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Automatic Database Diagnostic Monitor - BGOUG

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					Oracle Touch Count
Algorithm



     Edward Hayrabedian
    Semantec Bulgaria OOD
Presentation objectives:

 Why a new algorithm is needed


 General TC algorithm


 Oracle’s TC algorithm


 Optimizing Oracle’s buffer cache
New challenges …
       New algorithm
 Bigger caches. As more memory is
 available, buffer caches continue to
 grow uncomfortably huge.

 Better performance requirements
 dictate improved “big cache” memory
 management.

 More control of the buffer cache
 requires more granular tuning
 capabilities.
It’s all about staying in the
cache.


Oracle must find an efficient way to
 make it easy for popular blocks to
   stay in the cache and make it
 difficult for other blocks to stay in
               the cache.
Buffer cache mgt history

 Standard least recently used (LRU)
 algorithm

 Modified LRU algorithm


 Sub caches…


 Touch-count algorithm
Oracle’s standard LRU.

 Basically when a block is touched it is
 moved to the head of the LRU list.
                             MRU
 SELECT …




 Good: Pretty good chance popular blocks to
  stay in the cache.
 Bad: Full table scan blocks would flood the
  cache destroying a well developed cache.
Oracle’s modified LRU.

 Basically FTS blocks are placed at the tail
 of the LRU.
                             MRU
 NOT FTS …
    FTS …




 Good: FTS would not destroy the cache.
 Bad: Huge range scan would flood the
 cache with index leaf blocks destroying the
 cache.
Cache segmentation.

 Basically a DBA can segregate objects
  into sub-caches to improve performance.
          keep
          recycle
          default


 Good: Further preserved the main buffer cache
  while providing better caching for non-standard
  objects.
 Bad: A hassle and some “implementation risk”
  pressure.
Oracle’s touch-count LRU.

 Basically, each buffer is assigned a
 “touch-count” which assigns a kind of value
 or popularity.

             tch 1 tch 2 tch 3   …   tch n
Key TC components.

 Midpoint insertion.


 Touch-count incrementation.


 Buffer movement.


 Parameters for everything…
Midpoint insertion details.

 The buffer cache is divided into a hot region and
  a cold region.
                       midpoint pointer




 A midpoint pointer is maintained which moves to
  ensure proper regional block quantities.
 When a block is brought into the cache, it is
  placed in the “middle” of the LRU chain.
 A buffer naturally moves from the hot region into
  the cold region.
TC incrementation.

 In concept, whenever a block is touched its TC is
  incremented.
 In reality, it is NOT.
 No latching is used to reduce possible
  contention… so some incrementation may not
  occur.
 To reduce rapid fire buffer access cache stress, a
  touch count can only be incremented once every
  3 seconds (by default).
 Buffer movement and touch count incrementation
  are independent events.
Buffer movement.

 Keep in mind:
   TC incrementation is buffer movement
    independent.
   Blocks are inserted at the midpoint
 When:
   A server process is looking for a free buffer – OR –
   The DBWR process is looking for dirty blocks
   AND if the buffer’s touch count is greater than 2,
   ONLY, then the buffer is moved to the MRU head
 When buffer cache movement does occur, the
  buffer touch count is reset to “ZERO”!
Hot region to cold
movement.
 Regardless of its touch count,


 If a buffer crosses from the hot region into
  the cold region,

 Its touch count is reset to “ONE”!
Example
(midpoint insertion)


Midpoint insertion


      New     A        B        C        D       E       F
                  11       01       13       5       7       4
Example
(midpoint insertion)




     A        B        C        New   D       E       F
         11       01       13     0       5       7       4
Example
(buffer movement)

                       Buffer movement

                       from Cold to Hot

     A        B        C        New   D       E       F
         11       01       13     0       5       7       4



                       from Hot to Cold
Example
(buffer movement)




     B        C        New   D       E       F       A
         01       13     0       1       7       4       0
Example
(TCH incrementation)
   Incremented due to DMLs




      B        C        New   D       E       F       A
          01       13     7       1       7       4       0
Example
(buffer movement)

                             Buffer movement




     B        C        New   D       E       F       A
         01       13     7       1       7       4       0
Example
(buffer movement)




     B        C        D       E       F       A       New
         01       13       1       1       4       0     0
Buffer cache optimization.

 Solution should be based upon the
 problem.

 Problem determination should be Oracle
 Response Time Analysis based.

 Don’t mess with the TC related instance
 parameters unless you really understand
 what’s going on.
Severe contention based
change.
 For example, log buffer contention is not a good
  case for modifying TC related instance
  parameters.

 Significant buffer cache latching related
  contention is when TC related instance tuning
  may help

 Goal: Minimize latch contention (reduced of
  buffer movement) while keeping only the truly
  popular blocks cached.
Your instance parameter
options are many… (1/3)
 _db_percent_hot_default is the percentage of
 blocks in the hot region. If you want more blocks to
 be considered “hot”, increase this parameter.
 Decreasing this parameter give a buffer more TC
 incrementation time before it is scanned.


 _db_aging_touch_count is the time window in
 which a buffer’s TC can only be incremented by one.
 Increase this parameter to reduce sudden activity
 disruptions and to reduce buffer movement. You risk
 devaluing popular blocks.
Your instance parameter
options are many… (2/3)
 _db_aging_hot_criteria is the threshold when
 a buffer can be moved to the head of the LRU list. If
 you want to make it more difficult for a buffer to be
 moved (to MRU end of the list), then increase this
 value. Only really popular blocks will remain in the
 cache.


 _db_aging_stay_count is involved when a
 buffer’s TC is reset after it is moved to the head of
 the LRU list. If you want the hot buffer to remain
 really hot, increase this value.
Your instance parameter
options are many… (3/3)
 _db_aging_cool_count is the reassigned TC
 value when a buffer moves into the cool region.
 Increasing this value will slow the coolness (keep it
 “popular”) of blocks and make it easier for the buffer
 to be moved to the head of the LRU list.
Useful statistics

 X$bh – blocks inside the buffer cache
select obj object,
       file#,
       dbablk,
       lru_flag,
       status,
       tch touches,
       tim
  from x$bh
 where tch > &TCH_thresold
Resources

 Craig A. Shallahamer
“All about Oracle’s Touch Count Data Block Buffer Cache
   Algorithm” (original 2001, version 4a, Jan 5, 2004)
 Yoshihiro Uratsuji
  http://www.performance-
  insight.com/html/ora3/back/Oracle9i_1.html
 Oracle documentation
Thank you!

				
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posted:2/20/2013
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