Oracle7 Server Tuning

Tuning Your Operating System

You should begin tuning memory allocation by tuning your operating system with these goals:

to reduce paging and swapping

to fit the System Global Area (SGA) into main memory

to allocate enough memory to individual users

These goals apply in general to most operating systems. However, the details of tuning your operating system vary depending on which operating system you are using.

Additional Information: Refer to your operating system hardware and software documentation as well as your Oracle operating system-specific documentation for more information on tuning your operating system memory usage.

Reducing Paging and Swapping

Your operating system may store information in any of these places:

real memory

virtual memory

expanded storage

disk

Your operating system may also move information from one storage location to another. Depending on your operating system, this movement is called paging or swapping. Many operating systems page and swap to accommodate large amounts of information that do not fit into real memory. However, paging and swapping take time. Excessive paging or swapping can reduce the performance of many operating systems.

Monitor your operating system behavior with operating system utilities. Excessive paging or swapping indicates that new information is often being moved into memory. In this case, your system's total memory may not be large enough to hold everything for which you have allocated memory. You should either increase the total memory on your system or decrease the amount of memory you have allocated.

Tuning the System Global Area (SGA)

Since the purpose of the System Global Area (SGA) is to store data in memory for fast access, the SGA should always be contained in main memory. If the SGA is swapped to disk, its data is no longer so quickly accessible. On most operating systems, the disadvantage of excessive paging significantly outweighs the advantage of a large SGA, so you should ensure that the entire SGA always fits into memory and is not paged or swapped.

You can cause Oracle to read the entire SGA into memory when you start your instance by setting the value of the initialization parameter PRE_PAGE_SGA to YES. This setting may 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 that this setting does not prevent your operating system from paging or swapping the SGA after it is initially read into memory.

You can see how much memory is allocated to the SGA and each of its internal structures by issuing this Server Manager statement:

SVRMGR> SHOW SGA

The output of this statement might look like this:

Total System Global Area        3554188 bytes

              Fixed Size          22208 bytes

           Variable Size        3376332 bytes

        Database Buffers         122880 bytes

            Redo Buffers          32768 bytes

Information about the SGA can also be obtained through SNMP.

Some operating systems for IBM mainframe computers are equipped with expanded storage, or special memory, in addition to main memory to which paging can be performed very quickly. These operating systems may be able to page data between main memory and expanded storage faster than Oracle can read and write data between the SGA and disk. For this reason, allowing a larger SGA to be swapped may lead to better performance than ensuring that a smaller SGA stays in main memory. If your operating system has expanded storage, you can take advantage of it by allocating a larger SGA despite the resulting paging.

User Memory Allocation

On some operating systems, you may have control over the amount of physical memory allocated to each user. Be sure all users are allocated enough memory to accommodate the resources they need to use their application with Oracle. Depending on your operating system, these resources may include

the Oracle executable image

the SGA

Oracle application tools

application-specific data

On some operating systems, Oracle software can be installed so that a single executable image can be shared by many users. By sharing executable images among users, you can reduce the amount of memory required by each user.