Oracle Concepts - Guidelines for Tuning the Oracle Shared Pool
In Summary
I hope you nowunderstand that the old "just increase the shared pool" answer isn'tgood enough anymore when it comes to tuning problems. You must take an in depthlook at your shared pool and tune what needs to be tuned, not just throw memoryat a problem until it submerges. Indeed, I have shown that in some casesincreasing the size of the shared pool may harm performance and decreasing thesize may be advisable. The shared pool is vital to the proper performanceof your Oracle database, you must have it properly tuned or drown in badperformance. Next we will cover what to pin, the shared pool and multi-threadedserver, hashing and generalized library and dictionary cache tuning. We havealso discussed ways to monitor for what objects should be pinned, discussedmulti-threaded server, looked at hashing problems and their resolution as wellas examined classic library and data dictionary cache tuning. We haveestablished 8 guidelines for tuning the Oracle shared pool:
Guideline 1: Ifgross usage of the shared pool in a non-ad-hoc environment exceeds 95% (risesto 95% or greater and stays there) establish a shared pool size large enough tohold the fixed size portions, pin reusable packages and procedures. Graduallyincrease shared pool by 20% increments until usage drops below 90% on theaverage.
Guideline 2: Ifthe shared pool shows a mixed ad-hoc and reuse environment, establish a sharedpool size large enough to hold the fixed size portions, pin reusable packagesand establish a comfort level above this required level of pool fill. Establisha routine flush cycle to filter non-reusable code from the pool.
Guideline 3: Ifthe shared pool shows that no reusable SQL is being used establish a sharedpool large enough to hold the fixed size portions plus a few megabytes (usuallynot more than 40) and allow the shared pool modified least recently used (LRU)algorithm to manage the pool. (also see guideline 8)
Guideline 4:Determine usage patterns of packages, procedures, functions and cursors and pinthose that are frequently used.
Guideline 5: InOracle7when using MTS increase the shared pool size to accommodate MTSmessaging and queuing as well as UGA requirements. In Oracle8 use the LargePool to prevent MTS from effecting the shared pool areas.
Guideline 6: Usebind variables, PL/SQL (procedures or functions) and views to reduce the sizeof large SQL statements to prevent hashing problems.
Guideline 7: In asystem where there is no flushing increase the shared pool size in 20%increments to reduce reloads and invalidations and increase object cache hitratios.
Guideline 8: Inany shared pool, if the overall data dictionary cache miss ratio exceeds 1percent, increase the size of the shared pool.
Using theseguidelines and the scripts and techniques covered in this lesson, your shouldbe well on the way towards a well tuned and well performing shared pool.
Table 18:Initialization Parameters That Effect The Shared Pool
| NAME | DESCRIPTION |
|---|---|
| shared_pool_size | size in bytes of shared pool (7 and 8) |
| shared_pool_reserved_size | size in bytes of reserved area of shared pool (7 and 8) |
| shared_pool_reserved_min_alloc | minimum allocation size in bytes for reserved area of shared pool (7 and 8) |
| large_pool_size | size in bytes of the large allocation pool (8 only) |
| parallel_max_servers | Maximum number of parallel query slaves, if set forces calculation and setting of large pool size parameter (8i) |
| parallel_adaptive_multi_user | If set forces calculation of large pool size is over-ridden if size manually set(8i). Oracle DOES NOT recommend setting parallel_adaptive_multi_user. |
| parallel_automatic_tuning | If set forces calculation of large pool size is over-ridden if size is manually set (8i) |
| large_pool_min_alloc | minimum allocation size in bytes for the large allocation pool (8 only, obsolete in 8i) |
| parallel_min_message_pool | minimum size of shared pool memory to reserve for pq servers (8 only, obsolete in 8i) |
| backup_io_slaves | Number of backup IO slaves to configure (8 only) |
| temporary_table_locks | Number of temporary table locks to configure (7 and 8) |
| dml_locks | Number of DML locks to configure (7 and 8) |
| sequence_cache_entries | Number of sequence numbers to cache (7 and 8) |
| row_cache_cursors | Number of row caches to set up (7 and 8) |
| max_enabled_roles | Number of role caches to set up (7 and 8) |
| mts_dispatchers | Number of MTS dispatcher processes to start with (7 and 8) |
| mts_max_dispatchers | Maximum number of dispatcher processes to allow (7 and 8) |
| mts_servers | Number of MTS servers to start with (7 and 8) |
| mts_max_servers | Maximum number of MTS servers to allow (7 and 8) |
| open_cursors | Maximum number of open cursors per session (7 and 8) |
| Cursor_space_for_time | Hold open cursors until process exits (7 and 8) |
Table 19:Initialization Parameters Used In Tuning Shared Pool
| VIEW NAME | PURPOSE |
|---|---|
| V$PARAMETER | Contains current settings for all documented initialization parameters |
| V$SGASTAT | Contains sizing information for all SGA areas |
| V$SQLAREA | Contains information and statistics on the SQL area of the shared pool |
| V$DB_OBJECT_CACHE | Contains information on all cached objects in the database shared pool area |
| V$LIBRARYCACHE | Contains statistics on the library caches |
| V$ROWCACHE | Contains statistics on the data dictionary caches |
| DBA_USERS | Contains database user information |
| V$BUFFER_POOL | Oracle8 view showing pool areas |
| V$BUFFER_POOL_STATISTICS | Oracle8 buffer pool statistics |
| V$BH | View that monitors every buffer in buffer pool |
Table 20: ViewsDealing With Shared Pool and Buffer Tuning
| Software | Manufacturer | Purpose |
|---|---|---|
| Oracle Administrator | RevealNet, Inc. | Administration Knowledge base |
| Q Diagnostic | Savant, Corp. | Provide Oracle DB diagnostics |
Table 21: SoftwareMentioned in Lessons
