IBM 000-8697 Manuel D’Utilisation

IBM Informix OnLine Database Server Administrator’s Guide

The first shared-memory segment is attached to the virtual address space of
each process at the same virtual address defined as

SHMBASE

.

SHMBASE

identifies the specific virtual address where the database server processes
attach the first, or base, shared-memory segment. (Refer to

for an illustration of the virtual address space of a database server

process.)

The reason that all user processes share the same

SHMBASE

value is to speed

execution. All user processes can reference locations in shared memory
without recalculating shared-memory addresses because all addresses begin
at the same base address. All addresses assume that shared memory begins
at the address specified as

SHMBASE

. That is, all addresses are relative to

SHMBASE

. If each user process attached to shared memory at a different

location, shared-memory addresses would be relative to the start of shared
memory and would have to be recalculated for each user process, slowing
execution.

The specific value of

SHMBASE

 is often machine-dependent. It is not an

arbitrary number. Informix selects a value for

SHMBASE

 that will keep the

shared-memory segments safe in case the user process dynamically acquires
additional memory space.

Different

UNIX

systems accommodate additional memory at different virtual

addresses. Some

UNIX

architectures extend the highest virtual address of the

user process data segment to accommodate the next segment. In this case, it
is possible that the data segment could grow into the shared-memory
segment.

The server process function stack or (heap) can pose another threat. Some

UNIX

architectures begin the function stack at a high virtual address to keep

it clear of the growing data segments. If the stack begins too close to the
shared-memory segments, the stack can overwrite the end of shared memory.

Some versions of

UNIX

 require the user to specify a

SHMBASE

 of virtual

address of 0. The 0 address informs the

UNIX

 kernel that the kernel should

pick the best address at which to attach the shared-memory segments. This
kernel-selects option is an attempt to respond to the many different ways that
an application can affect the growth of the server process . However, all

UNIX

architectures do not support the kernel-selects option. Moreover, the kernel’s
selection is not always the best choice for all applications.

Informix recommends that you do not attempt to change the value of

SHMBASE

.