The document discusses buffer cache waits in Oracle. It describes how Oracle uses a hash table to quickly locate blocks in the buffer cache by file number and block number. It discusses how latches protect the hash buckets and linked lists that maintain the blocks in each bucket. The document also covers consistent read blocks, how they can cause contention, and how to identify hot blocks and SQL causing buffer cache chain latch waits.

1. Buffer Cache Waits

2. 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

3. Oracle Memory Structures
SGA
RAM : Log Library Buffer
Machine Buffer Cache Cache
Buffer Cache
Memory Log Buffer
DBWR
LGWR
User1
User3 User2
REDO Log Files Data Files

4. Buffer Cache Access
 Buffer Cache Management
 Locating Free blocks
 Finding data blocks
 Managing LRU lists
 Cleaning Dirty Blocks
 Buffer Cache management can cause contention
 Different from IO ( reading blocks of disk )

5. Is Block in cache?
Now you have a file# and block#
How do you know if a block is cached?
Shadow
Process
?
Do you search all the blocks?
Could be 1000s of blocks to search.
Buffer caches are in the multi Gig

6. Buffer Cache
Find a block by:
1) Hash of What is a hash value
Data file # What are Buckets
What is the linked list?
Block#
2) Result = Bucket #
3) Search linked list for that bucket #

7. Double Linked Lists
Address 03C38F60 03C39000 03C39478
Next 03C39000 03C39478
Previous 03C38F60 03C39000

8. 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

9. Hash Bucket Fill
Data Block
Header Hash Block’s
file#
block #’s
0 Result in a bucket#
? Put Block in bucket
1 ? Hash Block’s
1 file#
2 ? 437 block
#’s
3 ?
(1+437) mod 4 =
2
After a while the buckets become populated with blocks

10. Latches Protect Bucket Contents
latches Hash Buffer Data Blocks
bucket Headers
Buffer Headers contents described by X$BH

11. X$bh
 Describes Contents of Buffer Headers
SQL> desc x$bh
Name Type ADDR
------------ -------- DBARFIL
ADDR RAW(4) DBABLK
DBARFIL NUMBER OBJ
DBABLK NUMBER HLADDR
OBJ NUMBER NXT_HASH
HLADDR RAW(4) PRV_HASH
NXT_HASH RAW(4) …
PRV_HASH RAW(4) A each buffer header contains
… much more Information about the data block
It points to and the previous and next
Buffer header in a linked list

12. X$BH describes Headers
latches Hash Buffer Data Blocks
bucket Headers
HLADDR
ADDR ADDR
NXT_HASH
x$bh PRV_HASH
ADDR
DBARFIL
DBABLK
OBJ DBARFIL
HLADDR
NXT_HASH DBABLK
PRV_HASH OBJ

13. To Find a Block
latches Hash Buffer Data Blocks
bucket Headers
Shadow
Process
2 3 4 5
1
1. Hash the block address
2
2. Get Bucket latch
3
3. Look for header
4
4. Found, read block in cache
5
5. Not Found Read block off disk

14. Cache Buffers Chains
latches Hash Block
Buckets
Headers Data
Blocks
s5
s4
s3 Cache Buffer Chain
s2
s1
Sessions
Contention if too many accesses on a bucket

15. Examples
1. Look up Table
S1 S2 S3 S4
t1 t2
Index_t2
2. Nested Loops
Select t1.val, t2.val
from t1, t2
where t1.c1 = {value}
and t2.id = t1.id;

16. 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?

17. Which SQL
select
count(*),
sql_id
from v$active_session_history ash
where event
like 'latch: cache buffers chains'
group by sql_id
order by count(*)
/
CNT SQL_ID
84 a09r4dwjpv01q

18. CBC: OEM

19. CBC: ADDM
Problem
SQL Statement
Solution?

20. 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

21. 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

22. CBC – Real Time
select
count(*),
lpad(replace(to_char(p1,'XXXXXXXXX'),' ','0'),16,0) laddr
from v$active_session_history
where event='latch: cache buffers chains'
COUNT(*) LADDR
group by p1;
4933 00000004D8108330
select o.name, bh.dbarfil, bh.dbablk, bh.tch
from x$bh bh, obj$ o
where tch > 100
and hladdr='00000004D8108330'
NAME DBARFIL DBABLK TCH
and o.obj#=bh.obj
order by tch EMP_CLUSTER 4 394 120

23. Putting into one Query
select
name, file#, dbablk, obj, tch, hladdr This can be misleading, as
from x$bh bh TCH gets set to 0 ever rap
, obj$ o around the LRU and it only
where
o.obj#(+)=bh.obj and
gets updated once every 3
hladdr in seconds, so in this case DUAL
( was my problem table not
select ltrim(to_char(p1,'XXXXXXXXXX') ) MGMT_EMD_PING
from v$active_session_history
where event like 'latch: cache%'
NAME FILE# DBABLK OBJ TCH HLADDR
group by p1 ------------- ----- ------ ------ --- --------
having count(*) > 5 BBW_INDEX 1 110997 66051 17 6BD91180
) IDL_UB1$ 1 54837 73 18 6BDB8A80
and tch > 5 VIEW$ 1 6885 63 20 6BD91180
order by tch 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

24. Hot Block via Tanel Poder
Loop 100,000
times
Latch Holders
Event names
Sessions

25. Consistent Read Blocks
Current Consistent
Block Read
&
(XCUR) Undo (CR) Both have same file#
and block# and hash
to same bucket
s1 s2
Update Select

26. CBC: Consistent Read Blocks
Hash Block
latches Buckets Headers
s5
Cache Buffer Chain
s4
s3
Max length :
_db_block_max_cr_dba = 6
s2
(in 10g)
s1
Contention: Too Many Buffers in Bucket

27. Consistent Read Copies
select
count(*)
, name
, file#
, dbablk CNT NAME FILE# DBABLK HLADDR
, hladdr
from x$bh bh 14 MYDUAL 1 93170 2C9F4B20
, obj$ 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
)
group by name,file#, dbablk, hladdr
having count(*) > 1
order by count(*);

28. CBC : Solution
 Find the SQL causing the problem
 Change Application Logic
 Eliminate hot spots select
ash.sql_id,
 Look up tables count(*),
 Uses pl/sql functions sql_text
 Minimize data per block (pctfree=99) from v$active_session_history ash,
 Possibly using x$dual instead of dual v$sqlstats sql
 Oracle added fast dual in 10g , as long as the where
field “dummy” isn’t accessed uses fast dual event='latch: cache buffers chains'
 Index Nested loops and sql.sql_id(+)=ash.sql_id
 Hash join group by ash.sql_id, sql_text;
 Hash partition index
 Hash Cluster
 Updates, inserts , select for update
on blocks while reading those blocks
 Cause multiple copies

29. Block Pinning
latches Hash Buffer Data Blocks
bucket Headers

30. Free Buffer Wait
Finding a Free Block
 If the data block isn’t in cache
 Geta free block and header in the buffer cache
 Read it off disk
 Update the free header
 Read the block into the buffer cache
Need Free Block to Read in New Data Block
 Tune by
 Increase data blocks
 Try to tune DBWR

31. Finding a Free Block
When a session reads a block
Into the bufffer cache how does
it find a FREE spot? Shadow
Process

32. Finding a Free Block
latches Hash Buffer Data Blocks
bucket Headers
1. Arrange the Buffer Headers into an LRU List
2. Scan LRU for a free block

33. Cache Buffers LRU
= entry in x$bh

34. X$bh
 Describes Buffer Headers
SQL> desc x$bh
Name Type
---------------------- ---------
ADDR RAW(4)
DBARFIL NUMBER
DBABLK NUMBER
OBJ NUMBER
HLADDR
HLADDR Cache RAW(4)
RAW(4)
NXT_HASH
NXT_HASH buffer RAW(4)
RAW(4)
PRV_HASH
PRV_HASH chains RAW(4)
RAW(4)
NXT_REPL RAW(4)
LRU
PRV_REPL RAW(4)

35. LRU Chain
ADDR 03C38F60 03C39000 03C39478
NXT_HASH 03C39000 03C38F60
PRV_HASH 03C38F60 03C39000
NXT_REPL 03C39478 03C38638 03C385F4
PRV_REPL 03C38514 03C38620 03C38554

36. Cache Buffers LRU list

37. Cache Buffers LRU list
LRU Chain of Buffer Headers
Buffer Cache

38. Cache Buffers LRU Latch
Buffer Headers
MRU LRU latch LRU
“Hot” “Cold”
LRU = Least Recently Used
MRU = Most Recently Used
One LRU Latch protects the linked list during changes to the list

39. Session Searching for Free Blocks
MRU Buffer Headers LRU
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 Session
4. Free Buffer wait Shadow

40. Free Buffer Wait Solutions
 Tune by
 Increase data blocks
 Try 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

41. Session Finding a Free Block
LRU Latch
MRU LRU
Hot End
Mid-Point
Find Free
Insert Header Insertion
Block
Release LRU Latch
Get LRU Latch
Note: FTS read in at the cold end session
CR copies as well

42. DBWR taking Dirty Blocks off
MRU Buffer Headers LRU LRU
latch
Dirty List of Buffer Headers LRUW
LRU latch also covers
DBWR list of dirty blocs
DBWR

43. Cache Buffers LRU Latch
MRU LRU
Mid-Point
Insertion
Oracle Tracks the touch count of blocks. As the
block is pushed to the LRU end, if it’s touch count
is 3 or more, it’s promoted to the MRU end

44. Solution: Multiple Sets
Set 1
LRU Latch 1
Set 2
LRU Latch 2
_db_block_lru_latches = 8
10gR2 with cpu_count = 2
X$KCBWDS – set descriptor

45. 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”

46. simulator lru latch

47. CBC – Further Investigation
select * from v$event_name
where name = 'latch free'
PARAMETER1 PARAMETER2 PARAMETER3
address number tries
select p2, count(*)
from v$active_session_history
where event='latch free'
P2 COUNT(*)
group by p2 ---------- ----------
127 3556
select * from v$latchname where latch#=127
LATCH# NAME
127 simulator lru latch

48. db_cache_advice
Alter system set db_cache_advice=off;
Group “other” is
very small
compared to I/O
wait time – not a
problem

49. Cache Buffers LRU Latch : Solution Other
 Increase Size of Buffer Cache
 Using multiple cache buffers
 Keep, recycle
 Possibly increase _db_block_lru_latches
 Not supported

50. 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

51. Write Complete Waits
LRU
Dirty List of Buffer Headers LRUW
Session DBWR

52. Buffer Busy Waits
 User 1 tries to change a buffer header
 User 2 has buffer header “locked” (pinned)
0
1
2
3
User2
User1

53. 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)

54. 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

55. How to get Class Name
P3 = class#, how do we get class name? select rownum n,ws.class
from v$waitstat;
select * from v$event_name N CLASS
where name = 'buffer busy waits'
1 data block
2 sort block
NAME
NAME P1
P1 P2
P2 P3
P3 3 save undo block
buffer busy waits file# block# class#
buffer busy waits file# block# class# 4 segment header
5 save undo header
6 free list
7 extent map
Note: Before 10g, P3 was BBW type 8 1st level bmb
9 2nd level bmb
If P3 in 100,110,120,130 then read 10 3rd level bmb
11 bitmap block
Now “read by other session” 12 bitmap index block
13 file header block
Else Write, P3 in 200,210,220,230, 14 unused
231 15 system undo header
16 system undo block
17 undo header

56. Joining ASH with v$waitstat
select
o.object_name obj,
o.object_type otype,
ash.SQL_ID,
w.class
from v$active_session_history ash,
( select rownum class#, class from v$waitstat ) w,
all_objects o
where 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 block
TOTO1 TABLE 8gz51m9hg5yuf data block
TOTO1 TABLE 8gz51m9hg5yuf segment header
TOTO1 TABLE 8gz51m9hg5yuf data block

57. Example: Lack of Free List
4 Sessions running
Insert into toto1
values (null, ‘a’);
Commit;
S1 S2 S3 S4
OBJN OTYPE FILEN BLOCKN SQL_ID BLOCK_TYPE
54962 TOTO1 TABLE 16 45012 8gz51m9hg5yuf data block
54962 TOTO1 TABLE 16 161 8gz51m9hg5yuf segment header

58. Example: BBW with Insert
 Concurrent inserts will insert into the same block
 Each session has to wait for the previous session to
finish it’s write
 Usually pretty fast
 Contention builds on highly concurrent applications
 Lack of Free Lists
 Not Using ASSM (Automatic Segment Space
Management)

59. Solution1: Free Lists
4 Sessions running
Insert into toto
values (null, ‘a’);
S1 S2 S3 S4 Commit;

60. Solution2: ASSM
Header Level Level Level Level Level
and 2 1 1 1 1
Level 3 BMB BMB BMB BMB BMB
BMB
S1 S2 S3 S4

61. Tablespace Types : ASSM
select
tablespace_name,
extent_management LOCAL,
allocation_type EXTENTS,
TABLESPACE_NAME LOCAL
segment_space_management ASSM, EXTENTS ASSM
initial_extent
SYSTEM LOCAL SYSTEM MANUAL
from dba_tablespaces LOCAL
UNDOTBS1 SYSTEM MANUAL
SYSAUX LOCAL SYSTEM AUTO
TEMP LOCAL UNIFORM MANUAL
USERS create tablespace data2 SYSTEM
LOCAL AUTO
EXAMPLE LOCAL SYSTEM AUTO
datafile '/d3/kyle/data2_01.dbf'
DATA LOCAL SYSTEM MANUAL
size 200M
segment space management auto;

62. 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

63. BBW on Index
 Use Reverse Key indexes
 Breaks Index scans
 Hash Partition Index
 More IOs per index access
OBJN OTYPE FILEN BLOCKN SQL_ID BLOCK_TYPE
BBW_INDEX_INDEX 1 113599 97dgthz60u28d data block 1
Index
Session 1
Increasing index
key creates a hot
Session 2 spot on the leading
index leaf
Session 3

64. BBW on Index
Solutions
1. Hash Partitions
2. Reverse Keys

65. BBW on Index : ADDM Recs
Also consider “reversing” the key

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

67. Temporary File #’s
SQL> select file# from v$datafile;
FILE# Wait Temporary File#’s =
1
Db_files + file#
2
3
4 SQL> show parameters db_files
5
NAME VALUE
6
7 db_files 200
8
SQL> select file# from v$tempfile;
FILE#
2
1
File# 202 = v$tempfile 2
Copyright 2006 Kyle Hailey

68. BBW : File Header
Time P1 P2 OBJN OTYPE FN BLOCKN BLOCK_TYPE
11:44 202 2 TOTO TABLE 1 60218 file header block
ADDM doesn’t say much
Solution is make initial and next extent larger in Temp Table Space

69. Buffer Exterminate
 Buffer cache dynamically resized
Alter system set db_cache_size=50M;
 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.

70. Summary Buffer Cache Waits
1. latch: cache buffers chains - find SQL
 Eliminate hot spots
1. latch: cache buffers lru chain – increase sets
2. Free Buffer Wait - increase cache size
3. Buffer Busy Wait
 Index : alleviate hot spots, partition
 Table: add free lists or use ASSM
 File Segment Header : looked at high extent
allocations
1. Write Complete Waits - increase cache size