原文地址：
https://docs.oracle.com/database/121/REFRN/GUID-48C49E38-BE48-46FB-8654-493DE6D9987B.htm#REFRN10174
| Property | Description |
|---|---|
Parameter type |
Boolean |
Default value |
|
Modifiable |
No |
Modifiable in a PDB |
No |
Range of values |
|
Basic |
No |
PRE_PAGE_SGA determines whether Oracle reads the entire SGA into memory at instance startup. Operating system page table entries are then prebuilt for each page of the SGA. This setting can increase the amount of time necessary for instance startup, but it is likely to decrease the amount of time necessary for Oracle to reach its full performance capacity after startup.
Note:
This setting does not prevent your operating system from paging or swapping the SGA after it is initially read into memory.
PRE_PAGE_SGA can increase the process startup duration, because every process that starts must access every page in the SGA. The cost of this strategy is fixed; however, you might simply determine that 20,000 pages must be touched every time a process starts. This approach can be useful with some applications, but not with all applications. Overhead can be significant if your system frequently creates and destroys processes by, for example, continually logging on and logging off.
The advantage that PRE_PAGE_SGA can afford depends on page size. For example, if the SGA is 80 MB and the page size is 4 KB, then 20,000 pages must be touched to refresh the SGA (80,000/4 = 20,000).
If the system permits you to set a 4 MB page size, then only 20 pages must be touched to refresh the SGA (80,000/4,000 = 20). The page size is operating system-specific and generally cannot be changed. Some operating systems, however, have a special implementation for shared memory whereby you can change the page size.
在12c之前的版本中，这个参数默认是false，从12c开始，这个参数默认是true。
当然，在12c之前，设置这个参数为true，可能会引起连接的时候出现错误，或者连接连不上。所以建议设置成false。
从12c开始，默认是true，MOS上有一文档，建议 将该参数设置成true，而且MOS上进行了true和false的相关测试，测试结果表明，几乎没有影响。
而MOS PRE_PAGE_SGA Behaviour Change in Oracle Release 12c (文档 ID 1987975.1) 中。建议将该参数设置成默认的true 。
It is advised to leave PRE_PAGE_SGA to default value of TRUE in 12c.