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

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 )

Is Block in cache? Now you have a file# and block# How do you know if a block is cached? ShadowProcess ? 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 03C C C38F60 03C C38F60 03C39478 Address 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 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 = 2 After a while the buckets become populated with blocks

Latches Protect Bucket Contents Buffer Headers Data Blocks Hash bucket 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$BH describes Headers Buffer Headers Data Blocks Hash bucket latches HLADDR NXT_HASH PRV_HASH ADDR DBARFIL DBABLK OBJ x$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 ShadowProcess Buffer Headers Data Blocks Hash bucket latches

Cache Buffers Chains Hash Buckets s5 s4 s3 s2 s1 Sessions Contention if too many accesses on a bucket latches Block Headers 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; t1 Index_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_id from v$active_session_history ash where event like 'latch: cache buffers chains' group by sql_id order by count(*) / CNT SQL_ID 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 latch: cache buffers chains PARAMETER1 PARAMETER2 PARAMETER address number tries NOTE: _db_block_hash_buckets = # of hash buckets _db_block_hash_latches= # of hash latches

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

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$active_session_history where event like 'latch: cache%' group by p1 having count(*) > 5 ) and tch > 5 order by tch NAME FILE# DBABLK OBJ TCH HLADDR BBW_INDEX BD91180 IDL_UB1$ BDB8A80 VIEW$ BD91180 VIEW$ BDB8A80 DUAL BDB8A80 DUAL BD91180 MGMT_EMD_PING BDB8A80 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) s1 s2 Update Select Consistent Read (CR) Clone & 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 Bucket s5 s4 s3 s2 s1 Hash Buckets Block Headers Max length : _db_block_max_cr_dba = 6 (in 10g)

Consistent Read Copies select count(*), name, file#, dbablk, 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 ) group by name,file#, dbablk, hladdr having count(*) > 1 order by count(*); CNT NAME FILE# DBABLK HLADDR MYDUAL C9F4B20

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$dual instead of dual  Oracle added fast dual in 10g, as long as the field “dummy” isn’t accessed uses fast dual  Index Nested loops  Hash join  Hash partition index  Hash Cluster  Updates, inserts, select for update on blocks while reading those blocks  Cause multiple copies select ash.sql_id, count(*), sql_text from v$active_session_history ash, v$sqlstats sql where event='latch: cache buffers chains' and sql.sql_id(+)=ash.sql_id group by ash.sql_id, sql_text;

Block Pinning Buffer Headers Data Blocks Hash bucket 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 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

Finding a Free BlockShadowProcess When a session reads a block Into the bufffer cache how does it find a FREE spot?

Finding a Free Block Buffer Headers Data Blocks Hash bucket 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 03C C38F60 03C C38F60 03C39478 ADDR NXT_HASH PRV_HASH 03C C C C C385F4 03C38554 NXT_REPL PRV_REPL

Cache Buffers LRU list

LRU Chain of Buffer Headers Buffer Cache

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

Session Searching for Free BlocksMRU LRU Buffer 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 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

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

DBWR taking Dirty Blocks offMRU LRU Buffer Headers LRU DBWR Dirty List of Buffer Headers LRUW latch LRU latch also covers DBWR list of dirty blocs

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

Multiple Sets Solution: Multiple Sets _db_block_lru_latches = 8 10gR2 with cpu_count = 2 X$KCBWDS – set descriptor Set 1 Set 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(*) LATCH# NAME simulator lru latch select * from v$event_name where name = 'latch free' PARAMETER1 PARAMETER2 PARAMETER 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_latches  Not 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) User1 User2

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 iR2 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 Name select rownum n,ws.class from v$waitstat; NAME P1 P2 P buffer busy waits file# block# class# NAME P1 P2 P buffer busy waits file# block# class# select * from v$event_name where name = 'buffer busy waits' N CLASS 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$waitstat select o.object_name obj, o.object_type otype, ash.SQL_ID, w.class from v$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 segment header TOTO1 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 TOTO1 TABLE gz51m9hg5yuf data block TOTO1 TABLE gz51m9hg5yuf segment header

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)

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

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

Tablespace Types : ASSM select tablespace_name, extent_management LOCAL, allocation_type EXTENTS, segment_space_management ASSM, initial_extent from dba_tablespaces TABLESPACE_NAME LOCAL EXTENTS ASSM SYSTEM LOCAL SYSTEM MANUAL UNDOTBS1 LOCAL SYSTEM MANUAL SYSAUX LOCAL SYSTEM AUTO TEMP LOCAL UNIFORM MANUAL USERS LOCAL SYSTEM AUTO EXAMPLE LOCAL SYSTEM AUTO DATA LOCAL SYSTEM MANUAL create tablespace data2 datafile '/d3/kyle/data2_01.dbf' size 200M segment 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 dgthz60u28d data block 1  Use Reverse Key indexes  Breaks Index scans  Hash Partition Index  More IOs per index access

BBW on Index Solutions 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 : file header block 11: TOTO TABLE 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$datafile; FILE# SQL> select file# from v$tempfile; FILE# SQL> show parameters db_files NAME VALUE db_files 200 Wait Temporary File#’s = Db_files + file# File# 202 = v$tempfile 2

BBW : File Header Time P1 P2 OBJN OTYPE FN BLOCKN BLOCK_TYPE : TOTO TABLE 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