Performance Hit Ratios

Question

Hi
I would like calculate the following hit ratios against my instance performance metrics
1.Buffer Cache Hit %
2.Library Cache Hit %
3.Redo No Wait %
4.Memory Sort %
5.Soft Parse %
6.Commit %
And I need to know what would be the optimal values against this hit ratio for 900 GB database with Aix 5.3 platform.
if i dont have optimal values with query results. What are all the parameter should i need to tune and how much i need to tune?
Kindly help me on this.
Thanks
Ranga

Answer 1

V$PGASTAT
This view gives instance-level statistics on the PGA memory usage and the automatic PGA memory manager. For example:
SELECT * FROM V$PGASTAT;
The output of this query might look like the following:
NAME VALUE UNIT
aggregate PGA target parameter 41156608 bytes
aggregate PGA auto target 21823488 bytes
global memory bound 2057216 bytes
total PGA inuse 16899072 bytes
total PGA allocated 35014656 bytes
maximum PGA allocated 136795136 bytes
total freeable PGA memory 524288 bytes
PGA memory freed back to OS 1713242112 bytes
total PGA used for auto workareas 0 bytes
maximum PGA used for auto workareas 2383872 bytes
total PGA used for manual workareas 0 bytes
maximum PGA used for manual workareas 8470528 bytes
over allocation count 291
bytes processed 2124600320 bytes
extra bytes read/written 39949312 bytes
cache hit percentage 98.15 percent
The main statistics displayed in V$PGASTAT are as follows:
aggregate PGA target parameter: This is the current value of the initialization parameter PGA_AGGREGATE_TARGET. The default value is 20% of the SGA size. If you set this parameter to 0, automatic management of the PGA memory is disabled.
aggregate PGA auto target: This gives the amount of PGA memory Oracle can use for work areas running in automatic mode. This amount is dynamically derived from the value of the parameter PGA_AGGREGATE_TARGET and the current work area workload. Hence, it is continuously adjusted by Oracle. If this value is small compared to the value of PGA_AGGREGATE_TARGET, then a lot of PGA memory is used by other components of the system (for example, PL/SQL or Java memory) and little is left for sort work areas. You must ensure that enough PGA memory is left for work areas running in automatic mode.
global memory bound: This gives the maximum size of a work area executed in AUTO mode. This value is continuously adjusted by Oracle to reflect the current state of the work area workload. The global memory bound generally decreases when the number of active work areas is increasing in the system. As a rule of thumb, the value of the global bound should not decrease to less than one megabyte. If it does, then the value of PGA_AGGREGATE_TARGET should probably be increased.
total PGA allocated: This gives the current amount of PGA memory allocated by the instance. Oracle tries to keep this number less than the value of PGA_AGGREGATE_TARGET. However, it is possible for the PGA allocated to exceed that value by a small percentage and for a short period of time, when the work area workload is increasing very rapidly or when the initialization parameter PGA_AGGREGATE_TARGET is set to a too small value.
total freeable PGA memory: This indicates how much allocated PGA memory which can be freed.
total PGA used for auto workareas: This indicates how much PGA memory is currently consumed by work areas running under automatic memory management mode. This number can be used to determine how much memory is consumed by other consumers of the PGA memory (for example, PL/SQL or Java):
PGA other = total PGA allocated - total PGA used for auto workareas
over allocation count: This statistic is cumulative from instance start-up. Over-allocating PGA memory can happen if the value of PGA_AGGREGATE_TARGET is too small to accommodate the PGA other component in the previous equation plus the minimum memory required to execute the work area workload. When this happens, Oracle cannot honor the initialization parameter PGA_AGGREGATE_TARGET, and extra PGA memory needs to be allocated. If over-allocation occurs, you should increase the value of PGA_AGGREGATE_TARGET using the information provided by the advice view V$PGA_TARGET_ADVICE.
total bytes processed: This is the number of bytes processed by memory-intensive SQL operators since instance start-up. For example, the number of byte processed is the input size for a sort operation. This number is used to compute the cache hit percentage metric.
extra bytes read/written: When a work area cannot run optimally, one or more extra passes is performed over the input data. extra bytes read/written represents the number of bytes processed during these extra passes since instance start-up. This number is also used to compute the cache hit percentage. Ideally, it should be small compared to total bytes processed.
cache hit percentage: This metric is computed by Oracle to reflect the performance of the PGA memory component. It is cumulative from instance start-up. A value of 100% means that all work areas executed by the system since instance start-up have used an optimal amount of PGA memory. This is, of course, ideal but rarely happens except maybe for pure OLTP systems. In reality, some work areas run one-pass or even multi-pass, depending on the overall size of the PGA memory. When a work area cannot run optimally, one or more extra passes is performed over the input data. This reduces the cache hit percentage in proportion to the size of the input data and the number of extra passes performed. Example 7-3 shows how cache hit percentage is affected by extra passes.