How to identify blocks which cause latch contention on the 'cache buffers chains' latch

 Possible hot blocks in the buffer cache normally can be identified by a high or rapid increasing wait count on the CACHE BUFFERS CHAINS latch.This latch is acquired when searching for data blocks cached in the buffer cache. Since the Buffer cache is implemented
as a sum of chains of blocks, each of those chains is protected by a child of this latch when needs to be scanned. Contention in this latch can be caused by very heavy access to a single block. This can require the application to be reviewed.
To solve a hot block, the application may need to be reviewed because the contention may be caused by inefficient SQL that repeatedly accesses the same buffers unnecessarily

By examining the waits on this latch, information about the segment and the specific block can be obtained using the following queries.

First determine which latch id(ADDR) are interesting by examining the number of sleeps for this latch. The higher the sleep count, the more interesting the latch id(ADDR) is:

SQL> select CHILD#  "cCHILD"

     ,      ADDR    "sADDR"

     ,      GETS    "sGETS"

     ,      MISSES  "sMISSES"

     ,      SLEEPS  "sSLEEPS"

     from v$latch_children

     where name = 'cache buffers chains'

     order by 5, 1, 2, 3;
    

Run the above query a few times to to establish the id(ADDR) that has the most consistent amount of sleeps. Once the id(ADDR) with the highest sleep count is foundthen this latch address can be used to get more details about the blocks currently in the buffer
cache protected by this latch. The query below should be run just after determining the ADDR with the highest sleep count.

 

SQL> column segment_name format a35

     select /*+ RULE */

       e.owner ||'.'|| e.segment_name  segment_name,

       e.extent_id  extent#,

       x.dbablk - e.block_id + 1  block#,

       x.tch,

       l.child#

     from

       sys.v$latch_children  l,

       sys.x$bh  x,

       sys.dba_extents  e

     where

       x.hladdr  = '&ADDR' and

       e.file_id = x.file# and

       x.hladdr = l.addr and

       x.dbablk between e.block_id and e.block_id + e.blocks -1

     order by x.tch desc ;
    

Depending on the TCH column (The number of times the block is hit by a SQL statement), you can identify a hot block. The higher the value of the TCH column,the more frequent the block is accessed by SQL statements.

In order to reduce contention for this object the following mechanisms can be put in place:

1) Examine the application to see if the execution of certain DML and SELECT statements can be reorganized to eliminate contention on the object.

2) Decrease the buffer cache -although this may only help in a small amount of cases.

3) DBWR throughput may have a factor in this as well.If using multiple DBWR's then increase the number of DBWR's.

4) Increase the PCTFREE for the table storage parameters via ALTER TABLE or rebuild. This will result in less rows per block.

5) Consider implementing reverse key indexes (if range scans aren't commonly used against the segment)

 

The following query joins with DBA_OBJECTS to find the objects waiting, the misses, sleeps, etc:

SQL> with bh_lc as

(select /*+ ORDERED */

lc.addr, lc.child#, lc.gets, lc.misses, lc.immediate_gets,

lc.immediate_misses, lc.spin_gets, lc.sleeps,

bh.hladdr, bh.tch tch, bh.file#, bh.dbablk, bh.class,

bh.state, bh.obj

from

x$kslld ld,

v$session_wait sw,

v$latch_children lc,

x$bh bh

where lc.addr =sw.p1raw

and sw.p2= ld.indx

and ld.kslldnam='cache buffers chains'

and lower(sw.event) like '%latch%'

and sw.state='WAITING'

and bh.hladdr=lc.addr

)

select bh_lc.hladdr, bh_lc.tch, o.owner, o.object_name, o.object_type,

bh_lc.child#, bh_lc.gets,

bh_lc.misses, bh_lc.immediate_gets,

bh_lc.immediate_misses, spin_gets, sleeps

from

bh_lc,

dba_objects o

where bh_lc.obj = o.object_id(+)

union

select bh_lc.hladdr, bh_lc.tch, o.owner, o.object_name, o.object_type,

bh_lc.child#, bh_lc.gets, bh_lc.misses, bh_lc.immediate_gets,

bh_lc.immediate_misses, spin_gets, sleeps

from

bh_lc,

dba_objects o

where bh_lc.obj = o.data_object_id(+)

order by 1,2 desc;