Specify where to attach the first shared-memory segment

Each virtual processor attaches to the first shared-memory segment at the same virtual address. This situation enables all virtual processors within the same database server instance to reference the same locations in shared memory without calculating shared-memory addresses. All shared-memory addresses for an instance of the database server are relative to SHMBASE.

The value of SHMBASE is sensitive for the following reasons:

• The specific value of SHMBASE depends on the platform and whether the processor is a 32-bit or 64-bit processor. The value of SHMBASE is not an arbitrary number and is intended to keep the shared-memory segments safe when the virtual processor dynamically acquires additional memory space.
• Different operating systems accommodate additional memory at different virtual addresses. Some architectures extend the highest virtual address of the virtual-processor data segment to accommodate the next segment. In this case, the data segment might grow into the shared-memory segment.
• Some versions of UNIX™ require the user to specify an SHMBASE parameter of virtual address zero. The zero address informs the UNIX kernel that the kernel picks the best address at which to attach the shared-memory segments. However, not all UNIX architectures support this option. Moreover, on some systems, the selection that the kernel makes might not be the best selection.