Lock_sga和pre_page_sga参数详解(二)

No

LOCK_SGAlocks the entire SGA into physical memory. It is usually advisable to lockthe SGA into real (physical) memory, especially if the use of virtual memorywould include storing some of the SGA using disk space. This parameter isignored on platforms that do not support it.

PRE_PAGE_SGA

Property

Description

Parameter type

Boolean

Default value

false

Modifiable

No

Range of values

true | false

PRE_PAGE_SGAdetermines whether Oracle reads the entire SGA into memory at instancestartup. Operating system page table entries are then prebuilt for each page ofthe SGA. This setting can increase the amount of time necessary for instancestartup, but it is likely to decrease the amount of time necessary for Oracleto reach its full performance capacity after startup.

Note:

This setting does not prevent your operating system from paging orswapping the SGA after it is initially read into memory.

PRE_PAGE_SGAcan increase the process startup duration, because every process thatstarts 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 everytime a process starts. This approach can be useful with some applications, butnot with all applications. Overhead can be significant if your systemfrequently creates and destroys processes by, for example, continually loggingon and logging off.

The advantage thatPRE_PAGE_SGAcan afford depends on page size. For example, if the SGA is 80 MB in sizeand 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 mustbe touched to refresh the SGA (80,000/4,000 = 20). The page size is operatingsystem-specific and generally cannot be changed. Some operating systems,however, have a special implementation for shared memory whereby you can changethe page size.