Archive for August, 2011

Dear friends,
     Today I saw AWR report Top 5 time wait events. I saw one wait event “read by other session “.
Read by other session wait event & Buffer busy wait event are same.

Oracle 9i we called buffer busy wait event and oracle 10g/later we called “read by other session”

About “Read by other session wait event”

When a user issue the query in a database, oracle server processes will read the database blocks from disk to database buffer cache. When two or more session issue the same query/related query (access the same database blocks), the first session will read the data from database buffer cache while other sessions are in wait.

The resolution of a “buffer busy wait”  events is one of the most confounding problems with Oracle.  In an I/O-bound Oracle system, buffer busy waits are common, as evidenced by any system with read (sequential/scattered) waits in the top-five waits.

We simply say, several concurrent sessions will read the same blocks/same table or same index block.

How can we find the block contention?

AWR/Statspack report top 5 wait event shows the read by other session or Buffer busy wait event and also we can see wait event section.

SELECT p1 “file#”, p2 “block#”, p3 “class#”
FROM v$session_wait
WHERE event = ‘read by other session’;

SELECT p1 “file#”, p2 “block#”, p3 “wait class#”
FROM v$session_wait
WHERE event = ‘buffer busy waits’;

Also using v$segment_statistics or v$system_event we can see the buffer busy wait event.

How can we tune the Read by other session wait event?

Hot Objects/Blocks:

Number of concurrent session’s access single block in an object is known as hot object.

Using AWR report “Segment statistics” section shows the HOT objects list.

Or using below query we find the hot objects.

SELECT relative_fno, owner, segment_name, segment_type
FROM dba_extents
WHERE file_id = &file
AND &block BETWEEN block_id AND block_id + blocks – 1;

Why buffer busy wait/read by other session event happen? How we reduce the buffer busy waits?

Increasing INITRANS value method:

First we should know how concurrent sessions accessing a single block in an object?

Each db block having 3 layers.

1. Cache layer
2. Transaction layer
3. Data layer

Transaction layer is playing vital role for block contention.

Each block will have ITL (INTERESTED TRANSACTION LIST) slots. This ITL slots is required for any sessions that’s need to modify a block in an object.

INITRANS value for table having segment 1 & INITRANS for index segment having 2.

MAXTRANS value default is 255.
If there is no free ITL slot in a blocks, then transaction will waiting for serially for a free ITL slot. By increasing INITRANS value to avoid the serial transaction waiting. Concurrently number of session will perform the DML operation in single block.
Each ITL requires approximately 23 bytes in the block header.

Increasing PCTFREE method:

Suppose a single 8 KB block contains 1000 rows. We reducing the rows in a block can easily reduce the buffer busy wait.

PCTFREE space is used for future updates only. We have an 8 KB data block. Default PCTFREE value is 10%. If we increased the PCTFREE value is 20% automatically number of rows inserted in a block is automatically reduced.

Reducing database block size method:

It’s similar to PCTFREE method. Suppose a single 8 KB block contains 1000 rows. Using db multiblock size future we used 4 KB data block. Now 1000 rows should be stored two 4 KB blocks.

Our goal is to reduce the number of records stored in a block.

Tune the inefficient queries:

Reduce the number of blocks accessing for an objects in buffer cache. By tuning the query to minimize the number of blocks reads from disk to database buffer cache.

Example: I have a one huge table & it contains 10000 blocks. There is no index for this table.If we doing any operation against this table, it’s going full table scan & accessing all the blocks in a table (server process reads the 10000 blocks from disk to database buffer cache). We can put proper index for this table & avoid the full table scan.


Tune inefficient queries
Review the execution plan and make sure the plan chosen by Oracle is that read the fewest blocks possible. Optimize the SQL statement to reduce the number of physical and logical reads.

Adjusting the PCTFREE value downward for an object will reduce the number of rows physically stored in a block. Adjusting the PCTUSED value for an object keeps that object from getting prematurely put back on the freelist. Increase the number of FREELISTS and FREELIST GROUPS. Depending on the type of contention, adjusting these values could help distribute data among more blocks and reduce the hot block problem. Be careful to optimize these parameters so blocks do move in and out of the freelist too frequently.

Reduce the Block Size
This is very similar to adjusting the PCTFREE and PCTUSED parameters in that the goal is to reduce the amount of data stored within one block. In Oracle 9i and higher this can be achieved by storing the hot object in a tablespace with a smaller block size. In databases prior to Oracle 9i the entire database must be rebuilt with a smaller block size.

Optimize indexes
A low cardinality index has a relatively small number of unique values, e.g. a column containing state data with only 50 values. Similar to inefficient queries, the use of a low cardinality index could cause excessive number of blocks to be read into the buffer cache and cause premature aging out of “good” blocks.

I Hope this article helped to you. I am expecting your suggestions/feedback.
It will help to motivate me to write more articles….!!!!

Thanks & Regards,
“Key for suceess, always fight even knowing your defeat is certain….!!!!”

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Dear Friends,

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Thanks & Regards,
“Key for suceess, always fight even knowing your defeat is certain….!!!!”

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