBJ TCH HLADDR

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

BBW_INDEX 1 110997 66051 17 6BD91180

IDL_UB1$ 1 54837 73 18 6BDB8A80

VIEW$ 1 6885 63 20 6BD91180

VIEW$ 1 6886 63 24 6BDB8A80

DUAL 1 2082 258 32 6BDB8A80

DUAL 1 2081 258 32 6BD91180

MGMT_EMD_PING 3 26479 50312 272 6BDB8A80

This can be misleading, as TCH gets set to 0 ever rap around the LRU and it only gets updated once every 3 seconds, so in this case DUAL was my problem table not MGMT_EMD_PING

Hot Block via Tanel Poder

Sessions

Loop 100,000 times

Latch Holders

Event names

Consistent Read Blocks

Current

Block

(XCUR)

s1s1 s2s2

Update Select

Consistent

Read

(CR)& Undo Both have same file#

and block# and hash to same bucket

latches

CBC: Consistent Read Blocks

Cache Buffer Chain

Contention: Too Many Buffers in BucketContention: Too Many Buffers in Bucket

s5s5

s4s4

s3s3

s2s2

s1s1

Hash Buckets

BlockHeaders

Max length :_db_block_max_cr_dba = 6(in 10g)

Consistent Read Copies

select count(*) , name , file# , dbablk , hladdr from x$bh bh , obj$ owhere o.obj#(+)=bh.obj and hladdr in ( select ltrim(to_char(p1,'XXXXXXXXXX') ) from v$active_session_history where event like 'latch: cache%' group by p1 )group by name,file#, dbablk, hladdrhaving count(*) > 1order by count(*);

CNT NAME FILE# DBABLK HLADDR

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

14 MYDUAL 1 93170 2C9F4B20

CBC : Solution

Find the SQL causing the problem Change Application Logic

Eliminate hot spots Look up tables

Uses pl/sql functions Minimize data per block (pctfree=99) Possibly using x$dual instead of dual

Oracle added fast dual in 10g , as long as the field “dummy” isn’t accessed uses fast dual

Index Nested loops Hash join Hash partition index Hash Cluster

Updates, inserts , select for update on blocks while reading those blocks

Cause multiple copies

select ash.sql_id, count(*), sql_textfrom v$active_session_history ash, v$sqlstats sqlwhere event='latch: cache buffers chains'and sql.sql_id(+)=ash.sql_idgroup by ash.sql_id, sql_text;

Block Pinning

BufferHeaders

Data BlocksHashbucket

latches

Free Buffer Wait

Finding a Free Block

If the data block isn’t in cache Get a free block and header in the buffer cacheRead it off diskUpdate the free headerRead the block into the buffer cache

Need Free Block to Read in New Data Block

Tune by Increase data blocksTry to tune DBWR

Finding a Free Block

ShadowShadow

ProcessProcess

When a session reads a block When a session reads a block

Into the bufffer cache how does Into the bufffer cache how does

it find a it find a FREEFREE spot? spot?

Finding a Free Block

BufferHeaders

Data BlocksHashbucket

latches

1. Arrange the Buffer Headers into an LRU List

2. Scan LRU for a free block

Cache Buffers LRU

= entry in x$bh

X$bh Describes Buffer Headers

SQL> desc x$bh Name Type ---------------------- --------- ADDR RAW(4) DBARFIL NUMBER DBABLK NUMBER OBJ NUMBER HLADDR RAW(4) NXT_HASH RAW(4) PRV_HASH RAW(4) NXT_REPL RAW(4) PRV_REPL RAW(4)

NXT_REPL RAW(4) PRV_REPL RAW(4)

HLADDR RAW(4) NXT_HASH RAW(4) PRV_HASH RAW(4)

Cache buffer chains

LRU

LRU Chain

03C39000 03C38F60

03C38F60 03C39000

03C3900003C38F60 03C39478ADDR

NXT_HASH

PRV_HASH

03C39478

03C38514

03C38638

03C38620

03C385F4

03C38554

NXT_REPL

PRV_REPL

Cache Buffers LRU list

Cache Buffers LRU list

LRU Chain of Buffer Headers

Buffer Cache

Cache Buffers LRU Latch

MRUMRU LRULRUBuffer Headers

“Cold”

LRU = Least Recently UsedMRU = Most Recently Used

One LRU Latch protects the linked list during changes to the listOne LRU Latch protects the linked list during changes to the list

“Hot”

LRU latch

Session Searching for Free Blocks

MRUMRU LRULRUBuffer Headers

Session

Shadow

1. Go to the LRU end of data blocks

2. Look for first non-dirty block

3. If search too many post DBWR to make free

4. Free Buffer wait

Free Buffer Wait Solutions Tune by

Increase data blocksTry to tune DBWR

ASYNC If no ASYNC use I/O Slaves (dbwr_io_slaves)

Multiple DBWR (db_writer_processes) Direct I/O

Tune Inefficient SQL requesting large # of blocks

Session Finding a Free Block

MRUMRU LRULRU

Hot End

Mid-Point Mid-Point InsertionInsertion

Get LRU LatchGet LRU Latch

Find Free Find Free BlockBlockInsert HeaderInsert Header

Release LRU LatchRelease LRU Latch

session

LRU Latch

Note: FTS read in at the cold end CR copies as well

DBWR taking Dirty Blocks off

MRUMRU LRULRUBuffer Headers LRU

DBWR

Dirty List of Buffer Headers LRUW

latch

LRU latch also covers DBWR list of dirty blocs

Cache Buffers LRU Latch

MRUMRU LRULRU

Mid-Point Mid-Point InsertionInsertion

Oracle Tracks the touch count of blocks. As the Oracle Tracks the touch count of blocks. As the block is pushed to the LRU end, if it’s touch count block is pushed to the LRU end, if it’s touch count is 3 or more, it’s promoted to the MRU endis 3 or more, it’s promoted to the MRU end

Solution: Multiple SetsMultiple Sets

_db_block_lru_latches = 8_db_block_lru_latches = 810gR2 with cpu_count = 210gR2 with cpu_count = 2 X$KCBWDS – set descriptorX$KCBWDS – set descriptor

Set 1Set 1

Set 2Set 2

LRU Latch 1

LRU Latch 2

Test Case

8 Sessions reading separate tables Tables were too big to hold in cache cache option set on each table

Result : lots of buffer cache churn Expected to get “latch: cache buffer chains LRU”

simulator lru latch

CBC – Further Investigation

select p2, count(*)

from v$active_session_history

where event='latch free'

group by p2

select * from v$latchname where latch#=127

P2 COUNT(*)---------- ---------- 127 3556

LATCH# NAME

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

127 simulator lru latch

select * from v$event_name

where name = 'latch free'PARAMETER1 PARAMETER2 PARAMETER3

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

address number tries

db_cache_advice

Alter system set db_cache_advice=off;

Group “other” is very small compared to I/O wait time – not a problem

Cache Buffers LRU Latch : Solution Other

Increase Size of Buffer Cache Using multiple cache buffers

Keep, recycle

Possibly increase _db_block_lru_latchesNot supported

write complete waits

Usually happens in tandem with free buffer Tune by

Increase data block cache

Happens because shadow wants to access blocks that are currently being written to disk by DBWR

also seen it happen when there is a lot of write to sort the waits are on block 2 of the temp tablespace file

Write Complete Waits

LRU

DBWR

Dirty List of Buffer Headers LRUW

Session

Buffer Busy Waits

User 1 tries to change a buffer header User 2 has buffer header “locked” (pinned)

11

22

00

33

User1User1User2User2

10g Buffer Busy Waits

Mainly, on 10g (There are a few other cases )

BLOCK CLASS=data block (and some segment header)

Object Type = INDEX Object Type = TABLE

Indicates DML contention To verify use P3. P3 = Block Class (on 8.1.6-9iR2 was BBW type)

Other Class Types : Segment header

When also seeing “data block” on the same object and the object is of OTYPE= “TABLE” then confirms that the TABLE needs to use free lists or ASSM.

File Header Block Most likely extent allocation problems, look at extent size on tablespace and increase

the extent size

Free lists Add free list groups

Undo header Not enough UNDO segments, if using old RBS then switch to AUM “alter system set undo_management=auto scope=spfile;”

1st level bmb Contention on ASSM bitmap blocks, might do better using freelists.

Undo block Hot spot in UNDO, application issue

How to get Class Nameselect rownum n,ws.classfrom v$waitstat;

NAME P1 P2 P3

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

buffer busy waits file# block# class#

NAME P1 P2 P3

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

buffer busy waits file# block# class#

select * from v$event_namewhere name = 'buffer busy waits'

N CLASS

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

1 data block 2 sort block 3 save undo block 4 segment header 5 save undo header 6 free list 7 extent map

8 1st level bmb 9 2nd level bmb 10 3rd level bmb 11 bitmap block 12 bitmap index block 13 file header block 14 unused 15 system undo header 16 system undo block 17 undo header 18 undo block

Note: Before 10g, P3 was BBW type

If P3 in 100,110,120,130 then read

Now “read by other session”

Else Write, P3 in 200,210,220,230, 231

P3 = class#, how do we get class name?

Joining ASH with v$waitstatselect o.object_name obj, o.object_type otype, ash.SQL_ID, w.classfrom v$active_session_history ash, ( select rownum class#, class from v$waitstat ) w, all_objects owhere event='buffer busy waits' and w.class#(+)=ash.p3 and o.object_id (+)= ash.CURRENT_OBJ#Order by sample_time;

OBJ OTYPE SQL_ID CLASS

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

TOTO1 TABLE 8gz51m9hg5yuf data blockTOTO1 TABLE 8gz51m9hg5yuf data blockTOTO1 TABLE 8gz51m9hg5yuf segment headerTOTO1 TABLE 8gz51m9hg5yuf data block

Example: Lack of Free List

S1 S2 S3 S4

4 Sessions running

Insert into toto1 values (null, ‘a’);Commit;

OBJN OTYPE FILEN BLOCKN SQL_ID BLOCK_TYPE

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

54962 TOTO1 TABLE 16 45012 8gz51m9hg5yuf data block 54962 TOTO1 TABLE 16 161 8gz51m9hg5yuf segment header

Example: BBW with Insert

Concurrent inserts will insert into the same blockEach session has to wait for the previous session to

finish it’s writeUsually pretty fastContention builds on highly concurrent applications

Lack of Free Lists Not Using ASSM (Automatic Segment Space

Management)

Solution1: Free Lists

S1 S2 S3 S4

4 Sessions running

Insert into toto values (null, ‘a’);Commit;

Solution2: ASSM

S1 S2 S3 S4

HeaderandLevel 3 BMB

Level 1BMB

Level 1BMB

Level 1BMB

Level 2BMB

Level 1BMB

Tablespace Types : ASSM

select

tablespace_name, extent_management LOCAL, allocation_type EXTENTS, segment_space_management ASSM, initial_extentfrom dba_tablespaces

TABLESPACE_NAME LOCAL EXTENTS ASSM

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

SYSTEM LOCAL SYSTEM MANUALUNDOTBS1 LOCAL SYSTEM MANUALSYSAUX LOCAL SYSTEM AUTOTEMP LOCAL UNIFORM MANUALUSERS LOCAL SYSTEM AUTOEXAMPLE LOCAL SYSTEM AUTODATA LOCAL SYSTEM MANUAL

create tablespace data2 datafile '/d3/kyle/data2_01.dbf' size 200Msegment space management auto;

BBW: ASSM

Consider using Freelists instead of ASSM Normally waits on ASSM blocks should be too

small to warrant using Freelists ASSM is easier, automatically managed

BBW on Index

Index

Session 1

Session 2

Session 3

Increasing index key creates a hot spot on the leading index leaf

OBJN OTYPE FILEN BLOCKN SQL_ID BLOCK_TYPE

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

BBW_INDEX_INDEX 1 113599 97dgthz60u28d data block 1

Use Reverse Key indexesBreaks Index scans

Hash Partition IndexMore IOs per index access

BBW on IndexSolutions

1. Hash Partitions

2. Reverse Keys

BBW on Index : ADDM Recs

Also consider “reversing” the key

BBW: File Header Querying ASH, make sure

P1=current_file#P2=current_block#

If not, use p1, p2 and not current_object#Time P1 P2 OBJN OTYPE FN BLOCKN BLOCK_TYPE

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

11:44 202 2 -1 0 0 file header block11:44 202 2 TOTO TABLE 1 60218 file header block11:44 202 2 TOTO TABLE 1 60218 file header block11:44 202 2 TOTO TABLE 1 60218 file header block11:44 202 2 TOTO TABLE 1 60218 file header block

SELECT A.OBJECT_ID FROM ALL_OBJECTS A,

( SELECT * FROM ALL_OBJECTS WHERE ROWNUM < 1000) B ORDER BY A.OBJECT_NAME

Copyright 2006 Kyle Hailey

Temporary File #’s

SQL> select file# from v$datafile; FILE#

----------

1 2 3 4 5 6 7 8SQL> select file# from v$tempfile; FILE#

----------

2 1

SQL> show parameters db_files

NAME VALUE

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

db_files 200

Wait Temporary File#’s =

Db_files + file#

File# 202 = v$tempfile 2

BBW : File HeaderTime P1 P2 OBJN OTYPE FN BLOCKN BLOCK_TYPE

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

11:44 202 2 TOTO TABLE 1 60218 file header block

Solution is make initial and next extent larger in Temp Table Space

ADDM doesn’t say much

Buffer Exterminate

Buffer cache dynamically resized

V$SGA_DYNAMIC_COMPONENTS displays information about the dynamic SGA components. This view summarizes information based on all completed SGA resize operations since instance startup.

V$SGA_CURRENT_RESIZE_OPS displays information about SGA resize operations which are currently in progress. An operation can be a grow or a shrink of a dynamic SGA component.

V$SGA_DYNAMIC_FREE_MEMORY displays information about the amount of SGA memory available for future dynamic SGA resize operations.

Alter system set db_cache_size=50M;

Summary Buffer Cache Waits

1. latch: cache buffers chains - find SQL Eliminate hot spots

2. latch: cache buffers lru chain – increase sets

3. Free Buffer Wait - increase cache size

4. Buffer Busy Wait Index : alleviate hot spots, partition Table: add free lists or use ASSM File Segment Header : looked at high extent

allocations

5. Write Complete Waits - increase cache size