Maximum Shared-Memory Segment Size

When the database server creates the required shared-memory segments, it attempts to acquire as large an operating-system segment as possible. The first segment size that the database server tries to acquire is the size of the portion that it is allocating (resident, virtual, or communications), rounded up to the nearest multiple of 8 kilobytes.

The database server receives an error from the operating system if the requested segment size exceeds the maximum size allowed. If the database server receives an error, it divides the requested size by two and tries again. Attempts at acquisition continue until the largest segment size that is a multiple of 8 kilobytes can be created. Then the database server creates as many additional segments as it requires.

Using More Than Two Gigabytes of Memory (Windows)

The database server can access shared-memory segments larger than two gigabytes on Windows. However, you must enable this feature with an entry in the Windows boot file.

To add the entry, edit the boot.ini file (located in the top level, or root directory). You can either add a new boot option or use the currently existing boot option. To enable support for more than two gigabytes, add the following text to the end of the boot line:

/3GB 

The following example has support for more than two gigabytes enabled:

[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows NT
Workstation Version 4.00" 
/3GB

The maximum size of the shared-memory segment depends on the operating system, but it is approximately 3 gigabytes for Windows without additional drivers.

Maximum Number of Shared-Memory Identifiers (UNIX)

Shared-memory identifiers affect the database server operation when a virtual processor attempts to attach to shared memory. The operating system identifies each shared-memory segment with a shared-memory identifier. For most operating systems, virtual processors receive identifiers on a first-come, first-served basis, up to the limit that is defined for the operating system as a whole. For more information about shared-memory identifiers, refer to How Virtual Processors Attach to Shared Memory.

You might be able to calculate the maximum amount of shared memory that the operating system can allocate by multiplying the number of shared-memory identifiers by the maximum shared-memory segment size.