Settings for Oracle Library Cache Reload andLibrary Cache Hit Ratios
For optimal performance, you’ll want tokeep the library cache reload ratio [sum(reloads) / sum(pins)] at zero and thelibrary cache hit ratio greater than 95 percent. If the reload ratio is notzero, then statements are being “aged out” that are later needed and broughtback into memory. If the reload ratio is zero (0), it means items in thelibrary cache were never aged or invalidated. If the reload ratio is greaterthan 1 percent, the SHARED_POOL_SIZE parameter should probably be increased.Likewise, if the library cache hit ratio comes in less than 95 percent, thenthe SHARED_POOL_SIZE parameter may need to be increased. Also, if you are usingASMM, the SGA_TARGET includes both auto-tuned and manual parameters. When youdecide to raise a parameter specifically (such as SHARED_POOL_SIZE), itinfluences the auto-tuned part. (Other parameters are affected; see Metalinknotes 443746.1 and 295626.1, “How to Use Automatic Shared Memory (AMM) in 11gand How to Use Automatic Shared Memory Management (ASMM) in Oracle 10g.”)
You can monitor the library cache in acouple of ways. The first method is to execute the STATSPACK report. The secondis to use the V$LIBRARYCACHE view.
The following query uses theV$LIBRARYCACHE view to examine the reload ratio in the library cache:
This next query uses the V$LIBRARYCACHEview to examine the library cache’s hit ratio in detail:
This hit ratio is excellent (greater than99 percent) and does not require any increase in the SHARED_POOL_SIZEparameter.
Using Individual Library Cache Parametersto Diagnose Shared Pool Use
Using a modified query on the same table,you can see how each individual parameter makes up the library cache. This mayhelp diagnose a problem or reveal overuse of the shared pool.
Use the following list to help interpretthe contents of the V$LIBRARYCACHE view:
· namespace The object type stored in the librarycache. The values SQL AREA, TABLE/PROCEDURE, BODY, and TRIGGER show the keytypes.
· gets Shows the number of times an item inlibrary cache was requested.
· gethits Shows the number of times a requested itemwas already in the library cache.
· gethitratio Shows the ratio of gethits to gets.
· pins Shows the number of times an item in thelibrary cache was executed.
· pinhits Shows the number of times an item wasexecuted when that item was already in the library cache.
· pinhitratio Shows the ratio of pinhits to pins.
· reloads Shows the number of times an item had tobe reloaded into the library cache because it aged out or was invalidated.
Keeping the Pin Hit Ratio for LibraryCache Items Close to 100 Percent
The pin hit ratio for all library cacheitems—sum(pinhits)/sum(pins—should be close to 1 (or a 100 percent hit ratio).A pin hit ratio of 100 percent means that every time the system needs toexecute something, it is already allocated and valid in the library cache.Although you will always experience some misses the first time a request ismade, misses can be reduced by writing identical SQL statements.
TIP
Measure hit ratios for the library cacheof the shared pool with the V$LIBRARYCACHE view. A hit ratio of greater than 95percent should be achieved. However, when the database is initially started,hit ratios are around 85 percent.
Keeping the Miss Ratio Less Than 15Percent
The miss ratio for data dictionary cachesum(getmisses)/sum(gets) should be less than 10 to 15 percent. A miss ratio ofzero (0) means that every time the system went into the data dictionary cache,it found what it was looking for and did not have to retrieve the informationfrom disk. If the miss ratio “sum(getmisses)/sum(gets)” is greater than 10–15 percent,the initialization SHARED_POOL_SIZE parameter should be increased.
